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(43) A day in the life of an Astrophysicist at Oxford University - YouTube

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A day in the life of an Astrophysicist at Oxford University

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Day in the life of a Forensic Scientist - YouTube

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Forensic scientist | Explore careers | National Careers Service

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Forensic scientists prepare traces of physical evidence for use in courts of law.

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What is forensic science?

Any scientific process used as part of a criminal investigation is considered forensic science. This spans both the grim, grisly procedures of the autopsy room and the cutting-edge analysis of a crime scene.

But it also encompasses the less glamorous, painstaking lab work of DNA profiling, fingerprint analysis and the uncovering of hidden digital files. There is even such a thing as forensic accountancy.

What techniques are used to solve crimes today?

The bulk of modern forensic work involves the analysis of DNA or fingerprints left at a crime scene. In murder cases, forensic autopsies help work out how a person died.

A range of more specialised and elaborate forensic techniques can be used to identify suspects in the most serious cases, such as tracking serial killers or terrorists.
Read more about DNA and forensic science:
These methods include forensic ecology, where tiny traces of pollen or fungal spores can be used to tell where a suspect has been, or forensic entomology, where the presence of certain insects can help reveal how long a person has been dead.

As people spend more and more time on devices like smartphones and computers, so-called 'digital forensics' is playing an ever-greater role in criminal investigations, too.

The growth in this field of forensic science also reflects the fact that there are now over four million CCTV cameras in the UK.

Is forensic science anything like what we see on TV?

Rarely. According to forensic scientist Prof Sue Black, who has advised a number of crime writers throughout her career, "There is an element of truth in TV crime, but also an element of fantasy - the work is often long, slow, and laborious, and viewers quite rightly don't want to see us doing our double-blind trials."

Nathan Clarke, a professor of digital forensics and cyber security, says the depiction of tracking technology in spy movies, where security services miraculously enhance fuzzy images of suspects, is pure fiction.

"In reality, you need someone to sit there and watch hours of video," he says.

How does DNA profiling work?

Although 99.9 per cent of our DNA is the same in every person, the remaining 0.01 per cent is different enough to distinguish one individual from another.

Forensic DNA profiling looks specifically at highly variable stretches of DNA called 'variable number tandem repeats' (VNTRs).

These are short sequences of genetic code that may occur tens or hundreds of times at specific points in a person's DNA. VNTRs are often located in parts of the human genome with little or no known function.

Mutations in the genetic code here will not cause abnormalities, and so over many generations these sections of our genome have become hugely varied.

And because unrelated people will almost certainly have different numbers of VNTRs in different places, they can be used to discriminate between two people.

DNA found at a crime scene is processed so that these sections can be compared to those from a sample swabbed from a suspect, or compared to a huge number of DNA profiles held on police databases.

As well as helping to identify suspects, DNA profiling has helped prove the innocence of people incorrectly convicted, in some cases decades after the crime, and is often used to help identify victims, especially where people have been killed in large numbers or when their remains are badly damaged.

What's the smallest amount of DNA from which a suspect or victim can be identified?

As technology advances, scientists can process smaller and smaller samples to develop a DNA profile.

Modern techniques can 'amplify' tiny amounts of DNA from minute traces of any material that contains fragments of tissue or cells, such as blood, semen, saliva, urine, faeces, hair, teeth or bone.

'Low-level' or 'touch DNA' can sometimes even be collected from a few skin cells left behind after a person has touched an object or victim.

With a full sample and the latest DNA profiling techniques, investigators are able to generate a 'match probability' of up to one in a quintillion (10 ¹⁸).

The chance of a random person in the population having that DNA profile is infinitesimally small.

If a suspect's DNA is found at a crime scene, will it always lead to a conviction?

Not necessarily - there are all sorts of innocent reasons why a person's DNA could be at a crime scene or on a body.

And even when DNA found at a crime scene is clearly that of the perpetrator, the police still need to find a match - if the murderer is not already a suspect, and their DNA profile is not on file, the evidence is effectively useless. However, in such cases a person's own family can land them in it.

A serial killer known as the Grim Sleeper, who killed at least 10 people in Los Angeles between 1985 and 2007, eluded police for decades, despite them having a sample of his DNA. The culprit was finally apprehended when his son was arrested for weapons offences.

The son gave a regulation DNA sample, which partially matched the DNA profile found at all of the Grim Sleeper crime scenes, which led the police to investigate his relatives. Police posed as waiters to get the father's DNA from a pizza slice, and found that it matched the crime scene DNA. He was arrested in 2010.

Can DNA evidence be faked?

Although DNA profiling is an excellent way to distinguish between individuals, it is still not immune to falsification, errors or manipulation.

In 1992, a doctor and rapist called John Schneeberger evaded justice by injecting other people's blood into his arm just before his DNA was sampled by police. He was found guilty when forced to take another test years later.

It's also possible, although extremely rare, for a person to be a 'genetic chimera', meaning that they have cells in their body with different DNA from the rest of them.

Why do forensic scientists still use fingerprinting?

With fingerprints, police can often use their own in-house specialists rather than call on external forensic scientists. The digitization of fingerprint records means a photograph of a fingermark can be sent from a crime scene and compared to a database almost in real time.

According to the Fingerprint Society, fingerprints remain the number one ID metric for crime scenes in the UK, accounting for the identification of well over 100,000 suspects in 2012.

Prints can also help indicate what someone was doing or how they entered a building - for example, if they are found leading up to a broken window or in a grip figuration on a weapon. And unlike DNA, it's hard to plant a fingerprint at a crime scene.

How accurate is fingerprinting?

Fingerprints may be considered an 'older' forensic technique, having first been used in the 1890s, but the technology behind them is continuously being improved. As well as the well-known method of 'dusting' for fingerprints at the scene, forensics teams can also use chemical reagents and lasers to reveal extremely faint prints.

Even more sensitive tools can be deployed if objects are taken back to the lab. One method involves gold or silver particles being placed with a sample in a vacuum; the metal will settle on the faintest of marks.

Is everyone's fingerprint really unique?

The underlying patterns of ridges on people's digits are determined genetically, but the way individual ridges divide and break is dependent on conditions in the womb and the movement of the developing foetus.

Even the fingerprints of identical twins will be different, while their DNA will be the same.

What else might police look for at a crime scene?

Criminals often wear gloves while committing premeditated crimes, but they can't float in and out of the crime scene, so footprints can be crucial.

While footwear can't definitively identify an individual, knowing the exact model of shoe the perpetrator wore is still very useful intelligence when looking for a suspect.

In more serious crimes, forensic ecologists will look for traces of biological material that can link a suspect to a certain area, such as the type of woodland where a body has been found. Pollen and fungal spores are especially important as they are picked up easily but not easily shed, even from clothing and footwear that's been washed.

But suspects are likely to have walked through many types of soil, mud, or vegetation before and after being at a crime scene. It all adds up to a huge headache for those trying to analyse and compare all the biological material found on the suspect.

The rarer the pollen or spore that forensics can match, the more credible the case. A good example is also the first ever example of forensic ecology.

In 1959, a man was murdered while travelling down the river Danube in Austria, but a body had not been found. Mud on the suspect's shoes contained a type of pollen from ancient hickory trees.

Scientists concluded this could only have come from vegetation growing on exposed Miocene-age rocks, and the only place such soil had developed was a small section of the river 20km north of Vienna.

Presented with this theory, the suspect confessed and took police to the body - exactly where the scientists had predicted.

Other techniques often seen on TV, such as blood spatter and ballistics analysis, may give detectives an idea of what happened at a crime scene, but rarely help find the perpetrator.

What can forensics teams learn from human remains?

In the first 72 hours after death, a pathologist is usually able to provide a reasonably accurate determination of the time and cause of death.

If a person has been dead for longer, forensic entomologists may be called on to estimate the time of death, based on the number and type of insects feeding on the corpse. This method can be used to determine a period of hours, weeks or even years since death.
Read more about DNA and forensic science:
Blowflies are almost always the first insects to arrive and lay eggs on a corpse, as they are mobile, common and able to smell death from up to 10km away. Eventually other families of insects are attracted to the body, such as beetles.

For more heavily decomposed or damaged remains, forensic dentists can match remnants of teeth to known dental records, or even use what they find to draw conclusions about the victim's age, size, gender, race and socioeconomic status.

What is currently being developed at the cutting edge of forensic science?

Forensic scientists can use anything to link a suspect to a crime scene, as long as they can prove the samples are unlikely to match by chance.

In the US, scientists have looked at the atomic structure of fragments of glass to prove it was from the same sheet of glass as that found broken at a crime scene.

Pioneering techniques that mix digital forensics with anatomy are also now being used to identify people from small areas of their body seen in photos or videos.

Features such as vein patternation or knuckle marks can identify suspects from images showing only small areas of their hands or arms. Grimly, this is likely to be used in cases involving sexual abuse.

As our understanding of DNA improves, we may one day be able to create a photofit-style image of a suspect solely from DNA evidence.

However, such 'DNA phenotyping' is not yet accurate enough, and can't predict many aspects of how a person looks, such as whether they have a beard.

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Any scientific process used as part of a criminal investigation is considered forensic science. This spans both the grim, grisly procedures of the autopsy room and the cutting-edge analysis of a crime scene. But it also encompasses the less glamorous, painstaking lab work of DNA profiling, fingerprint analysis and the uncovering of hidden digital files.

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Forensic Science degree BSc (Hons) - BSc (Hons) - ARU

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Which job is best in forensic science?

As a forensic science graduate you'll be ready to work in a range of careers including laboratory analysis, quality control analysis, crime scene investigation and policing.

In fact, having gained a number of transferable skills, you'll find you can explore roles in broader scientific and analytical-based jobs. Or you can consider a career in areas such as research, teaching or further education.

Some of our recent graduates are working in local police forces, and in lab settings for companies such as LGC, Chemtest, Treatt, Anglian Water and Cellmark.

Graduation doesn't need to be the end of your time with us. If you'd like to continue your studies we offer a range of postgraduate courses including MSc Forensic Science. Take advantage of our Alumni Scholarship and get 20% off your fees.

Employability and personal career development

Informed by employers, our courses support an integrated approach to employability. You'll have opportunities to develop the skills and abilities they are looking for and gain a deeper understanding of how your academic learning relates to the world of work through Live Briefs and Ruskin Modules.

To amplify your career ambition, you will need to actively dedicate time outside of your studies, whether you choose to explore placements, internships or volunteering. Or you could make a social impact by collaborating with our partner organisations through our volunteer scheme Students at the Heart of Knowledge Exchange (SHoKE). These all give you an edge, providing you with experience that you will be able to evidence on your CV and talk about in interviews.

Our Employability and Careers Service offers a range of expert advice and support to build your unique professional profile, including tailored career appointments, advice on writing your CV, and help to complete job applications. You'll have 24/7 access to the Careers Centre, our comprehensive online digital resource, which empowers you to start building a Personal Career Development Plan from the very start. Design your future at ARU.

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Forensic Science degree BSc (Hons) - BSc (Hons) - ARU

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Study Forensic Science at Anglia Ruskin University and learn how to safely and intelligently get, analyse and report evidence used in criminal prosecutions.

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Radiotherapy - NHS (www.nhs.uk)

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Read about what radiotherapy is, how it works, when's used, how it's given and what the possible side effects are.

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Ultrasound scan - NHS (www.nhs.uk)

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An ultrasound scan, sometimes called a sonogram, is a procedure that uses high-frequency sound waves to create an image of part of the inside of the body.

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Non-ionising radiation (hse.gov.uk)

Health and Safety: Ionising Radiation (hse.gov.uk)

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Regulators of ionising and non-ionising radiation

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Regulators of ionising and non-ionising radiation

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Ionising Radiation (Medical Exposure) Regulations (IR(ME)R) - Care Quality Commission (cqc.org.uk)

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Ionising Radiation (Medical Exposure) Regulations (IR(ME)R)

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We enforce Ionising Radiation (Medical Exposure) Regulations (IR(ME)R) in England. This page outlines the regulations and links to legisation.

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What is Interventional Radiology? | BSIR

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Definition of IR:
"A clinical doctor who performs image guided procedures, fully interprets the imaging required to guide and monitor response of those procedures, as well as provides the pre and post procedural care for those patients receiving imaged guided surgery procedures."

"Interventional Radiology" (IR) refers to a range of techniques which rely on the use radiological image guidance (X-ray fluoroscopy, ultrasound, computed tomography [CT] or magnetic resonance imaging [MRI]) to precisely target therapy. Most IR treatments are minimally invasive alternatives to open and laparoscopic (keyhole) surgery. As many IR procedures start with passing a needle through the skin to the target it is sometimes called pinhole surgery!

This short film shows what interventional radiology is:

The essential skills of an interventional radiologist are in diagnostic image interpretation and the manipulation of needles and the use of fine catheter tubes and wires to navigate around the body under imaging control. Interventional radiologists are doctors who are trained in radiology and interventional therapy. No other specialty possesses this unique combination of skills!

There is hardly any area of hospital medicine where IR has not had some impact on patient management. Although the interventional radiologist has many techniques at his/her disposal these fall into 5 broad categories:

The range of conditions which can be treated by IR is enormous and continually expanding. You may have heard of some of the following: It is important to recognise that the interventional treatment is usually one of several treatment options available ranging from nothing, through drug treatment and up to surgery. Each case should be considered on its own merits.

Well recognised advantages of these minimally invasive techniques include reduced risks, shorter hospital stays, lower costs, greater comfort, quicker convalesence and return to work. The effectiveness of treatment is often be better than with traditional treatments. It should be noted that humans are not the only species to benefit from IR. Veterinary surgeons are also turning to interventional techniques so you may find both you and your animals offered similar treatments!

Blood vessel disease

Arteries

Narrowing of arteries leading to restricted blood flow (peripheral vascular disease): Interventional radiologists treat this by using balloons to stretch the vessel (balloon angioplasty, PTA) and sometimes metal springs called stents to hold them open. Sometimes arteries or bypass grafts block suddenly with a rapid loss of blood supply to the limb. Unless the blood supply is restored this can lead to amputation. Interventional radiologists can help by infusion of clot busting drugs directly into the artery via small catheters thus saving many limbs.

Expanded arteries (aneurysms) at risk of rupture and bleeding: IRs treat these by relining the vessel with a tube called a stent graft

Bleeding (haemorrhage). This is the most common vascular emergency treated by IR. Haemorrhage can come from almost anywhere e.g. from the gut, secondary to major injury or following birth. Bleeding can often permanently be stopped by blocking the vessel (embolization), relining the vessel with a stent graft or by blowing up a balloon in the vessel to stop the bleeding until emergency surgery can be performed. Interventional radiology is also used to prevent bleeding during some sorts of surgery e.g. during caesarean section in patients with a high risk of bleeding from an abnormal placenta (post partum haemorrhage).

Veins

Blood clots in the lung (pulmonary embolism, PE): interventional radiologists perform 2 different forms of treatment, placement of devices (inferior vena cava filters) to capture blood clots before they reach the lung preventing further PE. When there is a massive PE causing collapse an interventional radiologist may use small catheter tubes to break up the blood clot and restore blood flow.

Dilated veins (varicose veins): these most commonly occur in the legs but can occur in the pelvis or scrotum. These can be treated by blocking the vein by heat treatment (laser or microwave) or by the use of irritant drugs and embolisation techniques.

Blocked veins: this can occur in the context of blood clot in the veins (venous thrombosis, DVT) which is sometimes treated by the injection of blood clot dissolving medicines (thrombolysis) through a small catheter passed into the vein. Some patients develop blood clots as a result of a narrowing in a vein, when the clot has been broken down using balloons and stents. Sometimes tumours in the chest will compress a vein leading to facial swelling, headache and other symptoms which can usually be relieved with a stent.

Non vascular intervention

This is sometimes referred to as interventional oncology but the treatments are also effective in benign conditions. IR therapies are used for the following:

to treat the tumour / cancer (tumour ablation, embolization)

to relieve the effects of the cancer on other systems e.g. blockage of the gullet (oesophagus), bowel, kidney (nephrostomy) or liver (biliary drainage)

To drain collections of fluid or pus in the chest or abdomen

To place feeding tubes (gastrostomy, jejunostomy)

To treat collapsed spinal bones (vertebroplasty)

Tumour therapies: these treatments are intended to shrink or destroy tumours at their primary site or which have spread to other areas (metastases). This is an area of increasing interest and leading to improved survival with reduced morbidity.

Liver, kidney and other tumours (e.g. bone, lung): these can be treated by destructive therapies (ablation) usually involving heat (radiofrequency, laser, microwave, ultrasound) or cold damage (cryotherapy). The treatment is performed and monitored using imaging (ultrasound, computed tomography or magnetic resonance imaging).

Uterine fibroids : heavy menstrual bleeding and pain can be caused by benign tumours called fibroids. These can be treated by blocking blood vessels (uterine fibroid embolization, UFE) which leads to shrinkage. Embolization is sometimes combined with drug therapy (chemoembolization) or radiotherapy (radioembolization) which targets the effect to the tumour and limits some of the side effects of cancer therapy.

Stone Disease

Kidney stones are not uncommon and cause pain, infection and blockage of the kidney. Obstruction of the kidney in the presence of infection will rapidly cause irreversible kidney damage. Interventional techniques include placing a tube in the kidney (nephrostomy) to allow the urine to drain and removing the stones using a variety of instruments placed through the skin into the kidney. Large kidney stones are best dealt with by creating a tunnel into the kidney through a small skin incision and then passing an endoscope directly into the kidney, breaking the stone with special instruments and pulling the fragments out (percutaneous nephrolithotomy).

Gall stones are one of the most common upper abdominal disorders. Most are dealt with by laparoscopic surgery. When stones or tumour stop bile draining from the liver this causes jaundice, this is usually treated via a telescope passed down the throat (endoscopy) but sometimes requires an interventional radiologist to perform drainage by placing catheter tubes through the liver to either remove the stones or place stents to allow drainage.

David Kessel

Past President, BSIR

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Definition of IR:"A clinical doctor who performs image guided procedures, fully interprets the imaging required to guide and monitor response of those procedures, as well as provides the pre and post procedural care for those patients receiving imaged guided surgery procedures."

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Medical Lasers | FDA

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Center for Devices and Radiological Health

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FDA information about medical lasers including: use, risks and benefits, regulations, and industry guidance.

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MRI scan - NHS (www.nhs.uk)

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Magnetic resonance imaging (MRI) is a type of scan that uses strong magnetic fields and radio waves to produce detailed images of the inside of the body.
An MRI scanner is a large tube that contains powerful magnets. You lie inside the tube during the scan.

An MRI scan can be used to examine almost any part of the body, including the:

brain and spinal cord

bones and joints

breasts

heart and blood vessels

internal organs, such as the liver, womb or prostate gland

The results of an MRI scan can be used to help diagnose conditions, plan treatments and assess how effective previous treatment has been.

What happens during an MRI scan?

During an MRI scan, you lie on a flat bed that's moved into the scanner.

Depending on the part of your body being scanned, you'll be moved into the scanner either head first or feet first.

The MRI scanner is operated by a radiographer, who is trained in carrying out imaging investigations.

They control the scanner using a computer, which is in a different room, to keep it away from the magnetic field generated by the scanner.

You'll be able to talk to the radiographer through an intercom and they'll be able to see you on a television monitor and through the viewing window throughout the scan.

At certain times during the scan, the scanner will make loud tapping noises. This is the electric current in the scanner coils being turned on and off.

You'll be given earplugs or headphones to wear.

It's very important to keep as still as possible during your MRI scan.

The radiographer may ask you to hold your breath for a few seconds or follow other instructions during the scan.

The scan lasts 15 to 90 minutes, depending on the size of the area being scanned and how many images are taken.

Read more about how an MRI scan is performed.

How does an MRI scan work?

Most of the human body is made up of water molecules, which consist of hydrogen and oxygen atoms.

At the centre of each hydrogen atom is an even smaller particle called a proton. Protons are like tiny magnets and are very sensitive to magnetic fields.

When you lie under the powerful scanner magnets, the protons in your body line up in the same direction, in the same way that a magnet can pull the needle of a compass. You will not be able to feel this.

Short bursts of radio waves are then sent to certain areas of the body, knocking the protons out of alignment.

When the radio waves are turned off, the protons realign. This sends out radio signals, which are picked up by receivers.

These signals provide information about the exact location of the protons in the body.

They also help to distinguish between the various types of tissue in the body, because the protons in different types of tissue realign at different speeds and produce distinct signals.

In the same way that millions of pixels on a computer screen can create complex pictures, the signals from the millions of protons in the body are combined to create a detailed image of the inside of the body.

Safety

An MRI scan is a painless and safe procedure. You may find it uncomfortable if you have claustrophobia, but most people are able to manage it with support from the radiographer.

Most modern MRI scanners have a wider tunnel, which can help reduce claustrophobia. Going into the scanner feet first may be easier, although this isn't always possible.

Extensive research has been carried out into whether the magnetic fields and radio waves used during MRI scans could pose a risk to the human body.

No evidence has been found to suggest there's a risk, which means MRI scans are one of the safest medical procedures available.

But MRI scans may not be recommended in certain situations. For example, if you have a metal implant fitted, such as a pacemaker or artificial joint, you may not be able to have an MRI scan.

If you're pregnant, a doctor will discuss with you whether an MRI scan is suitable for you.

Read more about who can and can't have an MRI scan.

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Magnetic resonance imaging (MRI) is a type of scan that uses strong magnetic fields and radio waves to produce detailed images of the inside of the body.

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BBC iPlayer - Click - Mechanical Medical Marvels

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Click looks at robotic medicine dispensaries.

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BBC iPlayer - The Planets

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Brian Cox explores the lives of the eight majestic planets that make up our solar system.

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Missions – NASA Solar System Exploration

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More than 250 robotic spacecraft-and 24 humans-have ventured into space since we first began exploring beyond Earth's atmosphere in 1958. This section focuses on U.S. missions with science goals to study planets, moons, asteroids and comets beyond Earth orbit.

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Missions - NASA Solar System Exploration

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More than 250 robotic spacecraft-and 24 humans-have ventured into space since we first began exploring beyond Earth's atmosphere in 1958. This section focuses on U.S. missions with science goals to study planets, moons, asteroids and comets beyond Earth orbit.

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Basics of Space Flight - Solar System Exploration: NASA Science

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Basics of Space Flight - Solar System Exploration: NASA Science

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NASA's real-time science encyclopedia of deep space exploration. Our scientists and far-ranging robots explore the wild frontiers of our solar system.

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Overview | Asteroids, Comets & Meteors – NASA Solar System Exploration

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Don't let the name fool you. Our solar system's small bodies - asteroids, comets, and meteors - pack big surprises. These chunks of rock, ice, and metal are leftovers from the formation of our solar system 4.6 billion years ago. They are a lot like a fossil record of our early solar system. There are currently known asteroids and known comets.

Explore By Type

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Asteroids and comets - and the meteors that sometimes come from them - are leftovers from the formation of our solar system 4.6 billion years ago.

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Overview | Planets – NASA Solar System Exploration

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Our solar system can be divided into three regions: the inner solar system, the outer solar system, and the Kuiper Belt and Oort Cloud. The inner, rocky planets are Mercury, Venus, Earth, and Mars. These worlds also are known as terrestrial planets because they have solid surfaces. Mercury, Earth, a…

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In Depth | Sun – NASA Solar System Exploration

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Our Sun is a 4.5 billion-year-old yellow dwarf star - a hot glowing ball of hydrogen and helium - at the center of our solar system. It's about 93 million miles (150 million kilometers) from Earth and it's our solar system's only star. Without the Sun's energy, life as we know it could not exist on our home planet.

10 things

The part of the Sun we see from Earth - the part we call the surface - is the photosphere. The Sun doesn't actually have a solid surface because it's a ball of plasma.

The Sun would have been surrounded by a disk of gas and dust early in its history when the solar system was first forming 4.6 billion years ago. Some of that dust is still around today, in several dust rings that circle the Sun. They trace the orbits of planets, whose gravity tugs dust into place around the Sun.

As a star, the Sun doesn't have any moons, but the planets and their moons orbit the Sun.

Once material leaves the corona at supersonic speeds, it becomes the solar wind, which forms a huge magnetic "bubble" around the Sun, called the heliosphere. The heliosphere extends beyond the orbit of the planets in our solar system. Thus, Earth exists inside the Sun's atmosphere. Outside the heliosphere is interstellar space.

In one of the Sun's biggest mysteries, the corona is much hotter than the layers immediately below it. (Imagine walking away from a bonfire only to get warmer.) The source of coronal heating is a major unsolved puzzle in the study of the Sun.

Solar activity can release huge amounts of energy and particles, some of which impact us here on Earth. Much like weather on Earth, conditions in space - known as space weather - are always changing with the Sun's activity. "Space weather" can interfere with satellites, GPS, and radio communications. It also can cripple power grids, and corrode pipelines that carry oil and gas.

Another solar flare on March 13, 1989, caused geomagnetic storms that disrupted electric power transmission from the Hydro Québec generating station in Canada, plunging 6 million people into darkness for 9 hours. The 1989 flare also caused power surges that melted power transformers in New Jersey.

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From our vantage point on Earth, the Sun may appear like an unchanging source of light and heat in the sky. But the Sun is a dynamic star, constantly changing and sending energy out into space. The science of studying the Sun and its influence throughout the solar system is called heliophysics.

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Earth's Moon: Formation, Composition, Orbit & Exploration | Solar Eclipse, Solar System & Astronomy (space.com)

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Moon's formation: Scientists say a giant impact blasted the raw material for the moon off the primitive molten Earth and into orbit, forming the moon.

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Night Sky - Stargazing and Night Sky Watching (space.com)

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Night Sky - Stargazing and Night Sky Watching

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Learn about skywatching, astronomy, star gazing, and comets in the night sky at SPACE.com. View astrophotography and images of celestial bodies.

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Spaceflight - News About Space Travel and Exploration

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Spaceflight - News About Space Travel and Exploration

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Read the latest spaceflight news about manned and unmanned space-travel, launch dates, space exploration and news from the international space station.

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Solar system, our planets, asteroids and moons: Latest news (space.com)

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Solar system, our planets, asteroids and moons: Latest news

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From planets and asteroids to the sun and comets, see the latest discoveries from across the solar system.

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Space.com: NASA, Space Exploration and Astronomy News

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Get the latest space exploration, innovation and astronomy news. Space.com celebrates humanity's ongoing expansion across the final frontier.

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Electrical safety - HSE

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HSE provides information on electrical safety at work. Around 1000 electrical accidents at work are reported to HSE each year and about 25 people die of their injuries.

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Detailed Notes - Section 05 Electricity - AQA Physics A-level (physicsandmathstutor.com)

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Electricity (alevelphysicsonline.com)

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Electricity

When I studied A Level Physics I found this one of my trickiest areas. I found that analogies helped me vidualise what is happeneing in the circuits, then you've got to know the formulas and apply these in turn to solve any problems.

All exam boards (not AQA)

This is where it gets tricky! You may previously have used the term 'voltage' but now you need to understand the difference between the e.m.f. of a supply and the p.d. across a component. Please note that this analogy is not perfect, it doesn't really explain how energy can be transferred almost instantaneously or how alternating current works but it's a start...

​

Electromotive Force: The energy gained per unit charge by charges passing through a supply (from chemical to electrical).

Potential Difference: The energy lost per unit charge by charges passing through a component (from electrical to other forms).

How can you define the volt?

​

or

Interesting but not for any exam board

All exam boards (not Edexcel)

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Electricity When I studied A Level Physics I found this one of my trickiest areas. I found that analogies helped me vidualise what is happeneing in the circuits, then you've got to know the formulas and apply these in turn to solve any problems. All exam boards (not AQA) This is where it gets tricky!

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Case Study: 33-Year-Old Female Presents with Chronic SOB and Cough - StatPearls - NCBI Bookshelf (nih.gov)

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Your access to the NCBI website at www.ncbi.nlm.nih.gov has been temporarily blocked due to a possible misuse/abuse situation involving your site. This is not an indication of a security issue such as a virus or attack.

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Influenza: A case study: View as single page (open.edu)

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Influenza: A case study: View as single page

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Unless otherwise stated, copyright © 2023 The Open University, all rights reserved. Printable page generated Tuesday, 21 Feb 2023, 15:32

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Resources to find out how to treat different diseases

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Resources to find out how to treat different diseases

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Flu - NHS (www.nhs.uk)

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NHS information about flu, including symptoms, how to treat it yourself and when to get help.

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Methods by which infectious diseases can be spread

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Methods by which infectious diseases can be spread

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How Infections Spread | Infection Control | CDC

Methods of Disease Transmission (nationalgeographic.org)

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Germs are a part of everyday life and are found in our air, soil, water, and in and on our bodies. Some germs are helpful, others are harmful. Many germs live in and on our bodies without causing harm and some even help us to stay healthy. Only a small portion of germs are known to cause infection.

An infection occurs when germs enter the body, increase in number, and cause a reaction of the body.

Three things are necessary for an infection to occur:

Source: Places where infectious agents (germs) live (e.g., sinks, surfaces, human skin)

Susceptible Person with a way for germs to enter the body

Transmission: a way germs are moved to the susceptible person

Click on a tab below to learn more.

A Source is an infectious agent or germ and refers to a virus, bacteria, or other microbe.

In healthcare settings, germs are found in many places. People are one source of germs including:

Patients

Healthcare workers

Visitors and household members

People can be sick with symptoms of an infection or colonized with germs (not have symptoms of an infection but able to pass the germs to others).
Germs are also found in the healthcare environment. Examples of environmental sources of germs include:

Dry surfaces in patient care areas (e.g., bed rails, medical equipment, countertops, and tables)

Wet surfaces, moist environments, and biofilms (e.g., cooling towers, faucets and sinks, and equipment such as ventilators)

Indwelling medical devices (e.g., catheters and IV lines)

Dust or decaying debris (e.g., construction dust or wet materials from water leaks)

A susceptible person is someone who is not vaccinated or otherwise immune, or a person with a weakened immune system who has a way for the germs to enter the body. For an infection to occur, germs must enter a susceptible person's body and invade tissues, multiply, and cause a reaction.

Devices like IV catheters and surgical incisions can provide an entryway, whereas a healthy immune system helps fight infection.

When patients are sick and receive medical treatment in healthcare facilities, the following factors can increase their susceptibility to infection.

Patients in healthcare who have underlying medical conditions such as diabetes, cancer, and organ transplantation are at increased risk for infection because often these illnesses decrease the immune system's ability to fight infection.

Certain medications used to treat medical conditions, such as antibiotics, steroids, and certain cancer fighting medications increase the risk of some types of infections.

Lifesaving medical treatments and procedures used in healthcare such as urinary catheters, tubes, and surgery increase the risk of infection by providing additional ways that germs can enter the body.

Recognizing the factors that increase patients' susceptibility to infection allows providers to recognize risks and perform basic infection prevention measures to prevent infection from occurring.

Transmission refers to the way germs are moved to the susceptible person.

Germs don't move themselves. Germs depend on people, the environment, and/or medical equipment to move in healthcare settings.

There are a few general ways that germs travel in healthcare settings - through contact (i.e., touching), sprays and splashes, inhalation, and sharps injuries (i.e., when someone is accidentally stuck with a used needle or sharp instrument).

Contact moves germs by touch (example: MRSA or VRE). For example, healthcare provider hands become contaminated by touching germs present on medical equipment or high touch surfaces and then carry the germs on their hands and spread to a susceptible person when proper hand hygiene is not performed before touching the susceptible person.

Sprays and splashes occur when an infected person coughs or sneezes, creating droplets which carry germs short distances (within approximately 6 feet). These germs can land on a susceptible person's eyes, nose, or mouth and can cause infection (example: pertussis or meningitis).
Close range inhalation occurs when a droplet containing germs is small enough to breathe in but not durable over distance.

Inhalation occurs when germs are aerosolized in tiny particles that survive on air currents over great distances and time and reach a susceptible person. Airborne transmission can occur when infected patients cough, talk, or sneeze germs into the air (example: TB or measles), or when germs are aerosolized by medical equipment or by dust from a construction zone (example: Nontuberculous mycobacteria or aspergillus).

Sharps injuries can lead to infections (example: HIV, HBV, HCV) when bloodborne pathogens enter a person through a skin puncture by a used needle or sharp instrument.

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How Infections Spread | Infection Control | CDC

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The modes of infection transmission; how infections move from a source to a susceptible person.

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Genetic Disorders (genome.gov)

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A genetic disorder is a disease caused in whole or in part by a change in the DNA sequence away from the normal sequence. Genetic disorders can be caused by a mutation in one gene (monogenic disorder), by mutations in multiple genes (multifactorial inheritance disorder), by a combination of gene mutations and environmental factors, or by damage to chromosomes (changes in the number or structure of entire chromosomes, the structures that carry genes).

As we unlock the secrets of the human genome (the complete set of human genes), we are learning that nearly all diseases have a genetic component. Some diseases are caused by mutations that are inherited from the parents and are present in an individual at birth, like sickle cell disease. Other diseases are caused by acquired mutations in a gene or group of genes that occur during a person's life. Such mutations are not inherited from a parent, but occur either randomly or due to some environmental exposure (such as cigarette smoke). These include many cancers, as well as some forms of neurofibromatosis.

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A genetic disorder is a disease caused in whole or in part by a change in the DNA sequence away from the normal sequence. Genetic disorders can be caused by a mutation in one gene (monogenic disorder), by mutations in multiple genes (multifactorial inheritance disorder), by a combination of gene mutations and environmental factors, or by damage to chromosomes (changes in the number or structure of entire chromosomes, the structures that carry genes).

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Health and environmental impacts of diets worldwide - Global Nutrition Report

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What we eat matters: Health and environmental impacts of diets worldwide

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Current dietary patterns are neither healthy nor sustainable. We present new estimates of the associated health and environmental impacts of our diets today.

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Introduction to Pathogens - Molecular Biology of the Cell - NCBI Bookshelf (nih.gov)

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We normally think of pathogens in hostile terms-as invaders that attack our bodies. But a pathogen or a parasite, like any other organism, is simply trying to live and procreate. Living at the expense of a host organism is a very attractive strategy, and it is possible that every living organism on earth is subject to some type of infection or parasitism ( Figure 25-1). A human host is a nutrient-rich, warm, and moist environment, which remains at a uniform temperature and constantly renews itself. It is not surprising that many microorganisms have evolved the ability to survive and reproduce in this desirable niche. In this section, we discuss some of the common features that microorganisms must have in order to be infectious. We then explore the wide variety of organisms that are known to cause disease in humans.

Pathogens Have Evolved Specific Mechanisms for Interacting with Their Hosts

The human body is a complex and thriving ecosystem. It contains about 10 ¹³ human cells and also about 10 ¹⁴ bacterial, fungal, and protozoan cells, which represent thousands of microbial species. These microbes, called the normal flora, are usually limited to certain areas of the body, including the skin, mouth, large intestine, and vagina. In addition, humans are always infected with viruses, most of which rarely, if ever, become symptomatic. If it is normal for us to live in such close intimacy with a wide variety of microbes, how is it that some of them are capable of causing us illness or death?

Pathogens are usually distinct from the normal flora. Our normal microbial inhabitants only cause trouble if our immune systems are weakened or if they gain access to a normally sterile part of the body (for example, when a bowel perforation enables the gut flora to enter the peritoneal cavity of the abdomen, causing peritonitis). In contrast, dedicated pathogens do not require that the host be immunocompromised or injured. They have developed highly specialized mechanisms for crossing cellular and biochemical barriers and for eliciting specific responses from the host organism that contribute to the survival and multiplication of the pathogen.

In order to survive and multiply in a host, a successful pathogen must be able to: (1) colonize the host; (2) find a nutritionally compatible niche in the host body; (3) avoid, subvert, or circumvent the host innate and adaptive immune responses; (4) replicate, using host resources; and (5) exit and spread to a new host. Under severe selective pressure to induce only the correct host cell responses to accomplish this complex set of tasks, pathogens have evolved mechanisms that maximally exploit the biology of their host organisms. Many of the pathogens we discuss in this chapter are skillful and practical cell biologists. We can learn a great deal of cell biology by observing them.

The Signs and Symptoms of Infection May Be Caused by the Pathogen or by the Host's Responses

Although we can easily understand why infectious microorganisms would evolve to reproduce in a host, it is less clear why they would evolve to cause disease. One explanation may be that, in some cases, the pathological responses elicited by microorganisms enhance the efficiency of their spread or propagation and hence clearly have a selective advantage for the pathogen. The virus-containing lesions on the genitalia caused by herpes simplex infection, for example, facilitate direct spread of the virus from an infected host to an uninfected partner during sexual contact. Similarly, diarrheal infections are efficiently spread from patient to caretaker. In many cases, however, the induction of disease has no apparent advantage for the pathogen.

Many of the symptoms and signs that we associate with infectious disease are direct manifestations of the host's immune responses in action. Some hallmarks of bacterial infection, including the swelling and redness at the site of infection and the production of pus (mainly dead white blood cells), are the direct result of immune system cells attempting to destroy the invading microorganisms. Fever, too, is a defensive response, as the increase in body temperature can inhibit the growth of some microorganisms. Thus, understanding the biology of an infectious disease requires an appreciation of the contributions of both pathogen and host.

Pathogens Are Phylogenetically Diverse

Many types of pathogens cause disease in humans. The most familiar are viruses and bacteria. Viruses cause diseases ranging from AIDS and smallpox to the common cold. They are essentially fragments of nucleic acid ( DNA or RNA) instructions, wrapped in a protective shell of proteins and (in some cases) membrane ( Figure 25-2A). They use the basic transcription and translation machinery of their host cells for their replication.

Of all the bacteria we encounter in our lives, only a small minority are dedicated pathogens. Much larger and more complex than viruses, bacteria are usually free-living cells, which perform most of their basic metabolic functions themselves, relying on the host primarily for nutrition ( Figure 25-2B).

Some other infectious agents are eucaryotic organisms. These range from single-celled fungi and protozoa ( Figure 25-2C), through large complex metazoa such as parasitic worms. One of the most common infectious diseases on the planet, shared by about a billion people at present, is an infestation in the gut by Ascaris lumbricoides. This nematode closely resembles its cousin Caenorhabditis elegans, which is widely used as a model organism for genetic and developmental biological research (discussed in Chapter 21). C. elegans, however, is only about 1 mm in length, whereas Ascaris can reach 30 cm ( Figure 25-2D).

Some rare neurodegenerative diseases, including mad cow disease, are caused by an unusual type of infectious particle called a , which is made only of protein. Although the prion contains no genome, it can nevertheless replicate and kill the host.

Even within each class of pathogen, there is striking diversity. Viruses vary tremendously in their size, shape, and content ( DNA versus RNA, enveloped or not, and so on), and the same is true for the other pathogens. The ability to cause disease (pathogenesis) is a lifestyle choice, not a legacy shared only among close relatives ( Figure 25-3).

Each individual pathogen causes disease in a different way, which makes it challenging to understand the basic biology of infection. But, when considering the interactions of infectious agents with their hosts, some common themes of pathogenesis emerge. These common themes are the focus of this chapter. First, we introduce the basic features of each of the major types of pathogens that exploit features of host cell biology. Then, we examine in turn the mechanisms that pathogens use to control their hosts and the innate mechanisms that hosts use to control pathogens.

Bacterial Pathogens Carry Specialized Virulence Genes

Bacteria are small and structurally simple, compared to the vast majority of eucaryotic cells. Most can be classified broadly by their shape as rods, spheres, or spirals and by their cell-surface properties. Although they lack the elaborate morphological variety of eucaryotic cells, they display a surprising array of surface appendages that enable them to swim or to adhere to desirable surfaces ( Figure 25-4). Their genomes are correspondingly simple, typically on the order of 1,000,000-5,000,000 nucleotide pairs in size (compared to 12,000,000 for yeast and more than 3,000,000,000 for humans).

As emphasized above, only a minority of bacterial species have developed the ability to cause disease in humans. Some of those that do cause disease can only replicate inside the cells of the human body and are called obligate pathogens. Others replicate in an environmental reservoir such as water or soil and only cause disease if they happen to encounter a susceptible host; these are called facultative pathogens. Many bacteria are normally benign but have a latent ability to cause disease in an injured or immunocompromised host; these are called opportunistic pathogens.

Some bacterial pathogens are fastidious in their choice of host and will only infect a single species or a group of related species, whereas others are generalists. Shigella flexneri, for example, which causes epidemic dysentery (bloody diarrhea) in areas of the world lacking a clean water supply, will only infect humans and other primates. By contrast, the closely related bacterium Salmonella enterica, which is a common cause of food poisoning in humans, can also infect many other vertebrates, including chickens and turtles. A champion generalist is the opportunistic pathogen Pseudomonas aeruginosa, which is capable of causing disease in plants as well as animals.

The significant differences between a virulent pathogenic bacterium and its closest nonpathogenic relative may result from a very small number of genes. Genes that contribute to the ability of an organism to cause disease are called virulence genes. The proteins they encode are called virulence factors. Virulence genes are frequently clustered together, either in groups on the bacterial chromosome called pathogenicity islands or on extrachromosomal virulence plasmids ( Figure 25-5). These genes may also be carried on mobile bacteriophages (bacterial viruses). It seems therefore that a pathogen may arise when groups of virulence genes are transferred together into a previously avirulent bacterium. Consider, for example, Vibrio cholerae-the bacterium that causes cholera. Several of the genes encoding the toxins that cause the diarrhea in cholera are carried on a mobile bacteriophage ( Figure 25-6). Of the hundreds of strains of Vibrio cholerae found in lakes in the wild, the only ones that cause human disease are those that have become infected with this virus.

Many virulence genes encode proteins that interact directly with host cells. Two of the genes carried by the Vibrio cholerae phage, for example, encode two subunits of cholera toxin. The B subunit of this secreted, toxic protein binds to a glycolipid component of the plasma membrane of the epithelial cells in the gut of a person who has consumed Vibrio cholerae in contaminated water. The B subunit transfers the A subunit through the membrane into the epithelial cell cytoplasm. The A subunit is an enzyme that catalyzes the transfer of an ADP-ribose moiety from NAD to the trimeric G protein , which normally activates adenylyl cyclase to make cyclic AMP (discussed in Chapter 15). ADP-ribosylation of the G protein results in an overaccumulation of cyclic AMP and an ion imbalance, leading to the massive watery diarrhea associated with cholera. The infection is then spread by the fecal-oral route by contaminated food and water.

Some pathogenic bacteria use several independent mechanisms to cause toxicity to the cells of their host. Anthrax, for example, is an acute infectious disease of sheep, cattle, other herbivores, and occasionally humans. It is usually caused by contact with spores of the Gram-positive bacterium, Bacillus anthracis. Unlike cholera, anthrax has never been observed to spread directly from one infected person to another. Dormant spores can survive in soil for long periods of time and are highly resistant to adverse environmental conditions, including heat, ultraviolet and ionizing radiation, pressure, and chemical agents. After the spores are inhaled, ingested, or rubbed into breaks in the skin, the spores germinate, and the bacteria begin to replicate. Growing bacteria secrete two toxins, called lethal toxin and edema toxin. Either toxin alone is sufficient to cause signs of infection. Like the A and B subunits of cholera toxin, both toxins are made of two subunits. The B subunit is identical between lethal toxin and edema toxin, and it binds to a host cell-surface receptor to transfer the two different A subunits into host cells. The A subunit of edema toxin is an adenylyl cyclase that directly converts host cell ATP into cyclic AMP. This causes an ion imbalance that can lead to accumulation of extracellular fluid (edema) in the infected skin or lung. The A subunit of lethal toxin is a zinc protease that cleaves several members of the MAP kinase kinase family (discussed in Chapter 15). Injection of lethal toxin into the bloodstream of an animal causes shock and death. The molecular mechanisms and the sequence of events leading to death in anthrax remain uncertain.

These examples illustrate a common theme among virulence factors. They are frequently either toxic proteins (toxins) that directly interact with important host structural or signaling proteins to elicit a host cell response that is beneficial to pathogen colonization or replication, or they are proteins that are needed to deliver such toxins to their host cell targets. One common and particularly efficient delivery mechanism, called the type III secretion system, acts like a tiny syringe that injects toxic proteins from the cytoplasm of an extracellular bacterium directly into the cytoplasm of an adjacent host cell ( Figure 25-7). There is a remarkable degree of structural similarity between the type III syringe and the base of a bacterial flagellum (see Figure 15-67), and many of the proteins in the two structures are clearly homologous.

Because bacteria form a kingdom distinct from the eucaryotes they infect (see Figure 25-3), much of their basic machinery for DNA replication, transcription, translation, and fundamental metabolism is quite different from that of their host. These differences enable us to find antibacterial drugs that specifically inhibit these processes in bacteria, without disrupting them in the host. Most of the antibiotics that we use to treat bacterial infections are small molecules that inhibit macromolecular synthesis in bacteria by targeting bacterial enzymes that are either distinct from their eucaryotic counterparts or that are involved in pathways, such as cell wall biosynthesis, that are absent in humans ( Figure 25-8 and Table 6-3).

Fungal and Protozoan Parasites Have Complex Life Cycles with Multiple Forms

Pathogenic fungi and protozoan parasites are eucaryotes. It is therefore more difficult to find drugs that will kill them without killing the host. Consequently, antifungal and antiparasitic drugs are often less effective and more toxic than antibiotics. A second characteristic of fungal and parasitic infections that makes them difficult to treat is the tendency of the infecting organisms to switch among several different forms during their life cycles. A drug that is effective at killing one form is often ineffective at killing another form, which therefore survives the treatment.

The fungal branch of the eucaryotic kingdom includes both unicellular yeasts (such as Saccharomyces cerevisiae and Schizosaccharomyces pombe) and filamentous, multicellular molds (like those found on moldy fruit or bread). Most of the important pathogenic fungi exhibit dimorphism-the ability to grow in either yeast or mold form. The yeast-to-mold or mold-to-yeast transition is frequently associated with infection. Histoplasma capsulatum, for example, grows as a mold at low temperature in the soil, but it switches to a yeast form when inhaled into the lung, where it can cause the disease histoplasmosis ( Figure 25-9).

Protozoan parasites have more elaborate life cycles than do fungi. These cycles frequently require the services of more than one host. Malaria is the most common protozoal disease, infecting 200-300 million people every year and killing 1-3 million of them. It is caused by four species of Plasmodium, which are transmitted to humans by the bite of the female of any of 60 species of Anopheles mosquito. Plasmodium falciparum-the most intensively studied of the malaria-causing parasites-exists in no fewer than eight distinct forms, and it requires both the human and mosquito hosts to complete its sexual cycle ( Figure 25-10). Gametes are formed in the bloodstream of infected humans, but they can only fuse to form a zygote in the gut of the mosquito. Three of the Plasmodium forms are highly specialized to invade and replicate in specific tissues-the insect gut lining, the human liver, and the human red blood cell.

Because malaria is so widespread and devastating, it has acted as a strong selective pressure on human populations in areas of the world that harbor the Anopheles mosquito. Sickle cell anemia, for example, is a recessive genetic disorder caused by a point mutation in the gene that encodes the hemoglobin β chain, and it is common in areas of Africa with a high incidence of the most serious form of malaria (caused by Plasmodium falciparum). The malarial parasites grow poorly in red blood cells from either homozygous sickle cell patients or healthy heterozygous carriers, and, as a result, malaria is seldom found among carriers of this mutation. For this reason, malaria has maintained the sickle cell mutation at high frequency in these regions of Africa.

Viruses Exploit Host Cell Machinery for All Aspects of Their Multiplication

Bacteria, fungi, and eucaryotic parasites are cells themselves. Even when they are obligate parasites, they use their own machinery for DNA replication, transcription, and translation, and they provide their own sources of metabolic energy. Viruses, by contrast, are the ultimate hitchhikers, carrying little more than information in the form of nucleic acid. The information is largely replicated, packaged, and preserved by the host cells ( Figure 25-11). Viruses have a small genome, made up of a single nucleic acid type-either DNA or RNA-which, in either case, may be single-stranded or double-stranded. The genome is packaged in a protein coat, which in some viruses is further enclosed by a lipid envelope.

Viruses replicate in various ways. In general, replication involves (1) disassembly of the infectious virus particle, (2) replication of the viral genome, (3) synthesis of the viral proteins by the host cell translation machinery, and (4) reassembly of these components into progeny virus particles. A single virus particle (a virion) that infects a single host cell can produce thousands of progeny in the infected cell. Such prodigious viral multiplication is often enough to kill the host cell: the infected cell breaks open (lyses) and thereby allows the progeny viruses access to nearby cells. Many of the clinical manifestations of viral infection reflect this cytolytic effect of the virus. Both the cold sores formed by herpes simplex virus and the lesions caused by the smallpox virus, for example, reflect the killing of the epidermal cells in a local area of infected skin.

Viruses come in a wide variety of shapes and sizes, and, unlike cellular life forms, they cannot be systematically classified by their relatedness into a single phylogenetic tree. Because of their tiny sizes, complete genome sequences have been obtained for nearly all clinically important viruses. Poxviruses are among the largest, up to 450 nm long, which is about the size of some small bacteria. Their genome of double-stranded DNA consists of about 270,000 nucleotide pairs. At the other end of the size scale are parvoviruses, which are less than 20 nm long and have a single-stranded DNA genome of under 5000 nucleotides ( Figure 25-12). The genetic information in a virus can be carried in a variety of unusual nucleic acid forms ( Figure 25-13).

The capsid that encloses the viral genome is made of one or several proteins, arranged in regularly repeating layers and patterns. In enveloped viruses, the capsid itself is enclosed by a lipid bilayer membrane that is acquired in the process of budding from the host cell plasma membrane ( Figure 25-14). Whereas nonenveloped viruses usually leave an infected cell by lysing it, an enveloped virus can leave the cell by budding, without disrupting the plasma membrane and, therefore, without killing the cell. These viruses can cause chronic infections, and some can help transform an infected cell into a cancer cell.

Despite this variety, all viral genomes encode three types of proteins: proteins for replicating the genome, proteins for packaging the genome and delivering it to more host cells, and proteins that modify the structure or function of the host cell to suit the needs of the virus ( Figure 25-15). In the second section of this chapter, we focus primarily on this third class of viral proteins.

Since most of the critical steps in viral replication are performed by host cell machinery, the identification of effective antiviral drugs is particularly problematic. Whereas the antibiotic tetracycline specifically poisons bacterial ribosomes, for example, it will not be possible to find a drug that specifically poisons viral ribosomes, as viruses use the ribosomes of the host cell to make their proteins. The best strategy for containing viral diseases is to prevent them by vaccination of the potential hosts. Highly successful vaccination programs have effectively eliminated smallpox from the planet, and the eradication of poliomyelitis is imminent ( Figure 25-16).

Prions Are Infectious Proteins

All information in biological systems is encoded by structure. We are used to thinking of biological information in the form of nucleic acid sequences (as in our description of viral genomes), but the sequence itself is a shorthand code for describing nucleic acid structure. The replication and expression of the information encoded in DNA and RNA are strictly dependent on the structure of these nucleic acids and their interactions with other macromolecules. The propagation of genetic information primarily requires that the information be stored in a structure that can be duplicated from unstructured precursors. Nucleic acid sequences are the simplest and most robust solution that organisms have found to the problem of faithful structural replication.

Nucleic acids are not the only solution, however. Prions are infectious agents that are replicated in the host by copying an aberrant protein structure. They can occur in yeasts, and they cause various neurodegenerative diseases in mammals. The most well-known infection caused by prions is bovine spongiform encephalopathy (BSE, or mad cow disease), which occasionally spreads to humans who eat infected parts of the cow ( Figure 25-17). Isolation of the infectious prions that cause the disease scrapie in sheep, followed by years of painstaking laboratory characterization of scrapie-infected mice, eventually established that the protein itself is infectious.

Intriguingly, the infectious prion protein is made by the host, and its amino acid sequence is identical to a normal host protein. Moreover, the prion and normal forms of the protein are indistinguishable in their posttranslational modifications. The only difference between them appears to be in their folded three-dimensional structure. The misfolded prion protein tends to aggregate, and it has the remarkable capacity to cause the normal protein to adopt its misfolded prion conformation and thereby to become infectious (see Figure 6-89). This ability of the prion to convert the normal host protein to misfolded prion protein is equivalent to the prion's having replicated itself in the host. If eaten by another susceptible host, these newly-misfolded prions can transmit the infection.

It is not known how normal proteins are usually able to find the single, correct, folded conformation, among the billions of other possibilities, without becoming stuck in dead-end intermediates (discussed in Chapters 3 and 6). Prions are a good example of how protein folding can go dangerously wrong. But, why are the prion diseases so uncommon? What are the constraints that determine whether a misfolded protein will behave like a prion, or simply get refolded or degraded by the cell that made it? We do not yet have answers to these questions, and the study of prions remains an area of intense research.

Summary

Infectious diseases are caused by pathogens, which include bacteria, fungi, protozoa, worms, viruses, and even infectious proteins called prions. Pathogens of all classes must have mechanisms for entering their host and for evading immediate destruction by the host immune system. Most bacteria are not pathogenic. Those that are contain specific virulence genes that mediate interactions with the host, eliciting particular responses from the host cells that promote the replication and spread of the pathogen. Pathogenic fungi, protozoa, and other eucaryotic parasites typically pass through several different forms during the course of infection; the ability to switch among these forms is usually required for the parasites to be able to survive in a host and cause disease. In some cases, such as malaria, parasites must pass sequentially through several host species to complete their life cycles. Unlike bacteria and eucaryotic parasites, viruses have no metabolism of their own and no intrinsic ability to produce the proteins encoded by their DNA or RNA genomes. They rely entirely on subverting the machinery of the host cell to produce their proteins and to replicate their genomes. Prions, the smallest and simplest infectious agents, contain no nucleic acid; instead, they are rare, aberrantly folded proteins that happen to catalyze the misfolding of proteins in the host that share their primary amino acid sequence.

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We normally think of pathogens in hostile terms-as invaders that attack our bodies. But a pathogen or a parasite, like any other organism, is simply trying to live and procreate. Living at the expense of a host organism is a very attractive strategy, and it is possible that every living organism on earth is subject to some type of infection or parasitism (Figure 25-1).

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Structure and function of the digestive system

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Structure and function of the digestive system

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18: Digestive System - Biology LibreTexts

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This chapter outlines the structure and function of the gastrointestinal tract and accessory organs of digestion. It explains the processes of peristalsis, mechanical and chemical digestion of food, …

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Structure and function of the Lymphatic System

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Structure and function of the Lymphatic System

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19.1A: Structure of the Lymphatic System - Medicine LibreTexts

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19.1A: Structure of the Lymphatic System

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Structure and Function of the Musculoskeletal system

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Structure and Function of the Musculoskeletal system

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Musculoskeletal system: Anatomy and functions | Kenhub

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Author: Reviewer: Gordana Sendić MD * Jana Vasković MD
Last reviewed: November 03, 2023
Reading time: 28 minutes

Musculoskeletal system

The musculoskeletal system (locomotor system) is a human body system that provides our body with movement, stability, shape, and support. It is subdivided into two broad systems:

Muscular system, which includes all types of muscles in the body. Skeletal muscles, in particular, are the ones that act on the body joints to produce movements. Besides muscles, the muscular system contains the tendons which attach the muscles to the bones.

Skeletal system, whose main component is the bone. Bones articulate with each other and form the joints, providing our bodies with a hard-core, yet mobile, skeleton. The integrity and function of the bones and joints is supported by the accessory structures of the skeletal system; articular cartilage, ligaments, and bursae.

Besides its main function to provide the body with stability and mobility, the musculoskeletal system has many other functions; the skeletal part plays an important role in other homeostatic functions such as storage of minerals (e.g., calcium) and hematopoiesis, while the muscular system stores the majority of the body's carbohydrates in the form of glycogen.

This article will introduce you to the anatomy and function of the musculoskeletal system.

Definition

A human body system that provides the body with movement, stability, shape, and support

Components

Muscular system: skeletal muscles and tendons
Skeletal system: bones, joints; associated tissues (cartilage, ligaments, joint capsule, bursae)

Function

Muscles: Movement production, joint stabilization, maintaining posture, body heat production
Bones: Mechanical basis for movements, providing framework for the body, vital organs protection, blood cells production, storage of minerals

Muscular system

The muscular system is an organ system composed of specialized contractile tissue called the muscle tissue. There are three types of muscle tissue, based on which all the muscles are classified into three groups:

Based on their histological appearance, these types are classified into striated and non-striated muscles; with the skeletal and cardiac muscles being grouped as striated, while the smooth muscle is non-striated. The skeletal muscles are the only ones that we can control by the power of our will, as they are innervated by the somatic part of the nervous system. In contrast to this, the cardiac and smooth muscles are innervated by the autonomic nervous system, thus being controlled involuntarily by the autonomic centers in our brain.

Skeletal muscles

The skeletal muscles are the main functional units of the muscular system. There are more than 600 muscles in the human body. They vary greatly in shape in size, with the smallest one being the stapedius muscle in the inner ear, and the largest one being the quadriceps femoris muscle in the thigh.

The skeletal muscles of the human body are organized into four groups for every region of the body:

Muscles of the head and neck, which include the muscles of the facial expression, muscles of mastication, muscles of the orbit, muscles of the tongue, muscles of the pharynx, muscles of the larynx, and muscles of the neck

Muscles of the trunk, which include the muscles of the back, anterior and lateral abdominal muscles, and muscles of the pelvic floor

Muscles of the upper limbs, which include muscles of the shoulder, muscles of the arm, muscles of the forearm and muscles of the hand

Muscles of the lower limbs, which include hip and thigh muscles, leg muscles and foot muscles

The fact that there are more than 600 muscles in the body can be quite intimidating. If you're tired of all the big, comprehensive anatomy books, take a look at our condensed muscle anatomy reference charts, which contain all the muscle facts in one place organized into neat tables!

Structure

Structurally, the skeletal muscles are composed of the skeletal muscle cells which are called the myocytes (muscle fibres, or myofibrils). Muscle fibers are specialized cells whose main feature is the ability to contract. They are elongated, cylindrical, multinucleated cells bounded by a cell membrane called sarcolemma. The cytoplasm of skeletal muscle fibers ( sarcoplasm), contains contractile proteins called actin and myosin. These proteins are arranged into patterns, forming the units of contractile micro-apparatus called sarcomeres.

Each muscle fiber is enclosed with a loose connective tissue sheath called endomysium. Multiple muscle fibers are grouped into muscle fascicles or muscle bundles, which are encompassed by their own connective tissue sheath called the perimysium. Ultimately, a group of muscle fascicles comprises a whole muscle belly which is externally enclosed by another connective tissue layer called the epimysium. This layer is continuous with yet another layer of connective tissue called the deep fascia of skeletal muscle, that separates the muscles from other tissues and organs.

This structure gives the skeletal muscle tissue four main physiological properties:

Excitability - the ability to detect the neural stimuli (action potential);

Contractibility - the ability to contract in response to a neural stimulus;

Extensibility - the ability of a muscle to be stretched without tearing;

Elasticity - the ability to return to its normal shape after being extended.

Learn everything about the skeletal muscle structure with our articles, video tutorials, quizzes and labelled diagrams.

Muscle contraction

The most important property of skeletal muscles is its ability to contract. Muscle contraction occurs as a result of the interaction of myofibrils inside the muscle cells. This process either shortens the muscle or increases its tension, generating a force that either facilitates or slows down a movement.

There are two types of muscle contraction; isometric and isotonic. A muscle contraction is deemed as isometric if the length of the muscle does not change during the contraction, and isotonic if the tension remains unchanged while the length of the muscle changes. There are two types of isotonic contractions:

Concentric contraction, in which the muscle shortens due to generating enough force to overcome the imposed resistance. This type of contraction serves to facilitate any noticeable movement (e.g. lifting a barbell or walking on an incline).

Eccentric contraction, in which the muscle stretches due to the resistance being greater than the force the muscle generates. During an eccentric contraction, the muscle maintains high tension. This type of contraction usually serves to slow down a movement (e.g. lowering a barbell or walking downhill).

Eccentric and concentric muscle contractions (diagram)

The sequence of events that results in the contraction of a muscle cell begins as the nervous system generates a signal called the action potential. This signal travels through motor neurons to reach the neuromuscular junction, the site of contact between the motor nerve and the muscle. A group of muscle cells innervated by the branches of a single motor nerve is called the motor unit.

The incoming action potential from the motor nerve initiates the release of acetylcholine (ACh) from the nerve into the synaptic cleft, which is the space between the nerve ending and the sarcolemma. The ACh binds to the receptors on the sarcolemma and triggers a chemical reaction in the muscle cell. This involves the release of calcium ions from the sarcoplasmic reticulum, which in turn causes a rearrangement of contractile proteins within the muscle cell. The main proteins involved are actin and myosin, which in the presence of ATP, slide over each other and pull on the ends of each muscle cell together, causing a contraction. As the nerve signal diminishes, the chemical process reverses and the muscle relaxes.

Tendons

A tendon is a tough, flexible band of dense connective tissue that serves to attach skeletal muscles to bones. Tendons are found at the distal and proximal ends of muscles, binding them to the periosteum of bones at their proximal ( origin) and distal attachment ( insertion) on the bone. As muscles contract, the tendons transmit the mechanical force to the bones, pulling them and causing movement.

Being made of dense regular connective tissue, the tendons have an abundance of parallel collagen fibers, which provide them with high tensile strength (resistance to longitudinal force). The collagen fibers within a tendon are organized into fascicles, and individual fascicles are ensheathed by a thin layer of dense connective tissue called endotenon. In turn, groups of fascicles are ensheathed by a layer of dense irregular connective tissue called epitenon. Finally, the epitenon is encircled with a synovial sheath and attached to it by a delicate connective tissue band called mesotenon.

Learn more about the microstructure of tendon in this study unit:

Functions of the muscular system

The main function of the muscular system is to produce movement of the body. Depending on the axis and plane, there are several different types of movements that can be performed by the musculoskeletal system. Some of the most important ones include:

Flexion and extension: movement of decreasing or increasing the angle between the bones involved in the movement, respectively. This motion takes place in the sagittal plane around a frontal axis. An example of flexion is bending the leg at the knee joint, whereas extension would be straightening knee from a flexed position.

Adduction and abduction: movements of bringing the parts of the body towards or away from the midline, respectively. These movements are carried out in the frontal plane around a sagittal axis. For example, abduction of the arm at the shoulder joint involves moving the arm away from the side of the body, while adduction involves bringing it back towards the body.

Rotation is the movement in which a part of the body rotates around its vertical (longitudinal) axis in the transverse plane. This movement is defined relative to the midline, where internal rotation involves rotating the segment towards to the midline, while external rotation involves moving it away from the midline. Examples include lateral or medial rotation of the thigh.

Supination and pronation are special types of rotatory movements usually used to describe the movements of the forearm. Supination is essentially a lateral rotation of the forearm which turns the palms anteriorly (if the arm is anatomical position) or superiorly, when the elbow is flexed. These movements are also sometimes used to describe movements in the ankle and foot, in which supination means rolling the foot outwards, while pronation means rolling the foot inwards.

Both during movement and stationary positions, muscles contribute to the overall support and stability of joints. Many muscles and their tendons pass over joints and thereby stabilize the articulating bones and hold them in position. In addition, the muscles also play an important role in maintaining posture. While the movements occur mainly due to muscles intermittently contracting and relaxing, the posture is maintained by a sustained tonic contraction of postural muscles. These muscles act against gravity and stabilize the body during standing or walking. The postural muscles include the muscles of the back and abdominal muscles.

Another important function of muscles is heat production. Muscle tissue is one of the most metabolically active tissues in the body, in which approximately 85 percent of the heat produced in the body is the result of muscle contraction. This makes the muscles essential for maintaining normal body temperature.

Wondering what's the best way to learn and understand the functional anatomy of the muscles? Check out our 3D muscle anatomy videos!

To improve your understanding of muscular system terminology, take a closer look at some commonly used roots, prefixes and suffixes related to the muscular system in the video below.

How well do you know the main muscles of the body? Test your knowledge with our quiz in different difficulty levels!

Skeletal system

The adult human skeleton is composed of 206 bones and their associated cartilages. The bones are supported by ligaments, tendons, bursae, and muscles. The bones of the body are grouped within the two distinct divisions:

Bones

Bones are rigid structures made of calcified dense connective tissue. Bone tissue is composed of a mineralized bone matrix that consists of type 1 collagen fibers dispersed throughout the ground substance. The cellular component of the bones is represented by three types of specialized bone cells called osteocytes, osteoblasts and osteoclasts.

The bones consist of two distinct layers that differ in histological appearance and characteristics;

Types of bones

Bones can be classified according to their shapes as follows:

Short bones have a roughly cuboid or round shape, and only contain a thin layer of compact bone surrounding the spongy bone. Examples include the tarsal and carpal bones.

Sesamoid bones are small, rounded unique types of bones that are embedded in muscle tendons where the tendon passes over a joint. The largest sesamoid bone in the body is the patella, but several other smaller sesamoid bones can be found in the hand and foot, usually in close proximity to the joints.

Wondering how to cut time in learning the bones of the body? Try our skeletal system quizzes!

A typical long bone consists of a long shaft ( diaphysis) that extends into a neck ( metaphysis) and head ( epiphysis) on its proximal and distal ends. It also features various markings and formations that give passage to neurovascular structures, as well as the attachment sites to the ligaments and tendons. Some of those features include:

Sulcus - a shallow groove on the bone surface (e.g. radial sulcus of humerus)

Condyle - rounded articular area (e.g. lateral condyle of tibia)

Epicondyle - eminence superior to a condyle (medial epicondyle of femur)

Crest - ridge of bone (e.g. iliac crest)

Facet - smooth, flat area, usually covered with cartilage (e.g. articular facet on vertebrae)

Foramen - passage through a bone (e.g. foramen magnum on the occipital bone)

Cartilage

Cartilage is a flexible connective tissue found in multiple organ systems of the body. Cartilage is composed of specialized cells called chondrocytes, collagen fibers and abundant ground substance rich in proteoglycan and elastin fibers.

Cartilage is classified into the following types based on its composition:

The musculoskeletal system specifically contains articular cartilage, a type of cartilage that lines the articulating surfaces of bones. The articular cartilage provides congruence to the articulating bones and allows them to bear weight and glide over each other with very little friction.

Joints

Each bone of the musculoskeletal system is connected to one or more bones via a joint. Joints provide a fulcrum to the bones, on which they pivot and thereby allow movements of body parts. However, movement is not a necessary attribute of a joint as some joints do not move, such as joints between the bones of the skull. The integrity or stability of a joint is provided by several factors including the bony congruence and structures that cross the joint, such as tendons and ligaments.

Based on the type of tissue that holds the neighboring bones together and the range of motion they exhibit, joints can be classified into the following:

Synovial joints are freely mobile joints in which the bones are not in direct contact, but are separated by a potential space called the synovial cavity. The synovial cavity is lined by a synovial membrane that secretes the synovial fluid which nourishes and lubricates the articulating surfaces in order to reduce friction. The articulating bones in most synovial joints are lined with hyaline cartilage. These joints usually have a wide range of motion, which is defined by the joint capsule, the supporting ligaments and muscles that cross the joint. Examples of synovial joints include the knee, shoulder, sternoclavicular and elbow joints.

Cartilaginous joints are articulations in which the bones are connected by cartilage. The bones have a range of motion between synovial and fibrous joints. Cartilaginous joints are subdivided into synchondrosis (e.g. costochondral joints) and symphysis joints (e.g. pubic symphysis).

According to movements they allow, the synovial joints are further subdivided into:

Ligaments

Ligaments are fibrous bands made of dense regular connective tissue which are similar in structure to tendons. Unlike the tendons that connect muscles to bone, the ligaments connect bone to bone. Besides the musculoskeletal system, the ligaments are also found in many other parts of the body, where they usually stabilize and hold internal organs in place and transmit neurovascular structures.

In the musculoskeletal system, ligaments stabilize the articulating bones and reinforce the joints. Depending on their anatomic position relative to the joint capsule, ligaments are classified into:

Capsular ligaments are essentially thickenings of the joint capsule that form either elongated bands or triangular structures. These ligaments serve to reinforce the integrity of the joint capsule. An example of the capsular ligament is the iliofemoral ligament of the hip joint.

Intracapsular ligaments are the ligaments that lie internal to the joint capsule. These ligaments reinforce the connection of the articulating surfaces of the joint, but allow a far wider range of motion than other ligaments. Examples include anterior and posterior cruciate ligament of the knee joint.

Bursae

Bursae are small sac-like outpouchings of the joint cavity lined by synovial membrane. They are found around the joints, providing cushioning of the associated bones, tendons and muscles and reducing friction between adjacent structures.

The majority of synovial bursae are located near the large joints of the arms and legs. For example, one of the bursae of the knee joint is the suprapatellar bursa, found superior to the patella, between the femur and the tendon of the quadriceps femoris muscle. The suprapatellar bursa allows for these structures to slide over each other without friction during flexion and extension of the knee joint.

To improve your understanding of skeletal system terminology, take a closer look at some commonly used roots, prefixes and suffixes related to the skeletal system in the video below.

Time for a skeletal system workout with our integrated quiz!

Functions of the skeletal system

The skeletal system serves a variety of functions. The bones give the shape to the body and provide the site of attachment to muscles, tendons, ligaments and cartilage. These tissues function together as a whole to generate a force that provides the biomechanical basis of movement.

Due to its structural integrity, the skeletal system protects the internal organs, most importantly the brain, which is surrounded by the skull, as well as the heart and lungs, which are protected by the rib cage.

Moreover, the skeletal system serves several metabolic functions. The bones are the storage site of important minerals, most notably calcium and phosphorus. This makes the bones essential for balancing calcium levels in the blood, which is regulated by adjusting the rate of bone resorption.

Lastly, the bone marrow found in spongy bone is the site of hematopoiesis, which is a process of production of new blood cells. Cells that are produced in the bone marrow are red blood cells, platelets and white blood cells, such as monocytes, granulocytes and lymphocytes.

We created a custom summary quiz about the anatomy and histology of the main features of the musculoskeletal system. You can adjust and filter individual structures to make this quiz your own!

Clinical correlation

There is a variety of conditions that affect the muscles, bones, and joints. Disorders of the musculoskeletal system may range from diseases to minor physical disabilities. The following are some clinical conditions of the musculoskeletal system:

Osteoporosis

Osteoporosis is a condition that affects bone strength (the word osteoporosis literally means "porous bones"). It is a condition in which the bones become fragile and brittle, leading to a higher risk of fractures than in normal bone. As a result, even a minor bump or accident can cause serious fractures.

Osteoporosis is the "bone of the old", especially, in women. The hard, rock-like quality of bone is dependent upon calcium. When too much calcium is dissolved from bones or not enough is replaced, bones lose density and are easily fractured. Estrogen, the female sex hormone, helps maintain proper calcium levels in bones. Once the ovaries stop producing the hormone, women are at higher risk of developing osteoporosis. A collapse of bony vertebrae of the spinal column results in loss of height and stooped posture. Hip fractures are a common occurrence.

Sarcopenia

Sarcopenia is a syndrome characterized by progressive and generalized loss of skeletal muscle mass and strength with a risk of adverse outcomes such as physical disability, poor quality of life and death.

Arthritis

Arthritis is a group of conditions affecting the joints. These conditions cause damage to the joints, usually resulting in pain and stiffness due to aging. Arthritis can affect many different parts of the joint and nearly every joint in the body.

As an individual ages, the joint tissues become less resilient to wear and tear and start to degenerate. This degeneration manifest as swelling, pain, and often-times, loss of mobility of joints. Changes occur in both joint soft tissues and the articulating bones, a condition called osteoarthritis. A more serious form of disease is called rheumatoid arthritis. The latter is an autoimmune disease wherein the body produces antibodies against joint tissues causing chronic inflammation resulting in severe joint damage, pain and immobility.

Muscular dystrophy

Muscular dystrophy is a group of muscle diseases that weaken the musculoskeletal system and hamper locomotion. Muscular dystrophies are characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle fibres (muscle cells) and tissue.

It is a group of inherited diseases in which the muscles that control movement progressively weaken. The prefix, dys-, means abnormal, while the root, -trophy, refers to maintaining normal nourishment, structure and function. The most common form in children is called Duchenne muscular dystrophy and affects only males. It usually appears between the ages of 2 to 6 and the afflicted live typically into late teens to early 20s.

Other conditions involving the musculoskeletal system include:

Lupus erythematosus

Myasthenia gravis

Rotator cuff tear

Tendonitis

Carpal tunnel syndrome

Osteomalacia

Sources

All content published on Kenhub is reviewed by medical and anatomy experts. The information we provide is grounded on academic literature and peer-reviewed research. Kenhub does not provide medical advice. You can learn more about our content creation and review standards by reading our content quality guidelines.

References:

Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Clinically Oriented Anatomy (7th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.

Netter, F. (2019). Atlas of Human Anatomy (7th ed.). Philadelphia, PA: Saunders.

Standring, S. (2016). Gray's Anatomy (41tst ed.). Edinburgh: Elsevier Churchill Livingstone.

Ross, Lawrence M; Lamperti, Edward D, eds. (2006). Thieme Atlas of Anatomy: General Anatomy and Musculoskeletal System.

R.M.H McMinn: Last's anatomy (Regional and Applied), 9th edition, Ana-Maria Dulea (2014

Illustrations:

Musculoskeletal system - Irina Münstermann

Eccentric and concentric muscle contractions (diagram) - Yousun Koh

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Musculoskeletal (MSK) conditions affect many people and can affect your joints, bones and muscles and sometimes associated tissues such as your nerves. They can range from minor injuries to long-term conditions.

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The A Level teaching content is divided in two, which I have identified here as Core and Further. It is possible to take an AS Level examination based on the Core content. Some students may choose to do this at the end of their first year of studies, then either drop the subject or continue on to study the Further content.

AS Levels are graded from A to E (grade A is the highest), and A Levels from A* to E (grade A* is the highest).

The Department for Education publishes the required skills, knowledge and understanding required by all Chemistry AS and A Level courses. The examination boards base their specifications on this content, with some differences in approach and with varying additional content.

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The molecule chlorophyll a has a specific shape. This shape causes wavelengths of light that we see as a dark bluish green to be reflected back. Change the shape of that molecule by adding only two atoms, making it chlorophyll b, and the light that is reflected back is now less blue and more yellow.

What does this have to do with photosynthesis? Organisms that perform photosynthesis do so by absorbing light and converting it into usable energy. This absorbed light is not reflected back. This means that the color of the pigment(s) that the organism has will determine the wavelengths of light that the organism can use. For example, plants have two types of chlorophyll molecules, chlorophyll a & b. Each of these reflects green light, meaning that green light cannot be used for photosynthesis. To help capture a bit more of the spectrum, plants have accessory pigments called carotenoids that reflect yellow, orange, and red light, absorbing a portion of the green part of the spectrum.

Chlorophylls tend to mask most other pigments in plants, so to see these other pigments, we need to separate them. You will use a process called thin layer chromatography to extract pigments from leaves, then dissolve them in a solvent.

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Colorimetry - an overview | ScienceDirect Topics

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Abstract

Colorimetry is the science of the measurement of colour, replacing subjective responses of colours with an objective numerical system. It plays an important role in all areas where colour generation, rendition and perception are involved. Perception of the colour of an object is governed by three factors: the nature of the illumination, the optical properties of the object itself and the response of the human eye. Colorimetry quantitates these aspects and introduces the concepts of standard illumination and observers, leading to colour representations such as RGB, XYZ, L*a*b* and L*C*h to unambiguously define colour.

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URL: https://www.sciencedirect.com/science/article/pii/B9780128032244001242

The human eye

The colorimetry of a light is intrinsically linked to the receptor for which it is intended. In the case of lighting and displays, it is of course the human eye. As a spectrometer, the eye does not perceive all the wavelengths with the same sensitivity. The retina is covered by light receptors of two types: cones and rods. The cones are in charge of day vision. Their spectral sensitivity is located in the blue (426 nm), green (530 nm), and red (560 nm). The split of eye response in three spectral areas is the reason for the use of trichromatic systems, both for paintings and light sources.

The number and efficiency of the cones in the three regions give a resulting optimal perception around 555 nm, described by the photopic distribution curve V(λ). For night vision, the rods are the more active ones, with an optimal sensitivity around 505 nm described by the scotopic distribution curve.

The reference curves are determined by the CIE (Comité International de l'Eclairage), measured by a representative number of observers, in 1931, updated in 1964 and 1976 (Fig. A16.1).

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9.3.1 Psychophysical colorimetry of color stimuli

Psychophysical colorimetry is based on the phenomenon, termed color matching, according to which different visible electromagnetic radiation (lights, color stimuli) produce the same color sensation. Physiology explains that any color sensation is producible by mixing in a proper ratio three independent lights, and three lights are independent if none of these lights is matched by a mixture of the other two lights. Trichromacy means that three is the highest number of independent colors. Color matching is independent of the visual situation, but not the color sensation. The classical visual situation in laboratory is in aperture mode, that is, the pair of colors to be matched is viewed in a bipartite field through a hole in a dark room. The color specification in psychophysical colorimetry is given by three numbers, that represent the amounts of the three lights entering the mixture. Grassmann's laws state that these sets of three numbers, termed tristimulus values, are the elements of a three-dimensional linear vector space, termed tristimulus space. Many different reference frames are possible and therefore many different color specifications with different meanings. Typical specifications are outlined below:

*

( L, M, S)

Here L, M and S, termed cone activations, are proportional to the (positive) numbers of photons absorbed in the unit of time by the three kinds of photosensitive cells (cones) in the retina; L, M and S refer to Long, Medium and Short wavelength-sensitive cones, respectively. This reference frame is related to the physiology and is used mainly by physiologists.

Light is measured outside the eye and the visible range of wavelength is 360-760 nm. The light entering the eye is named color stimulus and is physically represented by a spectral radiance L e,λ [W/(sr m ² nm)]. Cone activation is based on Rushton's univariance principle, that says that the visual effect of a radiation depends only on the number of absorbed photons and is independent of the photon energy, that is, a photon, once absorbed by the pigment of a cone, triggers a photochemical process, which is independent of the photon energy. Photons with different energy have their own probability of being absorbed and consequently produce different visual effects. In a defined visual situation, the cone activation is in one-to-one correspondence with the color sensation, therefore the cone activations ( L, M, S) specify the color sensation.

The tristimulus values ( L, M, S) are defined by three integrals

[9.1] L =∫380 780 L e,λ l¯λ d λ for L cones,M=∫380 780 L e,λ m¯(λ)d λ for M cones,S =∫380 780 L e,λ s¯λ d λ for S cones

where the functions , and (Fig. 9.4), named color-matching functions (CMFs), are the spectral sensitivities of the cones L, M and S, respectively, and also take into account the light absorption of the lens of the eye and of the macula lutea. The CMFs are normalized to give S = M = L = 1 for the equal-energy radiance L e,λ = 1, therefore representing the monochromatic components of the equal-energy stimulus (, , ) in the tristimulus space (Figs 9.4 and 9.5), that is, at any given wavelength λ, the CMFs represent the cone activations produced by a monochromatic radiation at equal wavelength and with unitary radiance.

The addition of color stimuli is represented in the tristimulus space by the addition of the corresponding vectors.

The expressions [9.1] that define the ( L, M, S) vectors say that the correspondence between color stimuli and tristimulus vectors is many to one, that is, different radiances can produce equal color sensation. This phenomenon is named metamerism.

The length of the vectors is related to the intensity of the color stimuli and the direction is related to chromatic sensation. Since the vector directions are in one-to-one correspondence with the intersection points between the tristimulus vectors and a plane, these points constitute a diagram (Fig. 9.5), termed a chromaticity diagram, representing the chromaticity, the practical role of which is very important. The chromaticities of the monochromatic lights are points of a line named the spectrum locus and the segment between short wavelength and long wavelength regions relates to the purple hues.

*

( R, G, B)

Here R, G and B are the intensities of three lights, one red, one green and one blue; very saturated colors require that one of these numbers is negative; this specification is typical for monitors, computers, scanners, photocameras, and so on; many different RGB reference frames exist, that must not be confused.

Figure 9.5 shows the tristimulus space in the fundamental reference frame and the vectors X, Y, Z and R, G, B representing the axes of the XYZ and RGB reference frames, respectively. The RGB reference frame is defined by three lights, one red, one green and one blue, and is termed the instrumental reference frame because these three lights are used as reference and this choice is proper for the laboratory. The passage between the LMS reference frame and, for example, RGB is made by the matrix that renders mutually orthogonal the R, G, B vectors with the constraint that R = G = B = 1 for the equal-energy radiance L e,λ = 1. (In some cases, where the white light has a meaningful role, as in the television standards, the same constraint regards the radiance of the chosen white light.) The same linear transformation is made on the CMFs and (, , ) are transformed into r¯λ ,g¯λ ,b¯λ . The tristimulus values are defined by the integrals

[9.2] R=r¯λ d λ,G=g¯λ d λ,B=b¯λ d λ

and the chromaticity coordinates are represented by baricentric coordinates ( r, g, b)

Historically, the CMFs have been measured in an RGB reference frame by a color-matching technique, because in this kind of reference frame it is possible to match a unitary spectral light by a mixture of three reference spectral lights, one red, one green and one blue. The CMFs were first measured by J. C. Maxwell in 1860. Two sets of standard CMFs exist related to the 2° visual field CIE 1931 standard observer and denoted by r¯λ ,g¯λ ,b¯λ , and to the 10° visual field CIE 1964 standard observer and denoted by λ ,λ ,λ , respectively (CIE 15:2004). All the quantities related to the standard observer CIE 1931 have no denoting apex while those related to the standard observer CIE 1964 have the apex 10.

A linear transformation exists between the tristimulus values ( L, M, S) and ( R, G, B).

*

( X, Y, Z)

Here X, Y and Z are the tristimulus values (always positive) related to three standard 'imaginary' reference vectors X, Y, Z, properly defined by the CIE in 1931, where Y represents the luminance.

The CIE 1931 colorimetric system was realized by embedding photometry in colorimetry. The photometry relates the luminosity sensation of a light defining the luminance, the quantity that would represent the brightness associated the radiance L e, λ,

with

*

V( λ) = relative photopic luminous efficiency function that defines the standard photometric observer CIE 24 (Fig. 9.5)

*

K m = 683 lumen/watt

Abney's law states that the luminance of a color stimulus obtained as a sum of many stimuli is equal to the sum of the corresponding luminances. This law induced scientists to represent luminance as a linear weighted sum of the tristimulus values:

[9.5] L v=L L L+L M M+L S S=L R R+L G G+L B B

This equation, is known as Schrödinger's 'Helligkeit' equation (Schrödinger, 1926), where the weights LL, LM, LS, LR, LG and LB are known as Exner's coefficients. Consequently this led scientists to search for a reference frame XYZ such that

*

the Y component of the stimulus is proportional to the luminance L v, that is,

[9.6] L v=K m Y cd/m 2 and y¯λ =V λ

where is the second color-matching function of XYZ system,

*

the X, Y and Z axes are mutually orthogonal,

*

the X and Z axes belong to the zero luminance plane Y = 0,

*

the tristimulus vectors with physical meaning have all positive components,

*

the planes X = 0 and Z = 0 are tangent to the spectrum locus in the short and in the long wavelength region, respectively.

This reference frame is possible and is obtained from the RGB one by a linear transformation. This reference frame is the XYZ of the CIE 1931 standard colorimetric observer (Figs 9.6 and 9.7). The tristimulus values are

[9.7] X=x¯λ d λ,Y=y¯λ d λ,=V λ d λ=L v K m,Z=Z¯λ d λ

The usual chromaticity diagram is obtained from the diagram on the plane X + Y + Z = 1 by a projection from infinity on the plane Z = 0 (Fig. 9.7).

All the quantities related to the standard observer CIE 1931 have no denoting apex while those related to the standard observer CIE 1964 have the apex 10: as such, the CMFs are (x¯λ , y¯λ , z¯λ ) and λ ,λ ,λ , the tristimulus values ( X, Y, Z) and ( X ₁₀, Y ₁₀, Z ₁₀), and the chromaticities ( x, y, z) and ( x ₁₀, y ₁₀, z ₁₀), respectively. Comparison of the CMFs and of the chromaticity diagrams of these two standard observers are shown in Figs 9.6 and 9.7.

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Introduction

The objective of colorimetry is the numerical description of colors in such a way that two objects with the same specification, for a given set of conditions, are always perceived to have the same color under those conditions. Any difference in the numerical descriptions of two similar, but nonidentical, colors should also correlate with the actual color difference evaluated by the observer. Such a system of color specification allows rapid and objective communication of color information, the specification of acceptable color tolerances, and the resolution of disagreements between observers. Colorimetry is widely used in many industries.

Color is not an intrinsic property of an object; its perception involves three important factors. The first is the light source. Light may enter the eye directly from the light source, for example, traffic lights or after the colorants present in a material selectively absorb or transmit part of the light illuminating it. Thus, the second important factor is the reflection or transmission of light by the object. This modified light enters the eye of the observer and stimulates the sensation, which is called the color of the material. The observer is, therefore, the third key factor that must be considered. For colorimetric purposes, numerical descriptions of each of these three factors are necessary.

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10.7.1 Parameters for colour measuring instruments

When you are presenting any colorimetry data, you must mention ( Gangakhedkar 1986a, RPI 1978a & b) the following instrument parameters

*

Instrument model - make.

*

Geometry of instrument: d/8 or 0/45.

*

Specular component included (SCI) or Specular component excluded (SCE) mode. The gloss covers a complex set of effects related to surface structure. The samples with intermediate gloss exhibit different measurement problems compared to those with very high gloss or no gloss, respectively. Colour matches of highly glossy samples are to be performed without gloss (SCE mode) and for SCI measurement, one uses gloss correction by subtracting the effect of gloss.

*

Aperture size (large, small or ultra small area view).

*

UV included.

*

*

UV filters: 400 / 420 / 440 nm.

*

Gloss compensation (automatic/manual); surface correction (many programs use Sauderson's correction - using different values of internal and external reflection coefficients).

*

Instrument diagnostics.

*

Instrument metamerism.

Comparison of instrumental measurements is meaningless unless all above mentioned parameters are the same for the measurements being compared. There are instrument variables such as lamp technology, measurement interval, geometry, gloss, type of photo detectors and electronics. These parameters are mainly responsible for instrument metamerism.

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11.5.8 Colorimetry

For analysis of species in solution, a common method is colorimetry (or spectrophotometry). The species of interest undergoes a chemical reaction with a specific reagent to produce a coloured product which can then be measured using a colorimeter or spectrophotometer. These instruments use filters or a monochromator (respectively) to pass light of suitable wavelength(s) through the sample where it is partially absorbed by the coloured product, the degree of absorption being proportional to the concentration of the original species. The apparatus is normally calibrated by measuring absorbance with various standard solutions of the coloured derivative of the required species. As an illustration of these methods, analysis of two common fire effluents, hydrogen cyanide and hydrogen fluoride is summarised in Table 11.2.

Analysed gas

Analyte(s) in liquid phase

Analytical technique

Advantages and disadvantages

HCN

Picric acid colorimetry. In presence of cyanides, picric acid (2,4,6-trinitrophenol) produces a red olour linked to cyanide concentration.

HF

2-(p-sulphophenylazo)-1,8-dihydroxynaphthalene-3,6-disulphonic acid (SPADNS) inverse colorimetry. In presence of zirconium ions, SPADNS produces a red colour destroyed by presence of fluoride ions.

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Colorimetry is the science of the measurement of colour, replacing subjective responses of colours with an objective numerical system. It plays an important role in all areas where colour generation, rendition and perception are involved.

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Bonding and Intermolecular forces of attraction

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Bonding and Intermolecular forces of attraction

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Bonding menu (chemguide.co.uk)

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Includes a simple view of ionic bonding and the way you need to modify this for A'level purposes.

Includes a simple view of covalent bonding (single and double) and the modifications needed for A'level purposes.

Explains what co-ordinate (dative covalent) bonding is, and looks at a wide range of examples.

Explains what electronegativity is and how it varies around the Periodic Table. Describes and explains how electronegativity differences determine the type of bond formed. Looks at polar bonds and molecules.

Explains how to work out the shapes of a wide range of simple molecules and ions.

A simple explanation of the forces holding metals together.

A description of van der Waals forces (temporary fluctuating dipole and dipole-dipole interactions) causing attractions between individual molecules.

An explanation of how hydrogen bonding arises and its effect on boiling points.

This leads you to the bonding menu in the organic section of this site in case you are only interested in bonding in organic compounds.

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Includes a simple view of ionic bonding and the way you need to modify this for A'level purposes. Includes a simple view of covalent bonding (single and double) and the modifications needed for A'level purposes. Explains what co-ordinate (dative covalent) bonding is, and looks at a wide range of examples.

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solid-liquid phase diagrams: tin and lead (chemguide.co.uk)

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This page explains the relationship between the cooling curves for liquid mixtures of tin and lead, and the resulting phase diagram. It also offers a simple introduction to the idea of a eutectic mixture.

Important: This page is only really designed to be an introduction to the topic suitable for courses for 16-18 year olds such as UK A level chemistry. Be aware that the phase diagram used is a simplified version of the real thing. In particular, it ignores the formation of solid solutions of tin and lead. I will give a link to the correct phase diagram later.

Cooling curves Cooling curves for pure substances

Suppose you have some pure molten lead and allow it to cool down until it has all solidified, plotting the temperature of the lead against time as you go. You would end up with a typical cooling curve for a pure substance.

Note: Just before the liquid freezes, there is sometimes a slight dip in the curve below the freezing point. This is called supercooling. As soon as some solid forms, the temperature recovers to the normal freezing point. Supercooling isn't important for the present discussion, so I'm ignoring it on the diagrams.

Throughout the whole experiment, heat is being lost to the surroundings - and yet the temperature doesn't fall at all while the lead is freezing. This is because the freezing process liberates heat at exactly the same rate that it is being lost to the surroundings.
The phase diagram Constructing the phase diagram
You start from data obtained from the cooling curves. You draw a graph of the temperature at which freezing first starts against the proportion of tin and lead in the mixture. The only unusual thing is that you draw the temperature scale at each end of the diagram instead of only at the left-hand side.

Notice that at the left-hand side and right-hand sides of the curves you have the freezing points (melting points) of the pure lead and tin.

Energy is released when new bonds form - in this case, the strong metallic bonds in the solid lead.

If you repeated this process for pure liquid tin, the shape of the graph would be exactly the same, except that the freezing point would now be at 232°C. (The graph for this is further down the page.)
Cooling curves for tin-lead mixtures A sample curve
If you add some tin to the lead, the shape of the cooling curve changes. The next graph shows what happens if you cool a liquid mixture containing about 67% lead and 33% tin by mass.

There are lots of things to look at:

Notice that nothing happens at all at the normal freezing point of the lead. Adding the tin to it lowers its freezing point.

Freezing starts for this mixture at about 250°C. You would start to get some solid lead formed - but no tin. At that point the rate of cooling slows down - the curve gets less steep.

However, the graph doesn't go horizontal yet. Although energy is being given off as the lead turns to a solid, there isn't anything similar happening to the tin. That means that there isn't enough energy released to keep the temperature constant.

The temperature does stop falling at 183°C. Now both tin and lead are freezing. Once everything has solidified, the temperature continues to fall.

Changing the proportions of tin and lead
If you had less tin in the mixture, the overall shape of the curve stays much the same, but the point at which the lead first starts to freeze changes.

The less tin there is, the smaller the drop in the freezing point of the lead.

For a mixture containing only 20% of tin, the freezing point of the lead is about 275°C. That's where the graph would suddenly become less steep.

BUT . . . you will still get the graph going horizontal (showing the freezing of both the tin and lead) at exactly the same temperature: 183°C.

As you increase the proportion of tin, the first signs of solid lead appear at lower and lower temperatures, but the final freezing of the whole mixture still happens at 183°C.

That continues until you have added enough tin that the mixture contains 62% tin and 38% lead. At that point, the graph changes.

This particular mixture of lead and tin has a cooling curve which looks exactly like that of a pure substance rather than a mixture. There is just the single horizontal part of the graph where everything is freezing.

However, it is still a mixture. If you use a microscope to look at the solid formed after freezing, you can see the individual crystals of tin and lead.

This particular mixture is known as a eutectic mixture. The word "eutectic" comes from Greek and means "easily melted".

The eutectic mixture has the lowest melting point (which is, of course, the same as the freezing point) of any mixture of lead and tin. The temperature at which the eutectic mixture freezes or melts is known as the eutectic temperature.
What happens if there is more than 62% of tin in the mixture?
You can trace it through in exactly the same way, by imagining starting with pure tin and then adding lead to it.

The cooling curve for pure liquid tin looks like this:

It's just like the pure lead cooling curve except that tin's freezing point is lower.

If you add small amounts of lead to the tin, so that you have perhaps 80% tin and 20% lead, you will get a curve like this:

Notice the lowered freezing point of the tin. Notice also the final freezing of the whole mixture again takes place at 183°C.

As you increase the amount of lead (or decrease the amount of tin - same thing!) until there is 62% of tin and 38% of lead, you will again get the eutectic mixture with the curve we've already looked at.

Note: The two lines meeting at the eutectic point have been simplified slightly so that they are drawn as straight lines rather than slight curves. It doesn't affect the argument in any way. I haven't been able to find the actual data to plot them accurately, so the simplification is to avoid giving the impression that I actually know exactly what the curves look like!

To finish off the phase diagram, all you have to do is draw a single horizontal line across at the eutectic temperature. Then you label each area of the diagram with what you would find under the various different conditions.

Important: This is a simplified version of the real tin-lead phase diagram. In particular, it ignores the formation of solid solutions of tin and lead. You will find the correct diagram on this NIST web page. Beware that on that page, the tin-lead axis is reversed from the one I have drawn above - in other words 100% lead is on the right rather than the left.

If you are just using this page as an introduction to this sort of phase diagram, you don't need to worry about this.

Using the phase diagram

Important: One of the problems with the various sorts of phase diagrams is that they are all interpreted slightly differently. You can't assume that because you know how to use one sort of phase diagram that you can treat others exactly the same. If you already know about other phase diagrams, look at this one completely afresh.

Suppose you have a mixture of 67% lead and 33% tin. That's the mixture from the first cooling curve plotted above. Suppose it is at a temperature of 300°C.

That corresponds to a set of conditions in the area of the phase diagram labelled as molten tin and lead.

Now consider what happens if you cool that mixture. Eventually the temperature will drop to a point where it crosses the line into the next region of the diagram.

At that point, the mixture will start to produce some solid lead - in other words, the lead (but not the tin) starts to freeze. That happens at a temperature of about 250°C.

Now there is a bit of a problem, because you might come across two different ways of explaining what happens next. We'll look at both.
Thinking about changes in the composition of the liquid
When the first of the lead freezes, the composition of the remaining liquid changes. It obviously becomes proportionally richer in tin. That lowers the freezing point of the lead a bit more, and so the next bit of lead freezes at a slightly lower temperature - leaving a liquid still richer in tin.

This process goes on. The liquid gets richer and richer in tin, and the temperature needed to freeze the next lot of lead continues to fall. The set of conditions of temperature and composition of the liquid essentially moves down the curve - until it reaches the eutectic point.

Once it has reached the eutectic point, if the temperature continues to fall, you obviously just move into the region of a mixture of solid lead and solid tin - in other words, all the remaining liquid freezes.

If you haven't come across this way of looking at it before, then please don't bother to learn it now! The second way is more in line with how we look at other phase diagrams, and actually needs less thinking about.
Thinking about the composition of the system as a whole
We've seen that as the liquid gradually freezes, its composition changes. But if you look at the system as a whole, obviously the proportions of lead and tin remain constant - you aren't taking anything away or adding anything. All that is happening is that things are changing from liquids to solids.

So suppose we continue the cooling beyond the temperature that the first solid lead appears and the temperature drops to the point shown in the next diagram - a point clearly in the "solid lead and molten mixture" area.

What would you see in the mixture? To find out, you draw a horizontal tie line through that point, and then look at the ends of it.

At the left-hand end, you have 100% lead. That represents the solid lead that has frozen from the mixture. At the right-hand end, you have the composition of the liquid mixture. This is now much richer in tin than the whole system is - because obviously a fair bit of solid lead has separated out.

As the temperature continues to fall, the composition of the liquid mixture (as shown by the right-hand end of the tie line) will get closer and closer to the eutectic mixture.

It will finally reach the eutectic composition when the temperature drops to the eutectic temperature - and the whole lot then freezes.

At a temperature lower than the eutectic temperature, you are obviously in the solid lead plus solid tin region. That's fairly obvious!

If you cooled a liquid mixture on the right-hand side of the phase diagram (to the right of the eutectic mixture), everything would work exactly the same except that solid tin would be formed instead of solid lead. If you have understood what has gone before, it isn't at all difficult to work out what happens.

Finally . . . what happens if you cool a liquid mixture which has exactly the eutectic composition?

It simply stays as a liquid mixture until the temperature falls enough that it all solidifies. You never get into the awkward areas of the phase diagram.
Tin-lead mixtures as solder
Traditionally, tin-lead mixtures have been used as solder, but these are being phased out because of health concerns over the lead. This is especially the case where the solder is used to join water pipes where the water is used for drinking. New non-lead solders have been developed as safer replacements.

Typical old-fashioned solders include:

60% tin and 40% lead. This is close to the eutectic composition (62% tin and 38% lead), giving a low melting point. It will also melt and freeze cleanly over a very limited temperature range. This is useful for electrical work.

50% tin and 50% lead. This will melt and freeze over a wider range of temperatures. When it is molten it will start to freeze at about 220°C and finally solidify at the eutectic temperature of 183°C. That means that it stays workable for a useful amount of time. That's helpful if it is being used for plumbing joints.

Where would you like to go now?
To the phase equilibrium menu . . .To the Physical Chemistry menu . . .To Main Menu . . .

© Jim Clark 2004 (modified January 2014)

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solid-liquid phase diagrams: tin and lead

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This page explains the relationship between the cooling curves for liquid mixtures of tin and lead, and the resulting phase diagram. It also offers a simple introduction to the idea of a eutectic mixture. Important: This page is only really designed to be an introduction to the topic suitable for courses for 16-18 year olds such as UK A level chemistry.

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Melting and freezing stearic acid | Experiment | RSC Education

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A-level set practicals - TLC of plant photosynthetic pigments - Science & Plants for Schools (saps.org.uk)

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A-level set practicals - TLC of plant photosynthetic pigments - Science & Plants for Schools

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Students conduct Thin Layer Chromatography (TLC) individually or in pairs to separate and identify the photosynthetic pigments from plant material within half an hour.

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Aspirin screen experiment (rsc.org)

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Aspirin level 1 Aspirin level 2 Aspirin level 3 Aspirin level 4

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Support for students | RSC Education

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Institute of Physics - For physics • For physicists • For all : Institute of Physics (iop.org)

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Royal Society of Biology (rsb.org.uk)

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The leading professional body representing many of the learned societies and other organisations making up the diverse landscape of the biological sciences, as well as thousands of individuals

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Thin Layer Chromatography (TLC) | Facts, Principle & Summary (alevelbiology.co.uk)

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Thin Layer Chromatography (TLC) | Facts, Principle & Summary

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Thin Layer Chromatography is a refined technique for separating non-volatile mixtures. It uses a slide or plate, which has a layer of specific material.

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Titration screen experiment (rsc.org)

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Titration level 1 Titration level 2 Titration level 3 Titration level 4

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AQA | A Level Physics (alevelphysicsonline.com)

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Revision for AQA Biology AS and A Level Papers, including summary notes, worksheets and past exam questions for each topic.

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(43) BTEC Applied Science: Communicating with Waves - YouTube

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BTEC Applied Science: Communicating with Waves

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chemguide: helping you to understand Chemistry - Main Menu

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chemguide: helping you to understand Chemistry - Main Menu

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Main menu of a site aimed to help advanced level chemistry students to understand chemistry

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Periodic Table of Nottingham - University of Nottingham

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Periodic Table of Nottingham - University of Nottingham

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(43) BTEC Applied Science: Unit 1 Chemistry Electron Configuration - YouTube

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BTEC Applied Science: Unit 1 Chemistry Electron Configuration

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External Exam, you can find sample exam papers and past exam papers here :)

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BTEC Nationals | Applied Science (2016) | Pearson qualifications

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Pearson BTEC Nationals Applied Science (2016)

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BTEC Nationals | Applied Science (2016) | Pearson qualifications

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