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A-level Biology Sunday Sessions: Variation, Mutation and Meiosis | Revision Webclass

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You must add a doddle browser to use this and create an account. The attached PDF tells you how.

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13 minute video

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Required Practical CHLOROPHYLL CHROMATOGRAPHY for A-level Biology.  Do you know what an Rf value is?

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10 minute video

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How to answer EVALUATE exam questions. Evaluate the method, data and conclusion in A-level BIOLOGY.

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9 minute video

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How to Answer APPLICATION exam questions. Top tips to get full marks & how to revise these questions

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Ultrafiltration and reabsorption in the kidney (16mins)

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Ultrafiltration and reabsorption in the kidney (16mins)

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A-Level Biology- Structure of the NEPHRON. Ultrafiltration and selective reabsorption in the kidney

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Wordwall - Create better lessons quicker

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glomerulus, afferent arteriole, efferent arteriole, branch of renal artery, branch of renal vein, proximal convoluted tubule, distal convoluted tubule, ascending limb of loop, descending limb of loop, collecting duct.

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glomerulus, afferent arteriole, efferent arteriole, branch of renal artery, branch of renal vein, proximal convoluted tubule, distal convoluted tubule, ascending limb of loop, descending limb of loop, collecting duct.

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Wordwall - Create better lessons quicker

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Carbohydrates, Lipids, Proteins (excluding enzymes), Enzymes/Enzyme controlled reactions, Nucleic Acids (excluding replication), DNA Replication, Water, ATP, Inorganic Ions, Eukaryotic Cells (not measuring), Measuring Cells, Prokaryotic Cells and viruses, Viewing Cells and separating components, Cell Cycle and Mitosis, Investigating Mitosis, Cell Membrane Structure, Diffusion and Osmosis, Active Transport, Immune System , Immunity/vaccines/antibodies in medicine, HIV and viruses, Size and surface area, Gas Exchange (not mammalian), Gas Exchange (mammalian), Digestion and Absorption, Haemoglobin , The circulatory system (not heart), The Heart , Transport in plants (xylem), Transport in plants (phloem), DNA, Genes and Chromosomes, RNA and Protein Synthesis, Meiosis, Mutations, Genetic Diversity and Natural selection, Investigating Selection, Classification, Biodiversity.

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Carbohydrates, Lipids, Proteins (excluding enzymes), Enzymes/Enzyme controlled reactions, Nucleic Acids (excluding replication), DNA Replication, Water, ATP, Inorganic Ions, Eukaryotic Cells (not measuring), Measuring Cells, Prokaryotic Cells and viruses, Viewing Cells and separating components, Cell Cycle and Mitosis, Investigating Mitosis, Cell Membrane Structure, Diffusion and Osmosis, Active Transport, Immune System ,

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Wordwall - Create better lessons quicker

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Photosynthesis, Respiration, Energy Transfer, Farming Practices/fertilisers/eutrophication, Nutrient Cycles, Simple Responses, Nervous Communication, Plant Responses, Receptors, Control of Heart Rate, Muscle Contraction, Homeostasis (basics), Control of Blood Glucose, Diabetes, The Kidney (excluding water potential), Kidney- Control of Water Potential , Inheritance (not linkage and epistasis), Linkage and Epistasis, Chi Squared, Hardy-Weinberg, Variation/Selection/Speciation, Populations in Ecosystems, Mutations, Gene Expression (not cancer), Regulation of Transcription/Translation and Epigenetics, Gene Expression and Cancer, Genome Project, Recombinant DNA Technology, Diagnosis of Heritable Conditions, Genetic Fingerprinting.

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Photosynthesis, Respiration, Energy Transfer, Farming Practices/fertilisers/eutrophication, Nutrient Cycles, Simple Responses, Nervous Communication, Plant Responses, Receptors, Control of Heart Rate, Muscle Contraction, Homeostasis (basics), Control of Blood Glucose, Diabetes, The Kidney (excluding water potential), Kidney- Control of Water Potential , Inheritance (not linkage and epistasis), Linkage and Epistasis, Chi Squared, Hardy-Weinberg, Variation/Selection/Speciation, Populations in Ecosystems, Mutations, Gene Expression (not cancer), Regulation of Transcription/Translation and Epigenetics, Gene Expression and Cancer, Genome Project, Recombinant DNA Technology, Diagnosis of Heritable Conditions, Genetic Fingerprinting.

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Sliding Filament Model. 2.5 minutes, you may want to mute the music as can be a bit irritating.

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Muscle contraction: Sliding filament model animation for A level biology

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Short 6 minute video

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Taxes and Kinesis: Simple responses in organisms to their environment. A-level Biology

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A level You Tube channel for videos

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"a-level biology" "a-level chemistry" "a-level maths" "a-level revision" tutoring "a level revision" "a level notes" "a-level revision" "a-level notes" "alev...

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Tailored Tutors is the leading A Level online specialist for Biology, Chemistry & Maths. Subscribe to our channel for weekly updates on how to improve your subject knowledge and how to revise (amongst other things!) You can also follow out students as they negotiate through their A Levels and see how they use TT resources to improve their grades.

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Good explanation of CVhi Squared calculation too

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Required practical 10: Investigation into the effect of an environmental variable on the movement of an animal using either a choice chamber or a maze.

Notes & Flashcards for this practical

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RP 10: Investigating Simple Animal Responses - AQA Biology A-level - PMT

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Video and notes for AQA Biology A-level Required practical 10: Investigation into the effect of an environmental variable on the movement of an animal using either a choice chamber or a maze.

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Pacinian Corpuscle, rods and cones

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Learn A Levels, GCSE, KS3 & KS2 2x Faster with Seneca

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Learn A Levels, GCSE, KS3 & KS2 2x Faster with Seneca

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A Level Biology Revision | S-cool, the revision website

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A-level biology revision guides and question banks covering applications of genetic engineering, molecules and enzymes, photosynthesis and all core a-level topics.

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Website with information on all units - boundless biology

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Website with information on all units - boundless biology

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The Study of Life

The Chemical Foundation of Life

Biological Macromolecules

Cell Structure

Structure and Function of Plasma Membranes

Metabolism

Cellular Respiration

Photosynthesis

Cell Communication

Cell Reproduction

Meiosis and Sexual Reproduction

Mendel's Experiments and Heredity

Modern Understandings of Inheritance

DNA Structure and Function

Genes and Proteins

Gene Expression

Biotechnology and Genomics

Evolution and the Origin of Species

The Evolution of Populations

Phylogenies and the History of Life

Prokaryotes: Bacteria and Archaea

Seedless Plants

Seed Plants

Introduction to Animal Diversity

Invertebrates

Vertebrates

Plant Form and Physiology

Soil and Plant Nutrition

Plant Reproduction

The Animal Body: Basic Form and Function

Animal Nutrition and the Digestive System

The Nervous System

Sensory Systems

The Endocrine System

The Musculoskeletal System

The Respiratory System

The Circulatory System

Osmotic Regulation and the Excretory System

The Immune System

Animal Reproduction and Development

Ecology and the Biosphere

Population and Community Ecology

Ecosystems

Conservation Biology and Biodiversity

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The Study of Life The Chemical Foundation of Life Biological Macromolecules Cell Structure Structure and Function of Plasma Membranes Metabolism Cellular Respiration Photosynthesis Cell Communication Cell Reproduction Meiosis and Sexual Reproduction Mendel's Experiments and Heredity Modern Understandings of Inheritance DNA Structure and Function Genes and Proteins Gene Expression Biotechnology and

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Revision links and quizzes for year 12 and 13

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Revision links and quizzes for year 12 and 13

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Revision links and quizzes for year 12 and 13

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A level Biology Revision site covering all topics

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Free A Level Biology AQA Revision Content - Study Rocket

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Browse hundreds of lessons for A Level Biology AQA to study for free with assessment questions, text & videos.

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Link to list of videos on You Tube covering most of the A level course. Click on the '3 line' icon at the top of the screen to get the list of videos

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Introduction to Biological Molecules: Monomers & Polymers | A-level Biology | OCR, AQA, Edexcel

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Free on line materials for AS and A2

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Learn A Levels, GCSE, KS3 & KS2 2x Faster with Seneca

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Learn A Levels, GCSE, KS3 & KS2 2x Faster with Seneca

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Just to let you know...
We will be carrying out maintenance work on Tuesday 26th April between 6:00am and 9:00am (GMT). During this time you may not be able to access your Kerboodle. We aim to complete the work as quickly as possible and apologise for any inconvenience caused.

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Kerboodle is the digital teaching, learning and assessment service from Oxford University Press.

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Practical demo plus explanation

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Respiration A-level Required Practical: Effect of temperature on dehydrogenase in yeast using TTC

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Practical demo plus explanation

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A-level Biology PHOTOSYNTHESIS REQUIRED PRACTICAL - AQA required practical 8 aim, method, conclusion

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Types of competition

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Ecology: Interspecific and Intraspecific Interactions | Ecology & Environment | Biology | FuseSchool

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Glucose is needed for respiration so if the level falls below this, the normal body activities may not be able to continue. If the level rises too much the normal behaviour of cells is affected and serious problems can arise. The ideal level of blood glucose is about 1mg/cm ³.

There are natural situations that can affect the level of glucose. For example, eating will increase it and exercise will decrease it. Because of this fluctuation, there are 2 hormones that minimise these fluctuations in glucose levels. Their names are insulin and glucagon and they work antagonistically (against each other). Both are produced in the pancreas; α cells of the pancreas produce glucagon, β cells of the pancreas produce insulin.

Glucose is a small, soluble molecule that is carried in the blood plasma. The pancreas detects the level of glucose and the appropriate hormone is released to either increase or reduce the level of glucose.

In the next section you need to be very careful as the words used are very similar and can be confusing.

Try looking at the start of the words to see if it is referring to Glyco or Gluco and then at the ends of words to see if it is referring to genesis (meaning formation) or lysis (meaning splitting).

Note: Neogenesis means new formation.
Insulin reduces the level of glucose in the blood plasma by:

Using up the excess glucose (this is done by increasing the rate of respiration).

Increasing the amount of glucose absorbed into the body cells (particularly the liver) thus taking the glucose out of the blood plasma.

Turning the glucose into glycogen. This process is called Glycogenisis (remember, genesis means formation).

Glucagon increases the level of Glucose in the blood plasma by doing roughly the opposite to insulin...

Slowing the use of glucose (this is done by reducing the rate of respiration).

Reducing the amount of glucose absorbed into the body cells (particularly the liver).

Releasing glycogen. Glycogen is stored in the liver and muscles and is easily converted into glucose in a process called Glycogenolysis (remember, lysis means splitting).

In addition to this, Glucagon promotes the conversion of fatty acids. into glucose. This process is called Gluconeogenesis (remember, neogenesis means new formation).
This illustration shows the process your body goes through to react to changes in glucose levels:

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Glucose is needed for respiration so if the level falls below this, the normal body activities may not be able to continue. If the level rises too much the normal behaviour of cells is affected and serious problems can arise. The ideal level of blood glucose is about 1mg/cm3.There are natural situations that can affect the level of glucose.

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Ignore the Menstrual Cycle. Also covers some gene expression and technology for 3.8

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Published on

1. 1. Biology Unit 5 AQA

2. 2. Nervous & Hormonal Communication  Receptors detect stimuli and effectors produce a response: ◦ Receptors can be cells or proteins on cell surface membranes ◦ Effectors include muscle cells & cells found in glands  Nervous system sends information as electrical impulses: ◦ Sensory Neurone - Transmits impulses from receptor to CNS ◦ Motor Neurone - Transmits impulses from CNS to effector ◦ Relay Neurone - Transmits impulses between Sensory & Motor Neurones  Stimulus Receptor CNS Effector Response  Peripheral Nervous System: ◦ Made up of neurones that connect CNS to rest of body  Somatic Nervous System: ◦ Controls conscious activities e.g. running  Autonomic Nervous System: ◦ Controls unconscious activities e.g. digestion ◦ Sympathetic: - stimulates effectors/speeds things up ◦ Parasympathetic: - inhibits effectors/slows things down

3. 3. Nervous & Hormonal Communication  Hormonal system is made up of glands & hormones: ◦ Group of specialised cells that secrete a useful substance ◦ Hormones are 'chemical messengers' mostly proteins or peptides  Glands stimulated by: ◦ Change in concentration of a specific substance ◦ Electrical impulse  Hormones diffuse directly into the blood  Hormones usually trigger a response in the target cells

4. 4.  They're specific to one particular stimulus  They can be cells, or proteins on cell surface membranes  How they work: ◦ When nervous system receptor at resting state there's a difference in voltage between inside & outside of cell ◦ Resting potential is when a cell is at rest ◦ When a stimulus is detected, cell membrane becomes excited & more permeable. This allows ions to move in & out of cell. ◦ This alters the potential difference (PD) of cell ◦ Change in PD is called the generator potential (GP) ◦ Bigger stimulus = more ion movement = bigger GP ◦ If the GP is big enough & reaches threshold, a AP is produced Receptors

5. 5. Receptors in Skin & Eye  Pacinian Corpuscles - Pressure Receptor in Skin: ◦ They're mechanoreceptors - detect mechanical stimuli e.g. pressure & vibrations ◦ Contain the end of a sensory neurone - a sensory nerve ending ◦ Ending wrapped in layers of connective tissue - lamellae ◦ When stimulated the lamellae are deformed & press on the sensory nerve ending ◦ This deforms the stretch-mediated Na+ channels in the sensory neurone's cell membrane ◦ Allows Na+ to move into the cell creating a GP ◦ If the GP reaches the threshold then an AP is produced  Photoreceptors - Light Receptors in Eye: ◦ Light enters through pupil - amount of light let through controlled by iris muscles ◦ Light rays are focused onto the retina by the lens ◦ Retina contains the photoreceptor cells ◦ Fovea - lots of photoreceptor cells

6. 6. Receptors in Eye  Photoreceptors convert light to an electrical impulse: ◦ Light enters eye, hits photoreceptors & absorbed by light-sensitive pigments ◦ Light bleaches pigments, causes chemical change - membrane more permeable to sodium ◦ If GP reaches threshold an AP is produced & is sent down the bipolar neurone ◦ Bipolar neurones connect photoreceptors to the optic nerve ◦ Two type of photoreceptor:  Rods - Found in peripheral part of retina - Give information in black & white  Cones - Found packed together in fovea - Three types of cone - red, green & blue sensitive ◦ Sensitivity:  Rods very sensitive to light. Many rods join one neurone, so many weak GP's combine to reach threshold and trigger an AP  Cones less sensitive to light. One cone joins one neurone, so more light is needed to reach the threshold ◦ Visual Acuity:  Rods have low visual acuity, many rods join one neurone. This means light from two objects close together can't be told apart  Cones have high visual acuity because each cone joins one neurone and the cones are tightly

7. 7. Neurones  Resting potential - difference in voltage across the membrane when at rest  Resting potential maintained & created by Na+/K+ Pumps & K+ Channels  Stimulus: ◦ Bigger stimulus fires more frequently ◦ Causes Na+ channels to open ◦ Membrane more permeable to Na+ ◦ Na+ diffuses into neurone ◦ Inside of neurone more negative  Depolarisation: ◦ If PD reaches -55mV (threshold), more Na+ open ◦ More Na+ diffuses into neurone  Repolarisation: ◦ At +30mV Na+ channels close and K+ open ◦ K+ diffuse out of neurone  Hyperpolarisation: ◦ K+ slow to close ◦ Too many K+ diffuse out of neurone ◦ More -ve than resting potential  Resting Potential: ◦ Na+- K+ pump returns membrane to rest  Refractory Period: ◦ Makes sure no overlaps or travel in one direction K+ OpenNa+ Close Threshold Na+ Open K+ Close Refractory Period

8. 8. Factors Affecting Speed of Impulse  Myelination: ◦ Some neurones have a myelin sheath - electrical insulator ◦ Made up of Schwann cells ◦ Between Schwann cells are nodes of Ranvier ◦ Neurone's cytoplasm conducts enough electrical charge to depolarise next node ◦ Impulse 'jumps' from node to node - saltatory conduction  Axon Diameter: ◦ Faster with larger diameters - less resistance ◦ Depolarisation reaches other parts of neurone quicker  Temperature: ◦ Ions diffuse faster at higher temperature ◦ Up to 40°C - after that the proteins denature Motor Neurone Sensory Neurone Myelinated Axon Dendrites Cell Body Motor End Plate

9. 9. Synapses  Presynaptic neurone has a swelling - 'synaptic knob'  Synaptic knob contains vesicles - filled with neurotransmitter  When AP reaches end of neurone causes neurotransmitter to be released into synaptic cleft  Neurotransmitter diffuses across cleft & binds to receptor  Triggers AP, Muscle Contraction or Hormone Secretion  Neurotransmitter removed from receptor by enzymes  Cholinergic Synapse: ◦ AP arrives at knob ◦ Stimulates voltage-gated Na+ channels to open ◦ Ca2+ diffuse into knob ◦ Causes vesicles to fuse with presynaptic membrane ◦ Vesicles release ACh (Acetyl Choline) in cleft - exocytosis ◦ ACh binds to specific receptors on postsynaptic membrane ◦ Na+ channels open on postsynaptic membrane ◦ Na+ influx produces an AP ◦ ACh removed from receptor by AChE and reabsorbed into presynaptic membrane

10. 10. More Synapses  Neuromuscular Junction: ◦ Synapse between motor neurone & muscle cell ◦ Similar to cholinergic synapse, but a few differences ◦ Post-synaptic membrane has lots of folds forming clefts, containing AChE ◦ Post-synaptic membrane has more receptors ◦ AP from motor neurone always triggers a response in a muscle cell  Neurotransmitter are Excitatory or Inhibitory: ◦ Excitatory depolarise the post-synaptic membrane, making it fire an AP if threshold is reached e.g. ACh ◦ Inhibitory hyperpolarise the post-synaptic membrane preventing an AP being fired e.g. GABA  Summation: ◦ Effect of neurotransmitter released from many neurones is added together ◦ Spatial:  Many neurones connect to one neurones  Small amount of neurotransmitter from each can be enough to reach threshold & produce AP  If some release inhibitory neurotransmitter then total effect might not produce an AP ◦ Temporal  Two or more impulses arrive in quick succession from same pre-synaptic neurone

11. 11. Drugs Affecting Action of Neurotransmitter  Same shape as neurotransmitter, mimic their action so more receptors are activated  Block receptors so they can't be activated by neurotransmitter, less receptors activated  Inhibit enzyme that breaks down neurotransmitter, receptor is blocked by neurotransmitter  Stimulate release of neurotransmitter from pre-synaptic membrane  Inhibit release of neurotransmitter from pre-synaptic membrane

12. 12. Muscles  Skeletal muscles (striated, striped or voluntary muscles)  Made up of muscle fibres  Cell membrane of muscle fibre cells is the Sarcolemma  Bits of sarcolemma fold inwards across the muscle fibre and stick into the sarcoplasm (muscle cell's cytoplasm)  These folds are called transverse (T) tubules & help spread impulses throughout sarcoplasm  Network of internal membranes called Sarcoplasmic Reticulum  Sarcoplasmic reticulum stores Ca2+ needed for muscle contraction  Muscle fibres are multinucleate (contain many nuclei)  Muscle fibres have lots of long, cylindrical organelles called Myofibrils  Myofibrils: ◦ Contain thick & thin filaments ◦ Thick = made of myosin ◦ Thin = made of actin

13. 13. Muscle Contraction  Myosin has globular heads  With a binding site for ATP & actin  Tropomyosin & troponin found between actin filaments, helps filaments slide over each other  At rest actin-myosin binding site blocked by tropomyosin, held in place by troponin  AP depolarises sarcolemma, spreading down T tubules to sarcoplasmic reticulum  Sarcoplasmic reticulum releases Ca2+ into sarcoplasm  Ca2+ binds to troponin causing it's change shape  Pulls tropomyosin out of actin-myosin binding site, exposing site  Bond between myosin head & actin creating a actin-myosin cross bridge  Ca2+ also activates ATPase, providing energy  Energy released moves myosin head, pulling actin molecule  ATP provides energy to break cross bridge  Myosin head reattaches to different binding site, creating new cross bridge  When excitation stops Ca2+ leave binding site on troponin & moves into sarcoplasmic reticulum by AT

14. 14. More Muscle Contraction  Aerobic Respiration: ◦ Most ATP produced via oxidative phosphorylation in mitochondria ◦ Long periods of low-intensity exercise  Anaerobic Respiration: ◦ ATP made rapidly by glycolysis ◦ Produces pyruvate which converts into lactate ◦ Lactate builds up in muscles causing fatigue ◦ Short periods of high-intensity exercise  ATP-Phosphocreatine (PCr) System: ◦ ATP produced by phosphorylating ADP, adding a phosphate group from PCr ◦ PCr stored inside cells ◦ Quick production of ATP ◦ PCr runs out after a few seconds ◦ ATP-PCr is anaerobic & alactic (no lactate formed)  Slow-Twitch Fibres: ◦ Aerobic - Red Colour - high levels of Myoglobin  Fast-Twitch Fibres: ◦ Anaerobic - White Colour - low levels of Myoglobin

15. 15. Control of Heart Rate  SAN generates impulses controlling HR  Unconsciously controlled by Medulla  Stimuli are detected by baroreceptors & chemoreceptors: ◦ Baroreceptors in Carotid Arteries - stimulated by Blood Pressure ◦ Chemoreceptors in Carotid Arteries - monitor O2, CO2 & pH in blood ◦ Chemoreceptors detect change in pH ◦ More CO2 = More Carbonic Acid = Lower pH Stimulus Receptor Neurone & Neurotransmitter Effector Response High BP Baroreceptor Impulse to medulla, along parasympathetic to SAN Cardiac Muscle HR slows BP lowers Low BP Impulse to medulla, along sympathetic to SAN HR increases BP increases Low CO2 Chemoreceptor Impulse to medulla, along parasympathetic to SAN HR slows CO2 level returns to norm High CO2 Impulse to medulla, along sympathetic to SAN HR increases CO2 level returns to norm

16. 16. Reflexes  Involuntary rapid response to a stimuli  Thermoreceptors in skin detect heat stimulus  Sensory neurone carries impulse to Relay  Relay to Motor  Motor sends impulse to effector (e.g. biceps)  Effector muscle contracts stopping hand being damaged  If relay neurone involved you can override the reflex e.g. leave your hand on the heat

17. 17. Responses in Animals  Tactic response (taxes): ◦ Organisms move towards or away from a directional stimulus ◦ Phototaxis - Light ◦ Chemotaxis - Chemicals ◦ Aerotaxis - Air (O2) ◦ Geotaxis - Gravity  Kinetic response (kineses): ◦ Organisms' movement is affected by the intensity of the stimulus  Cells communicate with other cells with Chemical Mediators: ◦ Chemical mediator is a chemical messenger that acts locally ◦ Similar to hormones ◦ Chemical mediators can be secreted from cells not just from glands ◦ Target cells near to where mediator is produced - local response ◦ Only travel a short distance - quicker response  Histamines: ◦ Stored in mast cells & basophils. Released in response to infection or injury. Allows more immunity cells to move in & out of blood to the area  Prostaglandins: ◦ Produced by most cells of the body. Involved in inflammation, fever, BP regulation & blood clotting

18. 18. Responses in Plants  Tropism - Plants Growth Response to an External Stimulus  +ve tropism = growth towards the stimulus  Phototropism: ◦ Growth in response to Light ◦ Shoot grow towards light +vely phototropic ◦ Roots grow away from light -vely phototropic  Geotropism: ◦ Growth in response to Light ◦ Shoots are -vely geotropic, grow upwards ◦ Roots are +vely geotropic, grow downwards  Gibberellin is a growth stimulus flowering & seed germination  Auxins stimulates growth of shoots by cell elongation  Auxins inhibit growth in roots  IAA is an important Auxin - diffuses over short distances & via phloem over long distances Root Root Shoot Shoot

19. 19. Homeostasis  Homeostasis involves control systems that keep your internal environment roughly constant  Temperature: ◦ If too high enzymes denature, enzymes molecules vibrate, breaks H-bonds, shape of active site is altered ◦ Optimum - 37°C  pH: ◦ pH too high or low, enzymes denature ◦ Optimum usually - 7 but specialist enzymes can work higher or lower  Glucose: ◦ Too high H2O potential of blood is more -ve. H2O moves out of cells by Osmosis ◦ Too low, cells unable to function properly due to lack of energy  Negative Feedback: ◦ Mechanism that returns the level to normal but only works between certain limits. If change is too big then effectors may not be able to counteract it ◦ Multiple negative feedback mechanisms give more control  Positive Feedback: ◦ Mechanism that amplifies the change from the norm, effectors respond to further increase level away from norm. ◦ Blood Clotting - Platelets become activated - trigger more platelets to be activated

20. 20. Controlling Body Temperature  Mechanism of Heat Loss: ◦ Vasodilation -arteriole diameter near skin increases - warm blood passes near skin ◦ Increased Sweating - H2O lost by evaporation, body heat used to evaporate H2O ◦ Lowering of Body Hair - less insulating layer - hair erector muscles in skin relax ◦ Behavioural - sheltering in the shade/burrows  Mechanisms of Heat Gain: ◦ Vasoconstriction - arteriole diameter near skin decreases - blood passes under insulating fat ◦ Shivering - muscles involuntarily contract - produces metabolic heat ◦ Hairs Stand Up - hair erector muscles contract - traps layer of air next to skin ◦ Hormones - releases adrenaline & thyroxine - increase metabolic rate - more heat produced ◦ Less Sweating ◦ Behavioural - basking in sun / huddling / sheltering from windEctotherms e.g. reptiles Endotherms e.g. mammals Control temp by changing their behaviour e.g. basking in sun Control temp internally by homeostasis & behaviour Internal temp depends on external temp Internal temp less affected by external temp Activity depends on external temp Activity independent on external temp Variable metabolic rate High metabolic rate - produces heat

21. 21. Hypothalamus  Controls body temperature in mammals  Heat Loss Centre: ◦ Responds to rise in body temp  Heat Gain Centre: ◦ Responds to fall in body temp  Receives information about both internal & external temperature  Information produced by Thermoreceptors  Internal temperature: ◦ Thermoreceptors in hypothalamus detect blood temperature  External temperature: ◦ Thermoreceptors in skin detect skin temperature  Thermoreceptors send impulses to hypothalamus along autonomic nervous system  To effectors from hypothalamus along autonomic nervous system

22. 22. Hormonal Control of BGC  Insulin: ◦ Lowers BGC when too high ◦ Produced by Beta cells in the Islets of Langerhans (pancreas) ◦ Binds to receptors on cell membrane of liver & muscle cells ◦ These cells are more permeable to glucose, cell takes up more glucose ◦ Insulin activates enzyme converts glucose to glycogen ◦ Cells store glycogen in cytoplasm - energy source ◦ Glycogenesis = glucose  glycogen ◦ Insulin increases rate of respiration of glucose, especially in muscle cells  Glucagon: ◦ Raises BGC when too low ◦ Produced by Alpha cells in Islets of Langerhans (pancreas) ◦ Binds to receptors on membrane of liver cells ◦ Activates enzyme breaks down glycogen into glucose ◦ Glycogenolysis = breaking down glycogen ◦ Gluconeogenesis = forming glucose from non-carbohydrates ◦ Glucagon reduces rate of respiration of glucose in cells BGC - Blood Glucose Concentration

23. 23. Control of BGC  Adrenaline: ◦ Hormone secreted by adrenal glands ◦ Secreted if BGC is low ◦ Binds to receptors on liver cells  Activates glycogenolysis - glycogen  glucose  Inhibits glycogenesis - glucose  glycogen ◦ Activates glucagon secretion ◦ Inhibits insulin secretion ◦ Adrenaline & Glucagon bind to receptors activating Adenylate Cyclase (enzyme) ◦ This enzyme converts ATP into a chemical signal a 'second messenger' ◦ Second messenger is Cyclic AMP (cAMP) ◦ cAMP activates a chain of reactions breaking down glycogen into glucose (glycogenolysis)  Diabetes: ◦ Type 1:  No insulin produced, BGC stays high - hyperglycaemia, treated with insulin injection ◦ Type 2:  Don't produce enough insulin or don't respond to insulin  Treated by controlling simple carbohydrate intake

24. 24. Menstrual Cycle  Lasts 28 days  Follicle developing in the ovary  Ovulation - egg being released  Uterus lining thickens, to support implanted egg  Corpus luteum develops from follicle remains  FSH (Follicle-Stimulating Hormone) - stimulates follicle to develop  LH (Luteinising Hormone) - stimulates ovulation & corpus luteum development  FSH & LH from Pituitary Gland  Oestrogen - Stimulates uterus lining thickening  Progesterone - maintains uterus lining thickening  Oestrogen & Progesterone from Ovaries

25. 25. 1. High [FSH] in Blood: FSH stimulates follicle development Follicle releases oestrogen FSH stimulates oestrogen to be released by ovaries 2. Rising [Oestrogen]: Oestrogen stimulates uterus lining thickening Oestrogen inhibits FSH 3. [Oestrogen] Peaks: High [oestrogen] stimulates FSH & LH production 4. LH Surge: Ovulation stimulated by LH LH stimulates follicle  corpus luteum Corpus luteum releases progesterone 5. [Progesterone] Rises: Progesterone inhibits FSH & LH Maintains uterus thickening If no embryo implants, corpus 6. Falling [Progesterone]: FSH & LH increase (not inhibited by progesterone) Uterus lining not maintained so it breaks down - menstruation

26. 26. -VE & +VE Feedback - Hormones  Negative Feedback:  Example One: ◦ FSH stimulates Ovary to release Oestrogen ◦ Oestrogen inhibits release of FSH After FSH has stimulated Follicle development, -ve feedback keeps [FSH] low, so no more follicles develop  Example Two: ◦ LH stimulates Corpus Luteum, which produces Progesterone ◦ Progesterone inhibits release of LH -ve feedback makes sure no more Follicles develop when Corpus Luteum is developing Also makes sure Uterus Lining no maintained if no Embryo implants  Positive Feedback:  Example: ◦ Oestrogen stimulates Pituitary Gland to release LH ◦ LH stimulates Ovary to release Oestrogen ◦ Oestrogen further stimulates to release LH High Oestrogen concentration triggers +ve feedback to make Ovulation happen

27. 27. DNA  It's a large polymer of repeating Nucleotides.  G & A Large Bases (Purines) C & T Small Bases (Pyrimidines)  A - T, C - G are the complementary pairs  Small % of DNA are genes, the rest is junk (VNTRS) used to create a DNA fingerprint  DNA molecules found inside nucleus, but organelles for protein synthesis are found in cytoplasm  DNA too large to move out of nucleus  Sections are copied into RNA which then moves to ribosomes A - Adenine G - Guanine C - Cytosine T (U) - Thymine (Uracil in RNA) P - Phosphate Group S - Sugar N - Nitrogen Base (Varies) Gen e Loci Exo n Intron A & T have 2 H bonds G & C have 3 H bonds

28. 28. RNA - Single Stranded  Messenger RNA (mRNA): ◦ Made during transcription in the nucleus ◦ Carries genetic information from nucleus to the cytoplasm  Transfer RNA (tRNA): ◦ Clover-shaped ◦ Every molecule has a specific anticodon at one end ◦ Amino Acid binding site at other end ◦ Found in cytoplasm where it's involved in translation DNA mRNA tRNA Shape Double Stranded Helix Single-Stranded Single-Stranded Sugar Deoxyribose Ribose Ribose Bases A T C G A U C G A U C G Other... 3 Bases = 1 Codon 3 Bases = 1 Codon 3 Bases = Anticodon or AA Binding Site

29. 29. Protein Synthesis  Transcription:  RNA polymerase attaches to DNA double-helix  H-bonds between helix break  One strand is template for mRNA copy  RNA polymerase attaches free RNA nucleotides to template strand  RNA nucleotides then form mRNA molecule  RNA polymerase moves along strand of DNA  H-bonds reform in DNA, back to a double-helix  Stops making mRNA if reaches a stop signal (specific base sequence)  mRNA moves out through nuclear pore & attaches to ribosome  Splicing:  Pre-mRNA contains Introns & Exons  Introns are removed during splicing  In the nucleus  Only DNA that codes for AA's remains

30. 30.  Translation:  mRNA attaches to ribosome & tRNA carries AA to ribosomes  tRNA anticodon molecule attaches to mRNA codon (specific base sequence)  2nd tRNA attaches to next mRNA codon  2 AAs joined by peptide bond  1st tRNA molecule moves away leaving it's AA  3rd tRNA attaches to next codon, it's AA bonds to previous 2 AAs  Process continues producing a polypeptide chain until a stop signal on mRNA  Polypeptide chain moves away from ribosome Protein Synthesis

31. 31. Genetic Code  It's non-overlapping, degenerate & universal  The genetic code is the sequence of codons in mRNA which code for AAs  Each codon is read in sequence, each codon is read separately  Degenerate: ◦ Some AAs are coded for by more than one base triplet  Some codons are used to start & stop protein production  Same codons code for AAs in all organisms

32. 32. Regulation of Transcription & Translation Transcription Factors: ◦ They move from the cytoplasm into the nucleus ◦ Bind to specific DNA sites near start of target gene ◦ They control the rate of transcription ◦ Activators - increase the rate ◦ Repressors - decrease the rate  Oestrogen ◦ Binds to an oestrogen receptor and acts as a transcription factor ◦ Forms an oestrogen-oestrogen receptor complex which moves into the nucleus ◦ It binds near the start of the target gene ◦ Complex can act as an activator or a repressor ◦ Depends what type of cell & the target gene  siRNA (small interfering RNA): ◦ Short, double-stranded RNA molecules ◦ Their bases are complimentary to sections of target gene ◦ Interferes with transcription & translation ◦ Affects translation through RNA interference:  In cytoplasm, siRNA & proteins bind to target mRNA  Proteins cut mRNA into sections so it can't be translated  Prevents expression of specific genes as translation cannot occur

33. 33. Mutations  Change in the base sequence of DNA  Caused by: ◦ Errors during DNA replication ◦ Mutagenic agents  Substitution: ◦ One base is substituted for another - causes frame shift  Deletion: ◦ One base is deleted  But not all mutations affect order of AAs: ◦ Degenerate nature of DNA ◦ Substitution may not affect AA order but deletion always will  Mutagenic Agents: ◦ UV radiation, Ionising radiation, some chemicals & viruses ◦ Act as a base - chemicals called base analogues can substitute for a base during DNA replication ◦ Altering bases - some chemicals can delete or alter bases ◦ Changing DNA structure - some radiation can change shape of DNA

34. 34. Genetic Disorders & Cancer  Hereditary Mutations: ◦ Some mutations can cause genetic disorders ◦ Some mutations can increase risk of developing certain cancers ◦ If a gamete containing a gene mutation is fertilised it becomes a hereditary mutation  Acquired Mutations: ◦ Mutations that occur after fertilisation ◦ If mutations occur in genes controlling cell division, can cause uncontrollable cell division ◦ Produces a tumour - mass of abnormal cells ◦ Tumours invade & destroy surrounding tissues are Cancers ◦ Two genes control cell division:  Tumour-Suppressor Genes - mutation causes protein not to be produced  uncontrollable cell division  Proto-Oncogenes -mutation causes it to be overactive  cells to divide uncontrollably Proto-OncogenesTumour-Suppressor Genes

35. 35. Stem Cells  Able to mature into any body cell  Stem cells are unspecialised cells  Stem cells found in embryo and some adult tissues  Stem cells that can mature into any body cell are totipotent cells  Totipotent cells only found in early life of embryo  After this point they lose ability to become any cell  Totipotent found in plants: ◦ Mature plants have them where they grow e.g. roots & shoots ◦ All plants stem cells are totipotent ◦ Can be used for tissue cultures - cell placed in sterile growth medium - produces new plant  Totipotent become specialised because they only translate & transcribe part of their DNA ◦ Certain genes are expressed others are turned 'off'

36. 36. Making DNA Fragments  Using Reverse Transcriptase: ◦ Most cells contain 2 copies of each gene but contain many mRNA copies ◦ Enzyme converts RNA  DNA ◦ DNA produced from RNA is cDNA (complementary DNA)  Using Restriction Endonucleases: ◦ Enzymes recognise palindromic sequences of nucleotides ◦ DNA sample incubated with the specific restriction endonuclease ◦ Can leave sticky ends allowing the DNA fragment to anneal to another DNA molecule if they have a complementary base match  Using PCR (Polymerase Chain Reaction): ◦ Mixture with Primers, DNA sample, Free Nucleotides & DNA Polymerase ◦ Primer - short piece of DNA complementary to bases at start of fragment required ◦ DNA mixture heated to 95°C to break H-bonds in double strand ◦ Cooled to 60°C so primers can anneal to strand ◦ Mixture heated to 72°C so DNA polymerase can work ◦ DNA polymerase forms a new strand complementary to the template strand ◦ Two new copies are formed, one cycle of PCR complete ◦ Cycle starts again ◦ Each PCR cycle doubles the amount of DNA

37. 37. Gene Cloning  In Vitro: ◦ Where the gene copies are made outside of a living organism using PCR  In Vivo: ◦ Where the gene copies are made within a living organism  In Vivo: ◦ Gene Inserted into a Vector:  DNA fragment inserted into vector DNA  Vector - something used to transfer DNA into a cell  Vector DNA cut open using restriction endonuclease  DNA ligase joins sticky ends of DNA fragment to vector DNA, Ligation  New combination of bases in DNA - recombinant DNA ◦ Vector Transfers the Gene into Host Cells:  If a plasmid vector used, host cells won't easily take in the Plasmid vector & it's DNA  With a bacteriophage vector, it will infect the host bacterium by injecting it's DNA into it  Host cells that take up the vector containing the gene are Transformed ◦ Identifying Transformed Host Cells:  Marker genes inserted into vector along with gene  Host cells grown on agar - creates colony of cloned cells  Marker gene can code for antibiotic resistance - agar contains antibiotic - only transformed cells with survive  Can also code for fluorescence - will show up under UV light Organisms with altered DNA are called transformed organisms Genetic engineering - recombinant DNA technology

38. 38. Genetic Fingerprinting  Genomes contain repetitive, non-coding DNA sequences  Number of repeats of a sequence can be compared between individuals  Gel Electrophoresis: ◦ PCR is used to make copies of sample ◦ Primers bind to each end of the repeat ◦ Fluorescent tag added to DNA fragments - visible under UV light ◦ DNA mixture placed into a well in the gel ◦ Gel covered in a buffer that conducts electricity ◦ Electric current passed through the gel, DNA moves towards +ve electrode ◦ Small fragments move further  Used to determine: ◦ Relationships ◦ Variation ◦ Medical Diagnosis ◦ Forensic Science

39. 39. Locating & Sequencing Genes Look for genes using DNA Probes & Hybridisation: ◦ DNA probes locate gene or see if DNA contains a mutated gene ◦ DNA probes are short strands of DNA, they have a complementary base sequence to part of the target gene ◦ DNA probe will bind to target gene & can be detected if it is labelled (fluorescent or radioactive) ◦ DNA sample digested with restriction enzymes & separated by gel electrophoresis ◦ DNA fragments transferred onto a nylon membrane & incubated with labelled DNA probe ◦ Membrane exposed to UV or X-ray & bands will be visible  Restriction Mapping: ◦ Restriction enzymes cut labelled DNA into fragments ◦ Gel electrophoresis produces bands ◦ Compare DNA with different restriction enzymes  Gene Sequencing: ◦ Mixture of DNA template, polymerase, primer, nucleotides & Fluorescently labelled modified nucleotides ◦ Each tube contains A*, G*, C* or T* & undergoes PCR ◦ Different length fragments produced depending on where the modified fragments bind to the DNA e.g. ACTACG*, ACTACGATG* in tube with G*

40. 40. DNA Probes in Medical Diagnosis  They can screen for mutated genes e.g. Sickle-cell Anaemia: ◦ Probe labelled to look for a specific gene ◦ Or Probe used as part of microarray, which can screen many genes at the same time  DNA Microarray is a glass slide with microscopic spots of different DNA probes attached in rows  Sample of labelled human DNA washed over the Array  Any matches with the probes will stick to the array  Array washed to remove excess DNA  Array under UV light shows any labelled DNA  Any spot that fluoresces contains that specific gene  Results of screening used to decide what treatment to use

41. 41. Gene Therapy  Involves altering the defective genes inside cell to treat genetic disorders & cancers  Depends if it is caused by mutated dominant or double-recessive alleles  Allele inserted into cells using a vector  Somatic Therapy: ◦ Altering the alleles in body cells, particularly ones most affected by disorder ◦ Doesn't affect sex cells so offspring could still inherit disease  Germ Line Therapy: ◦ Alters sex cells ◦ Offspring will not be affected by the disease ◦ Illegal at the moment

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Transmission across a cholinergic synapse

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Transmission across a cholinergic synapse

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Synaptic Transmission at a Cholinergenic Synapse Animation

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Neurones send messages electrochemically; this means that chemicals (ions) cause an electrical impulse. Neurones and muscle cells are electrically excitable cells, which means that they can transmit electrical nerve impulses. These impulses are due to events in the cell membrane, so to understand the nerve impulse we need to revise some properties of cell membranes.
The Resting Membrane Potential [back to top]

When a neurone is not sending a signal, it is at 'rest'.The membrane is responsible for the different events that occur in a neurone.All animal cell membranes contain a protein pump called the sodium-potassium pump(Na⁺K⁺ATPase). This uses the energy from ATP splitting to simultaneously pump 3 sodium ions out of the cell and 2 potassium ions in.

The Sodium-Potassium Pump (Na+K+ATPase)
(Provided by: Doc Kaiser's Microbiology Website )

Three sodium ions from inside the cell first bind to the transport protein. Then a phosphate group is transferred from ATP to the transport protein causing it to change shape and release the sodium ions outside the cell. Two potassium ions from outside the cell then bind to the transport protein and as the phospate is removed, the protein assumes its original shape and releases the potassium ions inside the cell.

If the pump was to continue unchecked there would be no sodium or potassium ions left to pump, but there are also sodium and potassium ion channels in the membrane. These channels are normally closed, but even when closed, they "leak", allowing sodium ions to leak in and potassium ions to leak out, down their respective concentration gradients.

Concentration of ions inside and outside the neurone at rest:

Ion Concentration inside cell/mmol dm⁻³ Concentration outside cell/mmol dm⁻³ Why don'tthe ions move down their concentration gradient?

K+

150.0

2.5

K+ ions do not move out of the neurone down their concentration gradient due to a build up of positive charges outside the membrane.This repels the movement of any more K+ ions out of the cell.

Na+

15.0

145.0

Cl-

9.0

101.0

The chloride ions do not move into the cytoplasm as the negatively charged protein molecules that cannot cross the surface membrane repel them.

The combination of the Na⁺K⁺ATPase pump and the leak channels cause a stable imbalance of Na⁺ and K⁺ ions across the membrane.This imbalance of ions causes a potential difference (or voltage) between the inside of the neurone and its surroundings, called theresting membrane potential. The membrane potential is always negative inside the cell, and varies in size from -20 to -200 mV (milivolt) in different cells and species (in humans it is -70mV). The Na⁺K⁺ATPase is thought to have evolved as an osmoregulator to keep the internal water potential high and so stop water entering animal cells and bursting them. Plant cells don't need this as they have strong cells walls to prevent bursting.

Check Point g The Resting Membrane Potential is always negative (-70mV)

K+ pass easily into the cell

Cl- and Na+ have a more difficult time crossing

Negatively charged protein molecules inside the neurone cannot pass the membrane

The Na+K+ATPase pump uses energy to move 3Na+ out for every 2K+ into neuron

The imbalance in voltage causes a potential difference across the cell membrane - called the resting potential

The resting potential tells us about what happens when a neurone is at rest.An action potential occurs when a neurone sends information down an axon.This involves an explosion of electrical activity, where the nerve and muscle cells resting membrane potential changes.
In nerve and muscle cells the membranes are electrically excitable, which means they can change their membrane potential, and this is the basis of the nerve impulse. The sodium and potassium channels in these cells arevoltage-gated, which means that they can open and close depending on the voltage across the membrane. The normal membrane potential inside the axon of nerve cells is -70mV, and since this potential can change in nerve cells it is called the resting potential. When a stimulus is applied a brief reversal of the membrane potential, lasting about a millisecond, occurs. This brief reversal is called theaction potential:
An action potential has 2 main phases called depolarisation and repolarisation:

At rest, the inside of the neuron is slightly negative due to a higher concentration of positively charged sodium ions outside the neuron.

When stimulated past threshold (about -30mV in humans), sodium channels open and sodium rushes into the axon, causing a region of positive charge within the axon.This is called depolarisation

The region of positive charge causes nearby voltage gated sodium channels to close. Just after the sodium channels close, the potassium channels open wide, and potassium exits the axon, so the charge across the membrane is brought back to its resting potential.This is called repolarisation.

This process continues as a chain-reaction along the axon. The influx of sodium depolarises the axon, and the outflow of potassium repolarises the axon.

The sodium/potassium pump restores the resting concentrations of sodium and potassium ions

(provided by: Markham )

Check Point g Action Potential has two main phases:

Depolarisation. A stimulus can cause the membrane potential to change a little. The voltage-gated ion channels can detect this change, and when the potential reaches -30mV the sodium channels open for 0.5ms. The causes sodium ions to rush in, making the inside of the cell more positive. This phase is referred to as a depolarisation since the normal voltage polarity (negative inside) is reversed (becomes positive inside).

Repolarisation. At a certain point, the depolarisation of the membrane causes the sodium channels to close.As a result the potassium channels open for 0.5ms, causing potassium ions to rush out, making the inside more negative again. Since this restores the original polarity, it is called repolarisation.As the polarity becomes restored, there is a slight 'overshoot' in the movement of potassium ions (called hyperpolarisation).The resting membrane potential is restored by the Na+K+ATPase pump.

'All or Nothing' Law

The action potential only occurs if the stimulus causes enough sodium ions enter the cell to change the membrane potential to a certain threshold level.At the threshold, sodium gates open in the membrane and allow a sudden flood of sodium ions to enter the cell.If the depolarisation is not great enough to reach the threshold, then an action potential (and hence an impulse) will not be produced.This is calledthe all or nothing law. This means that the ion channels are either open or closed; there is no half-way position.This means that the action potential always reaches +40mV as it moves along an axon, and it is never attenuated (reduced) by long axons.Action potentials are always the same size, however the frequency of the impulse carrying the information can determine the intensity of the stimulus, i.e. strong stimulus = high frequency.

How do Nerve Impulses Start? [back to top]

We and other animals have several types of receptors of mechanical stimuli. Each initiates nerve impulses in sensory neurons when it is physically deformed by an outside force such as:

touch

pressure

stretching

sound waves

motion

Mechanoreceptors enable us to

detect touch

monitor the position of our muscles, bones, and joints - the sense of proprioception

detect sounds and the motion of the body.

E.g. Touch

Light touch is detected by receptors in the skin. These are often found close to a hair follicle so even if the skin is not touched directly, movement of the hair is detected.

In the mouse, light movement of hair triggers a generator potential in mechanically-gated sodium channels in a neuron located next to the hair follicle. This potential opens voltage-gated sodium channels and if it reaches threshold, triggers an action potential in the neuron.

Touch receptors are not distributed evenly over the body. The fingertips and tongue may have as many as 100 per cm ²; the back of the hand fewer than 10 per cm ².This can be demonstrated with the two-point threshold test. With a pair of dividers like those used in mechanical drawing, determine (in a blindfolded subject) the minimum separation of the points that produces two separate touch sensations. The ability to discriminate the two points is far better on the fingertips than on, say, the small of the back.

The density of touch receptors is also reflected in the amount of somatosensory cortex in the brain assigned to that region of the body.

Proprioception

Proprioception is our "body sense".

It enables us to unconsciously monitor the position of our body.

It depends on receptors in the muscles, tendons, and joints.

If you have ever tried to walk after one of your legs has "gone to sleep", you will have some appreciation of how difficult coordinated muscular activity would be without proprioception.

The Pacinian Corpuscle

Pacinian corpuscles are Pacinian corpuscles are fast-conducting, bulb-shaped receptors located deep in the dermis.They consist of the ending of a single neurone surrounded by pressure receptors. They are located in the skin and also in various internal organs. Each is connected to a sensory neuron. lamellae. They are the largest of the skin's receptors and are believed to provide instant information about how and where we move. They are also sensitive to vibration. Pacinian corpuscles are also located in joints and tendons and in tissue that lines organs and blood vessels.

Pressure on the skin changed the shape of the Pacinian corpuscle.This changes the shape of the pressure sensitive sodium channels in the membrane, making them open.Sodium ions diffuse in through the channels leading to depolarisation called a generator potential.The greater the pressure the more sodium channels open and the larger the generator potential.If a threshold value is reached, an action potential occurs and nerve impulses travel along the sensory neurone.The frequency of the impulse is related to the intensity of the stimulus.

Adaptation

When pressure is first applied to the corpuscle, it initiates a volley of impulses in its sensory neuron. However, with continuous pressure, the frequency of action potentials decreases quickly and soon stops. This is the phenomenon of adaptation.

Adaptation occurs in most sense receptors. It is useful because it prevents the nervous system from being bombarded with information about insignificant matters like the touch and pressure of our clothing.

Stimuli represent changes in the environment. If there is no change, the sense receptors soon adapt. But note that if we quickly remove the pressure from an adapted Pacinian corpuscle, a fresh volley of impulses will be generated.

The speed of adaptation varies among different kinds of receptors. Receptors involved in proprioception - such as spindle fibres - adapt slowly if at all.

Check Point g The Pacinian Corpuscle

Deforming the corpuscle creates a generator potential in the sensory neuron arising within it. This is a graded response: the greater the deformation, the greater the generator potential. If the generator potential reaches threshold, a volley of action potentials (also called nerve impulses) are triggered at the first node of Ranvier of the sensory neuron.

Overall:
In living cells nerve impulses are started by receptor cells. These all contain special sodium channels that are not voltage-gated, but instead are gated by the appropriate stimulus (directly or indirectly). For example:

C hemical-gated sodium channels in tongue taste receptor cells open when a certain chemical in food binds to them hemical-gated sodium channels in tongue taste receptor cells open when a certain chemical in food binds to them

M echanically-gated ion channels in the hair cells of the inner ear open when they are distorted by sound vibrations

In each case the correct stimulus causes the sodium channel to open

i

Causes sodium ions to flow into the cell

i

Causes a depolarisation of the membrane potential

i

Affects the voltage-gated sodium channels nearby and starts an action potential (providing the threshold point is reached)

Check Point g An action occurs due to an increase in membrane permeability to Na+

An action potential is initiated by receptor cells that cause sodium channels to open

An action potential can only occur if the depolarisation of the membrane reaches the threshold point. This is the all or nothing law.

How are Nerve Impulses Propagated? [back to top]
Once an action potential has started it is moved (propagated) along an axon automatically. The local reversal of the membrane potential is detected by the surrounding voltage-gated ion channels, which open when the potential changes enough. The ion channels have two other features that help the nerve impulse work effectively:

For an action potential to begin, then the depo larisation of the neurone must reach the threshold value, i.e. the all or nothing law.

After an ion channel has opened, it needs a "rest period" before it can open again. This is called the refractory period, and lasts about 2 ms. This means that, although the action potential affects all other ion channels nearby, the upstream ion channels cannot open again since they are in their refractory period, so only the downstream channels open, causing the action potential to move one-way along the axon.

The refractory period is necessary as it allows the proteins of voltage sensitive ion channels to restore to their original polarity.

The absolute refractory period = during the action potential, a second stimulus willnot cause a new action potential.

Exception: There is an interval in which a second action potential can be produced but only if the stimulus is considerably greater than the threshold =relative refractory period

The refractory period can limit the number o faction potentials in a given time.

Average = about 100 action potentials per second.

Check Point g Nerve Impulses travel in one direction

How Fast are Nerve Impulses? [back to top]
Action potentials can travel along axons at speeds of 0.1-100 m/s. This means that nerve impulses can get from one part of a body to another in a few milliseconds, which allows for fast responses to stimuli. (Impulses are much slower than electrical currents in wires, which travel at close to the speed of light, 3x10 ⁸ m/s.) The speed is affected by 3 factors:

Temperature - The higher the temperature, the faster the speed. So homoeothermic (warm-blooded) animals have faster responses than poikilothermic (cold-blooded) ones.

Axon diameter - The larger the diameter, the faster the speed. So marine invertebrates, who live at temperatures close to 0°C, have developed thick axons to speed up their responses. This explains why squid have their giant axons.

Myelin sheath - Only vertebrates have a myelin sheath surrounding their neurones. The voltage-gated ion channels are found only at the nodes of Ranvier, and between the nodes the myelin sheath acts as a good electrical insulator. The action potential can therefore jump large distances from node to node (1mm), a process that is called saltatory propagation. This increases the speed of propagation dramatically, so while nerve impulses in unmyelinated neurones have a maximum speed of around 1 m/s, in myelinated neurones they travel at 100 m/s.

[back to top]
Last updated 17/04/2004 [Back to Nervous System]

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Neurones send messages electrochemically; this means that chemicals (ions) cause an electrical impulse. Neurones and muscle cells are electrically excitable cells, which means that they can transmit electrical nerve impulses. These impulses are due to events in the cell membrane, so to understand the nerve impulse we need to revise some properties of cell membranes.

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In the surface membrane of a cell there are protein carriers.

These actively (Sodium) ions out of the cytoplasm to the outside of the cell. At the same time, (Potassium) ions are pumped from the outside in.

This active pumping of Na⁺ and K⁺ ions requires energy (in the form of ATP) because the ions are being moved against their concentration gradients (from where they are at a lower concentration to where they are at a higher concentration). K⁺ and Na⁺ ions diffuse back down their concentration gradient but K⁺ diffuses back out of the cell faster than Na⁺ can diffuse back in.

This means there is a net movement of positive ions out of the cell making the inside of the cell negatively charged, relative to the outside.

This charge is the resting potential of the cell and is about -70mV.

When a receptor is stimulated, it will create a positive environment inside the cell.

This is caused by a change in the concentrations of Na⁺ and K⁺ ions in the cell and happens in a number of steps:

1. There is a change in permeability (the ability of the cell membrane to let ions through it) to Na⁺ and K⁺ in the cell surface membrane at the area of stimulation, which causes Na⁺ channels in that area to open.

2. Na⁺ therefore floods into the cytoplasm down the concentration gradient.

3. As this happens the membrane depolarises (this means that the resting potential of the cells starts to decrease). If this depolarisation reaches a certain level, called the threshold level (about -55 to -50 mV), then an action potential has been generated and an impulse will be fired. If it does not reach this level, nothing will happen.

4. Once +40mV is reached the Na⁺ channels close and K⁺ channels open. K⁺ floods out of the cytoplasm so that the overall charge inside goes back down. This stage is called repolarisation.

5. The K⁺ channels then close, the sodium-potassium pump restarts, restoring the normal distribution of ions either side of the cell surface membrane and thus restoring the resting potential.

An example of an action potential being reached would be pressure receptors cells in the skin which. If your hand was squashed, the pressure receptors cells in your skin would be would be pressed out of shape (this would be the external stimuli).

In response to this the Na⁺ channels in that area would open up, allowing ions to flood into the cell and thus reducing the resting potential of the cells. If the resting potential of the cell drops to the threshold level, then an action potential has been generated and an impulse will be fired.

The above has only described one area of the neurone and not how the impulse is carried along the neuron, this happens by another chain reaction.

Once an impulse is made, a local current is set up between the area where there is an action potential and the resting area next to it. The flow of some Na⁺ sideways towards the negative area next to it causes the Na⁺ channels in that area to open and depolarisation to occur there. That way, the action potential is moved down the neurone.

There is a length of time called the refractory period when the resting potential is being re-established. During this time no new action potential can be generated.

In this way the action potential can only travel in one direction down the neurone because the area behind the action potential is in a state of recovery.

Generally cells are covered in a fatty myelin sheath and therefore the Na⁺ and K⁺ cannot flow through this. This means that the ions can only flow through unprotected cell-surface membrane.

In the case of a myelinated neurone, the ions can only move in and out of the cytoplasm at the nodes of Ranvier.

Because of this, the action potential will 'jump' from one node to the next, a process called saltatory conduction, and so will travel much faster than in an unmyelinated neurone.

Other factors that affect the speed of conduction are diameter of the axon (the bigger, the faster) and temperature (up to 40°C, the higher the faster).

Action potentials themselves do not change size as they move down the neurone. All stimuli, as long as they cause the threshold level to be reached, cause an action potential of +40mV, no more or less. The speed of conduction is not altered by the intensity of the stimulus either.

If the stimulus is large, it will produce a greater frequency of impulses. Another one will very quickly follow the previous action potential (i.e. the intensity is frequency modulated).

Another consequence of an intense stimulus is that more than one neurone is likely to be affected. That way the brain, receiving more action potentials from more neurones, will interpret the stimulus as being strong.

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Resting potential In the surface membrane of a cell there are protein carriers. These actively pump Na+ (Sodium) ions out of the cytoplasm to the outside of the cell. At the same time, K+ (Potassium) ions are pumped from the outside in.

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The Pacinian Corpuscle | A-Level Biology Tutorial

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The Reflex Arc | A-level Biology | OCR, AQA, Edexcel

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Gene Mutations | A-Level Biology Revision Notes.

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Stem cells - Post 16 Biology (A Level, Pre-U, IB, AP Bio)

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National Human Genome Research Institute

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Definitions and quizzes

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Key Terms

__Population __- Group of organisms of the same species living in the same habitat.

__Habitat __- Part on an ecosystem in which particular organisms live.

__Community __- All the populations of different species in the same area at the same time.

__Ecosystem __- A community and the non-living components of an environment (the biotic and abiotic factors). Ecosystems can range in size from the very small to the very large.

__Niche __- An organism's role within an ecosystem. Each species occupies their own niche governed by adaptation to both abiotic and biotic conditions.

Carrying __Capacity __- The maximum population size an ecosystem can support.

Abiotic __Factors __- Non-living conditions of an ecosystem.

Biotic Factors - Impact of the interactions between organisms.

Factors Affecting Population Size

The abiotic factors with an ecosystem can affect the size of the different populations. These factors can range from temperature, oxygen and carbon dioxide concentration, light intensity, pH and soil conditions. Plants and animals are adapted to the abiotic factors within their ecosystem. These adaptations develop through the process on natural selection over time. The less harsh the abiotic factors, such as plenty of water and light, the larger the range of species and the larger the population sizes.

Biotic factors and the living components of an ecosystem interacting together, such as interspecific and intraspecific competition and predation also affect population size.

Interspecific competition is when members of different species are in competition for the same resource that is in limited supply. This could be competition for a habitat, food or water. The individual better adapted to the environment is more likely to succeed in the competition.

Intraspecific competition is when members of the same species are in competition for resources and a mate. Competition for a mate links to the courtship rituals. Individuals that are fitter will have more energy to perform a more impressive courtship ritual, or may have fur or feathers in a better condition to attract a mate.

The interaction between predators and prey in a food web is represented in the graph below. Regardless of the species, the graph always follows the same pattern. The size of the predator and prey population both fluctuate, there will always be more prey then predators and the size of the population will always change in the prey and then the predators.

Estimating Population Size

It would be too time consuming and inaccurate to count every individual within a population, and therefore samples are taken instead to provide an estimate. Population sizes can be estimated via a range of technique, but there are a few aspects in common. For the sampling to provide an accurate estimate that is representative of the population size, many samples must be taken, and sampling should be random to avoid bias.

Quadrats are using to sample slow-moving or non-motile organisms (such as plants). A quadrat is a frame typically 0.5m x 0.5m in size and can be gridded or open.

To randomly sample an area you could place two 20m tape measures at right angles to create a gridded area to sample. Using a random number generator, such as a calculator, two values can be generated to use as coordinates in the gridded area created. The quadrat should be placed on the coordinate and either the number of the species you are sampling could be counted or a percentage cover could be estimated. This should be repeated at least 30 times to provide a representative estimate.

Sometimes a belt transect is more appropriate than random sampling. A belt transect is when a tape measure is placed through an ecosystem that is not uniform, such as a beach rocky shore. To ensure you gather data for each area, the quadrat can be placed at intervals along the transect to measure the impact of the changes in the environment.

The three ways to estimate the population in a quadrat are frequency, density and percentage cover. Frequency is when you record how many squares out of the 100 in a quadrat the species is present in. Density is when every individual of that species in the quadrat is counted. Percentage cover is when you estimate the percentage of the whole quadrat that is covered by the species, using the 100 grids in the quadrat to assist. As motile organism can move out of a quadrat the mark-release-recapture method is used. In this method animals are first trapped and then marked. The mark must be non-toxic, all weather resistant and not impact reproduction or predation.

The animals are then released and left to redistribute in the population. The traps are used a few days later and a second sample is taken. The re-sampling continues until at least 10 samples are collected. The population size can be estimated using the formula:
Population size = Size of 1st sample X size of second sample / Number or marked recapture
This method assumes that the marked individuals released distribute themselves evenly among the population after being released, that there is no migration and no births or deaths.

Succession

This is the change in an ecological community over time. A primary succession starts with a pioneer species colonising bare rock or sand. Pioneer species, such as lichen, are adapted to survive harsh abiotic factors and through their death and decomposition they change the abiotic factors to become less harsh and form a thin layer of soil, humus. Mosses and smaller plants can now survive, and they further increase the depth and nutrient content of soil. This pattern continues, and as the abiotic factors continue to be less harsh larger plants can survive and change the environment further. Each new species may change the environment in such a way that it becomes less suitable for the previous species, and they become out-competed. Changes that organisms produce in their abiotic environment can result in a less hostile environment and increases biodiversity. The final stage in a succession is known as the climax community, and this is dominated by trees.

Conservation of Habitats

The destruction of habitats, usually due to human activity, results in a loss of food and space for organisms and can lead to extinction. To conserve habitats succession is often managed. By maintaining earlier stages in succession, and preventing a climax community, a greater variety of habitats are conserved and therefore a greater range of species survive. This can lead to conflict between human needs and conservation in order to maintain the sustainability of natural resources. To try and manage this conflict compromises are needed, for example forests can be coppiced to provide timber for fuel and furniture, whilst still allowing the tree to survive.

What is an abiotic factor?

Your answer should include: non / living / factor

What is a community?

Your answer should include: individuals / species

What is an ecosystem?

Your answer should include: biotic / abiotic / factors

Why should you sample randomly?

Your answer should include: avoid / bias

Why is it important to take a large number of samples?

Your answer should include: representative / population / statistics

What is primary succession?

Your answer should include: succession / area / first

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Populations in Ecosystems - A Level Biology AQA Revision - Study Rocket

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Population __- Group of organisms of the same species living in the same habitat. __Habitat __- Part on an ecosystem in which particular organisms live. __Community __- All the populations of different species in the same area at the same time. __Ecosystem __- A community and the non-living components of an environment (the biotic and abiotic factors).

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Definitions of inter and intraspecific plus short quiz

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Competition Definition in Biology

Competition is a relationship between organisms in which one is harmed when both are trying to use the same resource related to growth, reproduction, or survivability. Competition stems from the fact that resources are limited. There are simply not enough of some resources for all individuals to have equal access and supply. Competition can occur between organisms of the same species, or between members of different species.

Competition between species can either lead to the extinction of one of the species, or a decline in both of the species. However, this process can often be interrupted by environmental disturbances or evolution, which can change the rules of the game. Competition is often involved when species are limited in their range, often by direct competition from other organisms.

Examples of Competition

Intraspecific Competition

Intraspecific competition is a density-dependent form of competition. "Intra" refers to within a species, as opposed to "inter" which means between. Intraspecific competition can be summed up in the image below.

In this image, two wild dogs known as Dholes fight over a carcass. The carcass is a resource, something both organisms need to survive. Intraspecific competition is density dependent for one reason. The more dholes you have, the less food each one gets. To the individual dhole, food is everything. With very few predators of their own, the most successful dholes (the ones who survive and reproduce the most) often are simply the ones who eat the most.

Thus, while these dholes may have coordinated to take down this deer, they are now competing to see which one will get to eat first. The one that eats first will get more, and be more likely to survive and reproduce. The other one (or the last one if there are many) will not get as much. This will lower its survivability and the chances it will get to reproduce. Since evolution relies mainly on which organisms reproduce, this form of competition can quickly lead to changes in a population if only a few of the individuals are surviving and reproducing.

Interspecific Competition

Interspecific competition is between individuals which are different species. This could be between any two species, as long as they are competing over a resource. An interesting example of interspecific competition is found in coastal marine environments, like the coral reef in the picture below.

In this picture, there are dozens of species. There are several species of fish. Behind them, as a backdrop many people would ignore, is a canvas of dozens of species of coral. Coral, while it may look like some sort of rock or plant, is actually a colony of tiny animals. These tiny animals filter organic material from the water, and use stored bacteria to photosynthesize sunlight for additional energy. Thus, each coral species is competing with not only the other corals, but also with the fish for available nutrients and sunlight.

While corals might not seem like a competitive bunch, they are actually directly competitive with other corals. When an enemy coral is encroaching on their space, they can deploy chemical warfare to counter their rival. Often, coral fights end in one of the corals being killed by the other. While the corals are not predators of each other, the competition still ends in the death of one of the corals. The victorious coral was simply fighting for the resources it needs.

Direct and Indirect Competition

There is also another aspect of competition that can be applied to scenarios of limited resources, and that is the idea of direct vs indirect competition. Direct competition is like both of the scenarios above, and there are many more examples of it. Any time two or more animals fight or have a symbolized confrontation, this is probably some sort of competition for a resource.

However, indirect competition is when the two animals do not interact, but the presence of both animals in the same territory causes the competition. Think of the fish in the example above. If those fish feed on the same resources used by the corals, then the fish are in competition for the limited resources. Coral, being more or less anchored to the ocean floor, have little chance of directly attacking the fish. Instead, this would be referred to as an asymmetrical indirect competition. The fish eat as much of the food as they want, and the coral are limited to scraps. The coral have no way of competing. Luckily for most coral reef systems around the world, the ocean has plenty of food for most.

Outcomes of Competition

Competition is not a static process. Once set in motion, it can go a number of different ways. While the models may show that it will eventually drive one species to extinction, in reality a number of things can happen. First, an environmental disturbance, such as a fire or large wave, can upset the ecosystem and destroy the advantage the best competitor had. Typically, a pinewood forest is made mostly of pine trees because they are the best competitors in the environment. However, after a forest fire the most populous plants are small, opportunistic plants that grow quickly. The fire causes a change in the environment, which completely changes the dynamics of competition.

Further, most competition is also an evolutionary pressure on both parties. Animals from both sides that compete the best are able to survive and reproduce. Thus, over time the competition tends to resolve itself. More often than not, the competition can devolve as the species adapt to use different resources or change the way it uses a resource. This is known as character displacement. It is most well-documented in finches. When two different species of finch live on separate islands, their beaks are the same size because they prefer similar seeds. When they occupy the same island, one of their beaks gets smaller while the other gets larger. This separates the resources they consume and alleviates the competition.

Quiz

1. Which of the following represents competition?
A. Two swans (male and female) doing a mating dance
B. A lion defends its kill from a pack of hyenas
C. A lion stalks a buffalo, ready to pounce

2. What is the difference between intra- and interspecific competition?
A. Intraspecific is between members of the same species
B. Interspecific is between members of the same species
C. They are the same

3. A bald eagle is flying over a field, and sees a smaller hawk. The hawk has a fresh kill, but the bald eagle swoops in, threatens the hawk, and steals it. Which term best describes this scenario?
A. Predation
B. Intraspecific, indirect competition
C. Interspecific, direct competition

References

Cain, M. L., Bowman, W. D., & Hacker, S. D. (2008). Sunderland, MA: Sinauer Associates, Inc.

Feldhamer, G. A., Drickamer, L. C., Vessey, S. H., Merritt, J. F., & Krajewski, C. (2007). Mammology: Adaptation, Diversity, Ecology (3rd ed.). Baltimore: The Johns Hopkins University Press.

Kaiser, M. J., Attrill, M. J., Jennings, S., Thomas, D. N., Barnes, D. K., Brierley, A. S., & Hiddink, J. G. (2011). Marine Ecology: Processes, Systems, and Impacts. New York: Oxford University Press.

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Competition is a relationship between organisms in which one is harmed when both are trying to use the same resource related to growth, reproduction, or survivability. Competition stems from the fact that resources are limited. There are simply not enough of some resources for all individuals to have equal access and supply.

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Detailed methods over 4 pages of different techniques

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How to carry out ecological sampling page 1.

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(see picture below). The purpose of using a quadrat is to enable comparable samples to be obtained from areas of consistent size and shape. Rectangular quadrats and even circular quadrats have been used in some surveys. It does not really matter what shape of quadrat is used, provided it is a standard sampling unit and its shape and measurements are stated in any write-up.

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Explanation plus an exam question

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Ecosystems: Succession | A-level Biology | OCR, AQA, EDEXCEL

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43 flashcards looking at definitions, sampling and competition

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Flashcards - Topic 7.4 - Populations in Ecosystems - AQA Biology A-level - PMT

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Flashcards for AQA Biology A-level Topic 7.4: Populations in Ecosystems

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Evolution - Natural selection, niches and types of selection - Post 16 Biology (A Level, Pre-U, IB,)

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Wordwall matching activity

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The Light Independent Reaction of Photosynthesis

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Good use of graphs to show how production of TP, GP etc are influenced

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High level- just for those wanting to know what really happens

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Photosynthetic Electron Transport and ATP Synthesis

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Approx 90 mins but includes exam questions and worked answers.

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Revision cards for how organisms exchange substances

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Revision cards for how organisms exchange substances

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DNA Technology - Producing DNA Fragments

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Student share (Z)> Network resources> Science > Biology > Boardworks

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https://www.kerboodle.com/users/login

Username: student number
Password: student number
Institution code: mvf0 (zero)

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The Light Dependent Reaction of Photosynthesis

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phosphorus cycle biology a level - Search