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cell membrane, thin membrane that surrounds every living cell, delimiting the cell from the environment around it. Enclosed by this cell membrane (also known as the plasma membrane) are the cell's constituents, often large, water-soluble, highly charged molecules such as proteins, nucleic acids, carbohydrates, and substances involved in cellular metabolism. Outside the cell, in the surrounding water-based environment, are ions, acids, and alkalis that are toxic to the cell, as well as nutrients that the cell must absorb in order to live and grow. The cell membrane, therefore, has two functions: first, to be a barrier keeping the constituents of the cell in and unwanted substances out and, second, to be a gate allowing transport into the cell of essential nutrients and movement from the cell of waste products.

Cell membranes are composed primarily of fatty-acid-based lipids and proteins. Membrane lipids are principally of two types, phospholipids and sterols (generally cholesterol). Both types share the defining characteristic of lipids-they dissolve readily in organic solvents-but in addition they both have a region that is attracted to and soluble in water. This "amphiphilic" property (having a dual attraction; i.e., containing both a lipid-soluble and a water-soluble region) is basic to the role of lipids as building blocks of cellular membranes. Membrane proteins are also of two general types. One type, called the extrinsic proteins, is loosely attached by ionic bonds or calcium bridges to the electrically charged phosphoryl surface of the bilayer. They can also attach to the second type of protein, called the intrinsic proteins. The intrinsic proteins, as their name implies, are firmly embedded within the phospholipid bilayer. In general, membranes actively involved in metabolism contain a higher proportion of protein.

The chemical structure of the cell membrane makes it remarkably flexible, the ideal boundary for rapidly growing and dividing cells. Yet the membrane is also a formidable barrier, allowing some dissolved substances, or solutes, to pass while blocking others. Lipid-soluble molecules and some small molecules can permeate the membrane, but the lipid bilayer effectively repels the many large, water-soluble molecules and electrically charged ions that the cell must import or export in order to live. Transport of these vital substances is carried out by certain classes of intrinsic proteins that form a variety of transport systems: some are open channels, which allow ions to diffuse directly into the cell; others are "facilitators," which help solutes diffuse past the lipid screen; yet others are "pumps," which force solutes through the membrane when they are not concentrated enough to diffuse spontaneously. Particles too large to be diffused or pumped are often swallowed or disgorged whole by an opening and closing of the membrane.

In bringing about transmembrane movements of large molecules, the cell membrane itself undergoes concerted movements during which part of the fluid medium outside of the cell is internalized ( endocytosis) or part of the cell's internal medium is externalized ( exocytosis). These movements involve a fusion between membrane surfaces, followed by the re-formation of intact membranes.

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Cell membrane | Definition, Function, & Structure

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Cell membrane, thin membrane that surrounds every living cell. The cell membrane functions as a barrier, keeping cell constituents in and unwanted substances out, and as a gate, allowing transport into the cell of essential nutrients and movement from the cell of waste products.

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Cell Defense: The Plasma Membrane

Check out this series of animations explaining Membranes and Transport on YouTube too!

Updated:12/6/2021

Topics Covered: Plasma membrane structure and function, roles of phospholipids, transport proteins (carrier and channel), cholesterol and carbohydrates, types of transport, diffusion, facilitated diffusion, active transport, osmosis.

Check out the Worksheet that goes along with the game (courtesy of Ms. Ginnie Crotts)!

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Video computer games, virtual labs and activities for learning and reviewing biology content. Great for students and teachers.

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This article was most recently revised and updated by

cytoplasm, the semifluid substance of a cell that is external to the nuclear membrane and internal to the cellular membrane, sometimes described as the nonnuclear content of protoplasm. In eukaryotes (i.e., cells having a nucleus), the cytoplasm contains all of the organelles. Among such organelles are the mitochondria, which are the sites of energy production through ATP ( adenosine triphosphate) synthesis; the endoplasmic reticulum, the site of lipid and protein synthesis; the Golgi apparatus, the site where proteins are modified, packaged, and sorted in preparation for transport to their cellular destinations; lysosomes and peroxisomes, sacs of digestive enzymes that carry out the intracellular digestion of macromolecules such as lipids and proteins; the cytoskeleton, a network of protein fibres that give shape and support to the cell; and cytosol, the fluid mass that surrounds the various organelles.

The Editors of Encyclopaedia Britannica Kara Rogers.

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Cytoplasm, the semifluid substance of a cell that is external to the nuclear membrane and internal to the cellular membrane, sometimes described as the nonnuclear content of protoplasm. In eukaryotes (i.e., cells having a nucleus), the cytoplasm contains all of the organelles.

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A plant cell vacuole is surrounded by a single membrane called the tonoplast. Vacuoles are formed when vesicles, released by the endoplasmic reticulum and Golgi complex, merge together. Newly developing plant cells typically contain a number of smaller vacuoles. As the cell matures, a large central vacuole forms from the fusion of smaller vacuoles. The central vacuole can occupy up to 90% of the cell's volume.

Vacuole Function

Plant cell vacuoles perform a number of functions in a cell including:

Turgor pressure control - turgor pressure is the force exerted against the cell wall as the contents of the cell push the plasma membrane against the cell wall. The water filled central vacuole exerts pressure on the cell wall to help plant structures remain rigid and erect.​

Growth - the central vacuole aids in cell elongation by absorbing water and exerting turgor pressure on the cell wall. This growth is aided by the release of certain proteins that reduce cell wall rigidity.​

Storage - vacuoles store important minerals, water, nutrients, ions, waste products, small molecules, enzymes, and plant pigments.​

Molecule degradation - the internal acidic environment of a vacuole aids in the degradation of larger molecules sent to the vacuole for destruction. The tonoplast helps to create this acidic environment by transporting hydrogen ions from the cytoplasm into the vacuole. The low pH environment activates enzymes, which degrade biological polymers.​

Detoxification - vacuoles remove potentially toxic substances from the cytosol, such as excess heavy metals and herbicides.​

Protection - some vacuoles store and release chemicals that are poisonous or taste bad to deter predators from consuming the plant.​

Seed germination - vacuoles are a source of nutrients for seeds during germination. They store the necessary carbohydrates, proteins, and fats needed for growth.

Plant vacuoles function similarly in plants as lysosomes in animal cells. Lysosomes are membranous sacs of enzymes that digest cellular macromolecules. Vacuoles and lysosomes also participate in programmed cell death. Programmed cell death in plants occurs by a process called autolysis ( auto- lysis). Plant autolysis is a naturally occurring process in which a plant cell is destroyed by its own enzymes. In an ordered series of events, the vacuole tonoplast ruptures releasing its contents into the cell cytoplasm. Digestive enzymes from the vacuole then degrade the entire cell.

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A plant cell vacuole is surrounded by a single membrane called the tonoplast. Vacuoles are formed when vesicles, released by the endoplasmic reticulum and Golgi complex, merge together. Newly developing plant cells typically contain a number of smaller vacuoles. As the cell matures, a large central vacuole forms from the fusion of smaller vacuoles.

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Biology4Kids.com! This tutorial introduces mitochondria. Other sections include plants, animal systems, invertebrates, vertebrates, and microorganisms.

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The nuclear envelope is a double membrane composed of an outer and an inner phospholipid bilayer. The thin space between the two layers connects with the lumen of the rough endoplasmic reticulum (RER), and the outer layer is an extension of the outer face of the RER.

The inner surface of the nuclear envelope has a protein lining called the nuclear lamina, which binds to chromatin and other contents of the nucleus. The entire envelope is perforated by numerous nuclear pores. These transport routes are fully permeable to small molecules up to the size of the smallest proteins, but they form a selective barrier against movement of larger molecules. Each pore is surrounded by an elaborate protein structure called the nuclear pore complex, which selects molecules for entrance into the nucleus. Entering the nucleus through the pores are the nucleotide building blocks of DNA and RNA, as well as adenosine triphosphate, which provides the energy for synthesizing genetic material. Histones and other large proteins must also pass through the pores. These molecules have special amino acid sequences on their surface that signal admittance by the nuclear pore complexes. The complexes also regulate the export from the nucleus of RNA and subunits of ribosomes.

DNA in prokaryotes is also organized in loops and is bound to small proteins resembling histones, but these structures are not enclosed by a nuclear membrane.

Genetic organization of the nucleus

The structure of DNA

Several features are common to the genetic structure of most organisms. First is the double-stranded DNA. Each strand of this molecule is a series of nucleotides, and each nucleotide is composed of a sugar-phosphate compound attached to one of four nitrogen-containing bases. The sugar-phosphate compounds link together to form the backbone of the strand. Each of the bases strung along the backbone is chemically attracted to a corresponding base on the parallel strand of the DNA molecule. This base pairing joins the two strands of the molecule much as rungs join the two sides of a ladder, and the chemical bonding of the base pairs twists the doubled strands into a spiral, or helical, shape.

DNA Encyclopædia Britannica, Inc.

The four nucleotide bases are adenine, cytosine, guanine, and thymine. DNA is composed of millions of these bases strung in an apparently limitless variety of sequences. It is in the sequence of bases that the genetic information is contained, each sequence determining the sequence of amino acids to be connected into proteins. A nucleotide sequence sufficient to encode one protein is called a gene. Genes are interspersed along the DNA molecule with other sequences that do not encode proteins. Some of these so-called untranslated regions regulate the activity of the adjacent genes, for example, by marking the points at which enzymes begin and cease transcribing DNA into RNA ( see below Genetic expression through RNA).

Rearrangement and modification of DNA

Rearrangements and modifications of the nucleotide sequences in DNA are exceptions to the rules of genetic expression and sometimes cause significant changes in the structure and function of cells. Different cells of the body owe their specialized structures and functions to different genes. This does not mean that the set of genetic information varies among the cells of the body. Indeed, for each cell the entire DNA content of the chromosomes is usually duplicated exactly from generation to generation, and, in general, the genetic content and arrangement is strikingly similar among different cell types of the same organism. As a result, the differentiation of cells can occur without the loss or irreversible inactivation of unnecessary genes, an observation that is reinforced by the presence of specific genes in a range of adult tissues. For example, normal copies of the genes encoding hemoglobin are present in the same numbers in red blood cells, which make hemoglobin, as in a range of other types of cells, which do not.

antibody structure Encyclopædia Britannica, Inc.

Despite the general uniformity of genetic content in all the cells of an organism, studies have shown a few clear examples in some organisms of programmed, reversible change in the DNA of developing tissues. One of the most dramatic rearrangements of DNA occurs in the immune systems of mammals. The body's defense against invasion by foreign organisms involves the synthesis of a vast range of antibodies by lymphocytes (a type of white blood cell). Antibodies are proteins that bind to specific invading molecules or organisms and either inactivate them or signal their destruction. The binding sites on each antibody molecule are formed by one light and one heavy amino acid chain, which are encoded by different segments of the DNA in the lymphocyte nucleus. These DNA segments undergo considerable rearrangements, resulting in the synthesis of a great variety of antibodies. Some invasive organisms, such as trypanosome parasites, which cause sleeping sickness, go to great lengths to rearrange their own DNA to evade the versatility of their hosts' antibody production. The parasites are covered by a thick coat of glycoprotein (a protein with sugars attached). Given time, host organisms can overcome infection by producing antibodies to the parasites' glycoprotein coat, but this reaction is anticipated and evaded by the selective rearrangement of the trypanosomes' DNA encoding the glycoprotein, thus constantly changing the surface presented to the hosts' immune system.

Careful comparisons of gene structure have also revealed epigenetic modifications, heritable changes that occur on the sugar-phosphate side of bases in the DNA and thus do not cause rearrangements in the DNA sequence itself. An example of an epigenetic modification involves the addition of a methyl group to cytosine bases. This appears to cause the inactivation of genes that do not need to be expressed in a particular type of cell. An important feature of the methylation of cytosine lies in its ability to be copied, so that methyl groups in a dividing cell's DNA will result in methyl groups in the same positions in the DNA of both daughter cells.

Genetic expression through RNA

The transcription of the genetic code from DNA to RNA, and the translation of that code from RNA into protein, exerts the greatest influence on the modulation of genetic information. The process of genetic expression takes place over several stages, and at each stage is the potential for further differentiation of cell types.

As explained above, genetic information is encoded in the sequences of the four nucleotide bases making up a DNA molecule. One of the two DNA strands is transcribed exactly into messenger RNA (mRNA), with the exception that the thymine base of DNA is replaced by uracil. RNA also contains a slightly different sugar component ( ribose) from that of DNA ( deoxyribose) in its connecting sugar-phosphate chain. Unlike DNA, which is stable throughout the cell's life and of which individual strands are even passed on to many cell generations, RNA is unstable. It is continuously broken down and replaced, enabling the cell to change its patterns of protein synthesis.

Apart from mRNA, which encodes proteins, other classes of RNA are made by the nucleus. These include ribosomal RNA ( rRNA), which forms part of the ribosomes and is exported to the cytoplasm to help translate the information in mRNA into proteins. Ribosomal RNA is synthesized in a specialized region of the nucleus called the nucleolus, which appears as a dense area within the nucleus and contains the genes that encode rRNA. This is also the site of assembly of ribosome subunits from rRNA and ribosomal proteins. Ribosomal proteins are synthesized in the cytoplasm and transported to the nucleus for subassembly in the nucleolus. The subunits are then returned to the cytoplasm for final assembly. Another class of RNA synthesized in the nucleus is transfer RNA (tRNA), which serves as an adaptor, matching individual amino acids to the nucleotide triplets of mRNA during protein synthesis.

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Cell - Cell - The nuclear envelope: The nuclear envelope is a double membrane composed of an outer and an inner phospholipid bilayer. The thin space between the two layers connects with the lumen of the rough endoplasmic reticulum (RER), and the outer layer is an extension of the outer face of the RER.

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Cells need to make proteins. Enzymes made of proteins are used to help speed up biological processes. Other proteins support cell functions and are found embedded in membranes. Proteins even make up most of your hair. When a cell needs to make proteins, it looks for ribosomes. Ribosomes are the protein builders or the protein synthesizers of the cell. They are like construction guys who connect one amino acid at a time and build long chains.

Ribosomes are special because they are found in both prokaryotes and eukaryotes. While a structure such as a nucleus is only found in eukaryotes, every cell needs ribosomes to manufacture proteins. Since there are no membrane-bound organelles in prokaryotes, the ribosomes float free in the cytosol.

Ribosomes are found in many places around a eukaryotic cell. You might find them floating in the cytosol. Those floating ribosomes make proteins that will be used inside of the cell. Other ribosomes are found on the endoplasmic reticulum. Endoplasmic reticulum with attached ribosomes is called rough ER. It looks bumpy under a microscope. The attached ribosomes make proteins that will be used inside the cell and proteins made for export out of the cell. There are also ribosomes attached to the nuclear envelope. Those ribosomes synthesize proteins that are released into the perinuclear space.

There are two pieces or subunits to every ribosome. In eukaryotes, scientists have identified the 60-S (large) and 40-S (small) subunits. Even though ribosomes have slightly different structures in different species, their functional areas are all very similar.

For example, prokaryotes have ribosomes that are slightly smaller than eukaryotes. The 60-S/ 40-S model works fine for eukaryotic cells while prokaryotic cells have ribosomes made of 50-S and 30-S subunits. It's a small difference, but one of many you will find in the two different types of cells. Scientists have used this difference in ribosome structure to develop drugs that can kill prokaryotic microorganisms which cause disease. There are even structural differences between ribosomes found in the mitochondria and free ribosomes.

When are ribosomes used in the process of protein synthesis? When the cell needs to make a protein, mRNA is created in the nucleus. The mRNA is then sent out of the nucleus and to the ribosomes. When it is time to make the protein, the two subunits come together and combine with the mRNA. The subunits lock onto the mRNA and start the protein synthesis.

The process of making proteins is quite simple. First, you need an amino acid. Another nucleic acid that lives in the cell is transfer RNA. tRNA is bonded to the amino acids floating around the cell. With the mRNA offering instructions, the ribosome connects to a tRNA and pulls off one amino acid. The tRNA is then released back into the cell and attaches to another amino acid. The ribosome builds a long amino acid (polypeptide) chain that will eventually be part of a larger protein.

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Cells need to make proteins. Enzymes made of proteins are used to help speed up biological processes. Other proteins support cell functions and are found embedded in membranes. Proteins even make up most of your hair. When a cell needs to make proteins, it looks for ribosomes.

# data[].attributes.attachment_link.readable_text
The nuclear envelope is a double membrane composed of an outer and an inner phospholipid bilayer. The thin space between the two layers connects with the lumen of the rough endoplasmic reticulum (RER), and the outer layer is an extension of the outer face of the RER.

The inner surface of the nuclear envelope has a protein lining called the nuclear lamina, which binds to chromatin and other contents of the nucleus. The entire envelope is perforated by numerous nuclear pores. These transport routes are fully permeable to small molecules up to the size of the smallest proteins, but they form a selective barrier against movement of larger molecules. Each pore is surrounded by an elaborate protein structure called the nuclear pore complex, which selects molecules for entrance into the nucleus. Entering the nucleus through the pores are the nucleotide building blocks of DNA and RNA, as well as adenosine triphosphate, which provides the energy for synthesizing genetic material. Histones and other large proteins must also pass through the pores. These molecules have special amino acid sequences on their surface that signal admittance by the nuclear pore complexes. The complexes also regulate the export from the nucleus of RNA and subunits of ribosomes.

DNA in prokaryotes is also organized in loops and is bound to small proteins resembling histones, but these structures are not enclosed by a nuclear membrane.

Genetic organization of the nucleus

The structure of DNA

Several features are common to the genetic structure of most organisms. First is the double-stranded DNA. Each strand of this molecule is a series of nucleotides, and each nucleotide is composed of a sugar-phosphate compound attached to one of four nitrogen-containing bases. The sugar-phosphate compounds link together to form the backbone of the strand. Each of the bases strung along the backbone is chemically attracted to a corresponding base on the parallel strand of the DNA molecule. This base pairing joins the two strands of the molecule much as rungs join the two sides of a ladder, and the chemical bonding of the base pairs twists the doubled strands into a spiral, or helical, shape.

DNA Encyclopædia Britannica, Inc.

The four nucleotide bases are adenine, cytosine, guanine, and thymine. DNA is composed of millions of these bases strung in an apparently limitless variety of sequences. It is in the sequence of bases that the genetic information is contained, each sequence determining the sequence of amino acids to be connected into proteins. A nucleotide sequence sufficient to encode one protein is called a gene. Genes are interspersed along the DNA molecule with other sequences that do not encode proteins. Some of these so-called untranslated regions regulate the activity of the adjacent genes, for example, by marking the points at which enzymes begin and cease transcribing DNA into RNA ( see below Genetic expression through RNA).

Rearrangement and modification of DNA

Rearrangements and modifications of the nucleotide sequences in DNA are exceptions to the rules of genetic expression and sometimes cause significant changes in the structure and function of cells. Different cells of the body owe their specialized structures and functions to different genes. This does not mean that the set of genetic information varies among the cells of the body. Indeed, for each cell the entire DNA content of the chromosomes is usually duplicated exactly from generation to generation, and, in general, the genetic content and arrangement is strikingly similar among different cell types of the same organism. As a result, the differentiation of cells can occur without the loss or irreversible inactivation of unnecessary genes, an observation that is reinforced by the presence of specific genes in a range of adult tissues. For example, normal copies of the genes encoding hemoglobin are present in the same numbers in red blood cells, which make hemoglobin, as in a range of other types of cells, which do not.

antibody structure Encyclopædia Britannica, Inc.

Despite the general uniformity of genetic content in all the cells of an organism, studies have shown a few clear examples in some organisms of programmed, reversible change in the DNA of developing tissues. One of the most dramatic rearrangements of DNA occurs in the immune systems of mammals. The body's defense against invasion by foreign organisms involves the synthesis of a vast range of antibodies by lymphocytes (a type of white blood cell). Antibodies are proteins that bind to specific invading molecules or organisms and either inactivate them or signal their destruction. The binding sites on each antibody molecule are formed by one light and one heavy amino acid chain, which are encoded by different segments of the DNA in the lymphocyte nucleus. These DNA segments undergo considerable rearrangements, resulting in the synthesis of a great variety of antibodies. Some invasive organisms, such as trypanosome parasites, which cause sleeping sickness, go to great lengths to rearrange their own DNA to evade the versatility of their hosts' antibody production. The parasites are covered by a thick coat of glycoprotein (a protein with sugars attached). Given time, host organisms can overcome infection by producing antibodies to the parasites' glycoprotein coat, but this reaction is anticipated and evaded by the selective rearrangement of the trypanosomes' DNA encoding the glycoprotein, thus constantly changing the surface presented to the hosts' immune system.

Careful comparisons of gene structure have also revealed epigenetic modifications, heritable changes that occur on the sugar-phosphate side of bases in the DNA and thus do not cause rearrangements in the DNA sequence itself. An example of an epigenetic modification involves the addition of a methyl group to cytosine bases. This appears to cause the inactivation of genes that do not need to be expressed in a particular type of cell. An important feature of the methylation of cytosine lies in its ability to be copied, so that methyl groups in a dividing cell's DNA will result in methyl groups in the same positions in the DNA of both daughter cells.

Genetic expression through RNA

The transcription of the genetic code from DNA to RNA, and the translation of that code from RNA into protein, exerts the greatest influence on the modulation of genetic information. The process of genetic expression takes place over several stages, and at each stage is the potential for further differentiation of cell types.

As explained above, genetic information is encoded in the sequences of the four nucleotide bases making up a DNA molecule. One of the two DNA strands is transcribed exactly into messenger RNA (mRNA), with the exception that the thymine base of DNA is replaced by uracil. RNA also contains a slightly different sugar component ( ribose) from that of DNA ( deoxyribose) in its connecting sugar-phosphate chain. Unlike DNA, which is stable throughout the cell's life and of which individual strands are even passed on to many cell generations, RNA is unstable. It is continuously broken down and replaced, enabling the cell to change its patterns of protein synthesis.

Apart from mRNA, which encodes proteins, other classes of RNA are made by the nucleus. These include ribosomal RNA ( rRNA), which forms part of the ribosomes and is exported to the cytoplasm to help translate the information in mRNA into proteins. Ribosomal RNA is synthesized in a specialized region of the nucleus called the nucleolus, which appears as a dense area within the nucleus and contains the genes that encode rRNA. This is also the site of assembly of ribosome subunits from rRNA and ribosomal proteins. Ribosomal proteins are synthesized in the cytoplasm and transported to the nucleus for subassembly in the nucleolus. The subunits are then returned to the cytoplasm for final assembly. Another class of RNA synthesized in the nucleus is transfer RNA (tRNA), which serves as an adaptor, matching individual amino acids to the nucleotide triplets of mRNA during protein synthesis.

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Cell - Cell - The nuclear envelope: The nuclear envelope is a double membrane composed of an outer and an inner phospholipid bilayer. The thin space between the two layers connects with the lumen of the rough endoplasmic reticulum (RER), and the outer layer is an extension of the outer face of the RER.

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Open Educational Resources / Open Source Physics @ Singapore

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Factors Affecting Photosynthesis JavaScript HTML5 Applet Simulation Model

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You walk into a coffee shop and immediately smell coffee. Why?

One reason is convection. Moving currents of air carry coffee smell particles all around the coffee shop.

But even without convection, the smell would still reach you eventually - not by convection, but by diffusion. In diffusion, coffee particles move from the coffee machine (an area of high concentration) to the rest of the coffee shop (an area of low concentration).

Making a cup of coffee also involves the diffusion of coffee particles through hot water:

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You walk into a coffee shop and immediately smell coffee. Why? One reason is convection. Moving currents of air carry coffee smell particles all around the coffee shop. But even without convection, the smell would still reach you eventually - not by convection, but by diffusion.

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Osmosis

Osmosis is a biophysical phenomenon occurring commonly in biologic systems, in which cells of fluid compartments are separated by semipermeable membranes. Osmosis describes the diffusion of the solvent through a semipermeable membrane. The driving force of the solvent shift is the concentration difference of solutes in the solutions separated by the semipermeable membrane. In contrast to solvent, solutes cannot pass this barrier. Water, the usual solvent in biologic systems, migrates from the compartment with lower concentration to the compartment with higher concentration of solutes. The net fluid flux ends when the concentration of osmotic active molecules is equal in the two compartments. Therefore the distribution of water is a matter of osmosis and not transport of solutes.

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URL: https://www.sciencedirect.com/science/article/pii/B9780323449427001394

1.2 Osmosis

Osmosis is a special type of diffusion, namely the diffusion of water across a semipermeable membrane. Water readily crosses a membrane down its potential gradient from high to low potential ( Fig. 19.3) [4]. Osmotic pressure is the force required to prevent water movement across the semipermeable membrane. Net water movement continues until its potential reaches zero. An early application of the basic principles of osmosis came from the pioneering work on hemolysis of red blood cells by William Hewson in the 1770s (see Chapter 2). It has also been discussed that MLVs (multilamellar vesicles, liposomes) behave as almost perfect osmometers, swelling in hypotonic solutions and shrinking in hypertonic solutions (see Chapter 3) [5,6]. Liposome swelling and shrinking can be easily followed by changes in absorbance due to light scattering using a simple spectrophotometer. Therefore, osmosis has been investigated for many years using common and inexpensive methodologies and a lot is known about the process.

Membranes are rarely, if ever, perfectly semipermeable. Deviation from ideality is defined by a reflection coefficient ( σ). For an ideal semipermeable membrane where a solute is totally impermeable, σ = 1. If a solute is totally permeable (its permeability is equal to water), σ = 0. Biological membranes are excellent semipermeable barriers with σ = 0.75 to 1.0.

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URL: https://www.sciencedirect.com/science/article/pii/B9780444637727000191

IIIA. Definition of Osmosis

Osmosis refers to the movement of fluid across a membrane in response to differing concentrations of solutes on the two sides of the membrane. Osmosis has been used since antiquity to preserve foods by dehydration with salt or sugar. The removal of water from a tissue by salt was referred to as imbibition. This description comes from the notion that these solutes attracted water from material they touched. In 1748, J.A. Nollet used an animal bladder to separate chambers containing water and wine. He noted that the volume in the wine chamber increased and, if this chamber was closed, a pressure developed. He named the phenomenon osmosis from the Greek ωσμoς, meaning thrust or impulse.

Pfeffer (1877) provided early quantitative observations on osmosis. He made an artificial membrane in the walls of an unglazed porcelain vessel by reacting copper salts with potassium ferrocyanide to form a copper ferrocyanide precipitation membrane on the surface of the vessel. He used this membrane to separate a sucrose solution inside the vessel from water outside and found a volume flow from the water side to the sucrose side. Pfeffer observed that the flow was proportional to the sucrose concentration. Further, a pressure applied inside the vessel produced a filtration flow proportional to the pressure. He found that a closed vessel containing a sucrose solution would develop a pressure proportional to the concentration of sucrose. He recognized this as an equilibrium state in which the pressure balanced the osmosis caused by the sucrose solution. Pfeffer's original data for the osmotic pressure of sucrose solutions are plotted in Fig. 16.2. He defined osmotic pressure as the hydrostatic pressure necessary to stop osmotic flow across a barrier (e.g. a membrane) that is impermeable to the solute. This concept is illustrated in Fig. 16.3. Osmotic pressure is a property intrinsic to the solution and is measured at equilibrium, when the pressure-driven flow exactly balances the osmotic-driven flow. By defining osmotic pressure in this way, we assign a positive value to an apparent reduction in pressure brought about by dissolving the solute. Thus, fluid movement occurs from the solution of low osmotic pressure (water) to the solution of high osmotic pressure, opposite in direction to the hydraulic flow of water from high to low hydrostatic pressure.

An ideal semipermeable membrane is required for determining osmotic pressure. These membranes are permeable to water but absolutely impermeable to solute. The concept of osmotic pressure differs from tonicity in that tonicity compares two solutions separated by a specific non-ideal membrane. If the membrane is highly permeable to solute as well as to water, no water flow will occur and, therefore, the externally applied pressure required to stop osmosis is zero. This observation makes it plain that the effective osmotic pressure, which is measured with a real membrane, must be due to some interaction of the membrane with the solute because pressure depends on both the specific solute and the specific membrane.

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URL: https://www.sciencedirect.com/science/article/pii/B9780123877383000160

Summary

Osmosis refers to the movement of fluid across a membrane in response to different concentrations of solutes on the two sides of the membrane. The movement of fluid is toward the more concentrated solution. Osmotic pressure is defined as the pressure that must be applied to the solution side to stop fluid movement when a semipermeable membrane separates a solution from pure water. Here, the semipermeable membrane is permeable to water but not to solute. The osmotic pressure for dilute ideal solutions obeys van't Hoff's Law:

which can be derived on thermodynamic grounds. Because the solutions are not ideal, the equation is refined by including an osmotic coefficient, φ s, characteristic of each solute:

Defined in this way, the osmotic pressure is a pressure deficit caused by dissolving solutes. However, membranes that are somewhat permeable to the solute develop a transient osmotic pressure that is less than that predicted by van't Hoff's Law. Thus the actual pressure developed across a membrane that separates a solution from pure water depends on the interaction of the solute with the membrane. The correction for partially permeable membranes requires the reflection coefficient, σ:

The magnitude of the pressure tells us nothing of the flow. Osmotic pressure and hydrostatic pressure add to drive fluid flow across the membrane, with a proportionality constant, L p. The phenomenological equation describing fluid flow is

Osmosis and osmotic pressure is a thermodynamic concept which exists independently of mechanism. In microporous membranes, osmosis is caused by a momentum deficit within the pores due to the reflection of solute molecules by the membrane. This reduces the pressure on the solution side of the pore by π for a semipermeable membrane.

Thus there are three characteristic parameters for describing passive material transfer across membranes: the permeability, p, the hydraulic conductivity, L p, and the reflection coefficient, σ.

Osmolarity is a kind of concentration measure, distinct from molarity. It is related to other colligative properties of solutions including freezing point depression, vapor pressure depression, and boiling point elevation. Tonicity is a related concept but involves a real, biological membrane that may not be semipermeable. Tonicity makes reference to a particular cell and its membrane. Solutions may be isoosmotic but not isotonic.

Cells respond to swelling or shrinking according to the empirical relation:

where V ₀ is the volume of the cell under isotonic conditions. V b is interpreted as the osmotically inactive volume. If V b=0, the cell would behave like an ideal osmometer.

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15.4.1.3 Osmotic pump systems

Osmosis, the natural movement of water into a solution through a semipermeable membrane, has been employed in the development of zero-order release drug delivery systems ( Fig. 15.3). Only water, not solutes, can diffuse through the semipermeable membrane (Herbig et al., 1995). For different polymeric membranes, their water vapor transmission value is distinguishable, and the selection of a semipermeable membrane depends on the nature of application. The rate of release of drug from these products is determined by the constant flow of water across a semipermeable membrane into a reservoir that contains an osmotic agent called as osmogen. The rate of release is constant and can be controlled within fixed limits, yielding relatively constant drug concentration. The osmotic pump is similar to a reservoir device but it contains an osmogen. Osmogen is the active agent in salt form, which acts to imbibe water from the surrounding medium through a semipermeable membrane (Gupta et al., 2011). Pressure is developed within the reservoir which forces the active agent out of the reservoir device through an orifice. The rate of release can be adjusted by changing the osmogen and size of the orifice. The constant release is unaffected by the environment of the gastrointestinal tract and relying simply on the passage of water into the dosage form, acts as an advantage for this type of system (Lachman et al., 1991). Consequently, osmotically controlled systems can be classified in to single chamber osmotic pump (elementary osmotic pump) and multichamber osmotic pump (push-pull osmotic pump, osmotic pump with nonexpanding chamber) (Fig. 15.3). Specific types: are controlled porosity osmotic pump, monolithic bursting osmotic pump, osmotic bursting osmotic pump, osmotic release oral system-colon targeted (OROS-CT), multiparticulate delayed release systems, and liquid oral osmotic system (Vyas and Khar, 2000; Bechgaard and Nielson, 1978).

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Definitions

: This is the hydrostatic force acting to equalize the concentration of water on both sides of the membrane that is impermeable to substances dissolved in that water. Water will move along its concentration gradient. This force is termed osmotic pressure or, in the case of colloids, e.g., albumin, oncotic pressure. It is proportional to the number of atoms/ions/molecules in solution and is expressed as mOsm/liter (osmolarity) or mOsm/kg (osmolality) of solution. The cell membrane and the capillary membrane are both partially permeable membranes, although not strictly semipermeable in the chemical sense. They act, however, as partial barriers dividing the ECF from the ICF space, and the intravascular from the interstitial space. Osmotic or oncotic shifts occur across these membranes, modified by physiological as well as pathological mechanisms.

Osmolarity describes the molar number of osmotically active particles per liter of solution. In practice, the osmolarity of a solution can be "calculated" by adding up the milliequivalent concentrations of the various ions in the solution.

Osmolality describes the molar number of osmotically active particles per kilogram of solvent. This value is directly "measured" by determining either the freezing point or the vapor pressure of the solution. For most dilute salt solutions, osmolality is equal to or slightly less than osmolarity (Box 49.2).

The molecular weight of NaCl is 58.43

0.9% NaCl contains 9 g NaCl/1000 mL solution

Molarity of 0.9% solution = 9 g/L/58.43 g/mol = 0.154 mol/L or a 154 mmol/L solution of NaCl

Since each molecule of NaCl dissociates into Na⁺ and Cl − ions

Molar value is multiplied by 2 = 308 = osmolarity of 0.9% saline.

Colloid osmotic pressure: Osmolarity/osmolality is determined by the total number of dissolved "particles" in a solution, regardless of their size. Colloid osmotic pressure is the osmotic pressure generated by large molecules (e.g., albumin, hetastarch, dextran).

The colloid osmotic pressure becomes particularly important in biological systems where vascular membranes are often permeable to small ions, but not to large molecules (Box 49.3).

Box 49.3

Calculation of Osmotic Pressure

Osmotic pressure π = CRT

C = 0.001 mmol/L

R = 0.08206

T = 273K+36K = 309K(body temperature)

Hence π = CRT = (0.001)(0.08206)(309°)° = 0.02535 atm or 19.27 mmHg

Hemodilution: The beneficial effects of hemodilution are based on the correlation of hematocrit and whole blood viscosity. As the hematocrit and viscosity decrease, the cerebrovascular resistance correspondingly decreases and cerebral blood flow increases. However, one argument against hemodilution is that the oxygen carrying capacity of the blood also decreases. However, experimental evidence suggests that a hematocrit of 33% provides an optimal balance between viscosity and oxygen carrying capacity, and this has been applied clinically.

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1.1.3 Membrane proteins

Beyond motion via osmosis, DIB networks have no function without incorporated membrane proteins. So far, there are two general strategies for incorporating membrane proteins into DIBs, namely, reconstitution or in situ synthesis. In reconstitution, membrane proteins are first purified from some form of biological synthesis, such as expression in bacteria or cell-free expression. On the other hand, proteins may be produced directly in the droplet by replacing the aqueous solution with a cellular expression extract and a DNA plasmid containing the gene of interest ( Syeda, Holden, Hwang, & Bayley, 2008). Each method has unique advantages and disadvantages. For example, purified proteins are easier to work with and can be diluted to known concentrations with relatively controlled activity. Also, reconstitution essentially guarantees that only one type of protein will be incorporated into the DIB. However, very few membrane proteins are robust enough to be purified from an expression system intact. Indeed, this process has been restricted to mostly prokaryotic small channels and toxins so far. It can also be difficult to control the orientation of incorporated species using reconstitution techniques. While some protein toxins like α-hemolysin (αHL) incorporate unidirectionally, others, like OmpG can insert with the intracellular or extracellular domain facing the membrane leaflet where the proteins were added (Hwang et al., 2008). Expression of proteins within the droplet offers the unique advantage of bypassing the tedium associated with protein expression and purification in bacterial systems. The expression of any protein is as simple as including the desired gene/genes into the expression extract. However, the rate of synthesis and incorporation is relatively uncontrolled and there is a substantial lag time between droplet formation and protein expression. Also, synthesis extracts tend to have a detrimental effect on DIB stability. Our experience with extracts from Escherichia coli and HeLa cells demonstrates that DIBs tend to coalesce within 30 min (Syeda et al., 2008). The instability of DIBs in the presence of extracts is a significant hurdle for the continued development of DIB techniques, though some interesting solutions are on the horizon. Though in-droplet protein expression has been shown to work for several proteins, it has yet to be used in droplet networks primarily because of stability issues. Therefore, the examples discussed herein exclusively use protein reconstitution as the source of membrane protein. The majority of examples use engineered forms of the αHL toxin from Staphylococcus aureus.

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A Movement of Water between Intracellular Fluid and Extracellular Fluid Regulated by Osmotic Pressure

The principle of osmosis states that when a semipermeable membrane separates two fluid spaces, water will flow from an area of lower solute concentration to one of higher solute concentration to achieve equilibrium so that the osmotic pressures are balanced. In biologic systems, the cell membranes allow water to diffuse across them, but they are relatively impermeable to charged solutes (e.g., sodium, potassium) and large organic molecules (e.g., glucose). Cell membranes have special transport systems (proteins or channels) that regulate the movement of these ions and molecules across the cell membrane. Small, uncharged molecules (e.g., urea, ammonia) diffuse across cell membranes quickly. The concentration of solutes in the compartment (molecules or ions dissolved in the water) determines the osmolality of the fluid, and this is expressed as mOsm/kg of water. In the steady-state condition, body water is distributed between ICF and ECF compartments so that the osmolality is the same in each compartment. Therefore, the plasma osmolality is a fairly accurate estimate of the total body osmolality.

The plasma osmolality can be measured directly or estimated from the following equation:

Plasma osmolality  2  (Sodium)  (Glucose in mg/dl)/18  (Blood urea nitrogen in mg/dl)/2.8

The sodium concentration is doubled because each sodium ion is balanced, usually with a monovalent anion (e.g., chloride, bicarbonate). Although glucose and urea contribute to plasma osmolality, their contribution to the calculation is small compared with that of the plasma sodium concentration. Exceptions to this statement are the patient with hyperglycemia from uncontrolled diabetes mellitus and the patient with uremia (an extremely high blood urea nitrogen level).

Now we will consider the distribution of water between the intracellular and extracellular compartments. In the steady state, the osmolality of both compartments is the same, so the size of the compartment (or volume of fluid) is a function of how many dissolved solutes are in it (Box 2-3).

Box 2-3

Referring to the case of the 70-kg woman (described in Box 2-1), assume that her serum osmolality is 280 mOsm/kg water. If a normal distribution of total body water is assumed, the solute content of each compartment can be calculated as follows:

Her total body solute content would be 280 mOsm/kg water × 35 L (1 L of water weighs 1 kg) = 9800 mOsm.

Her intracellular fluid solute content is 280 mOsm/kg water × 23.5 L = 6580 mOsm.

Her extracellular fluid solute content is 280 mOsm/kg water × 11.5 L = 3220 mOsm.

The solutes in each compartment generate the osmotic pressure. This pressure is created only by solutes that do not readily cross the membrane that defines the compartment. Solutes that freely cross the membrane (e.g., urea) cannot generate a gradient or osmotic pressure. When the solute concentration on one side of the membrane is different from the solute concentration on the other side and when the solutes cannot diffuse across the membrane, then an osmotic pressure gradient is established. Water will move quickly from the side of the lower osmotic concentration toward the side with the higher osmotic concentration until the osmotic gradient is abolished. Thus, the volume of a compartment reflects the percentage of osmotic solutes in that compartment and the total amount of water available (Fig. 2-2). In Section IV, the ways in which changes in compartment size occur will be discussed.

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12.12.3.4 Hyper- and Hypoosmolality

The principle of osmosis is central to all of biology, yet the potential osmotic load associated with administration of high doses of small molecules is rarely considered in toxicology. In addition to the direct hyperosmotic effects of chemical exposure ( Glasser et al. 1973), hyperosmolality can also result indirectly from dehydration, increased salt intake, loss of body water through vomiting, or inhibition of water reabsorption in the renal collecting duct (e.g., diabetes insipidus). Conversely, compounds such as morphine, nicotine, certain tranquilizers, and anesthetics increase water reabsorption in the kidney and thus have the potential to induce hypoosmolality.

As with acid-base balance, there are many maternal systems that tightly regulate extracellular fluid osmolality, among them the thirst centers in the brain that affect water intake, and arginine vasopressin that controls reabsorption of water in the renal collecting ducts. At the cellular level, sodium and chloride ion concentrations are the major determinants of osmotic pressure. Proteins in plasma and interstitial fluid are also an important source of osmotic pressure, usually referred to as oncotic or colloid osmotic pressure. This is particularly important in the movement of solutes across capillary walls.

Studies on preimplantation embryos have demonstrated increased expression of genes such as p38 MAPK and CCM2 in response to hyperosmotic environments (Fong Carney Rogers Daston et al. 2007), although we know that embryos at these early stages can be very sensitive to changes in osmolality. Limited data are available concerning osmoregulation during organogenesis, although one study ( et al. 1996) showed that GD 10.5 rat conceptuses cultured with intact visceral yolk sac placenta grew normally in culture media that was extremely hypertonic (418 mOsmol kg −1 H ₂O, vs 299 for control media). The visceral yolk sac placenta may play a crucial role in the maintenance of embryonic osmolality. For example, trypan blue treatment of embryos in vivo and in vitro caused lysosomal damage to the visceral yolk sac endoderm, a decrease in exocoelomic fluid osmolality, and a corresponding increase in the incidence of embryonic edema ( et al. 1985). The opposite effect was observed following treatment with leupeptin, an inhibitor of yolk sac lysosomal proteinase activity, which caused an increase in exocoelomic fluid osmolality and malformations characterized by decreased neural tube volume ( et al. 1991b). Embryonic edema, blistering, and hemorrhage are common responses to many teratogenic agents, suggesting that the embryo may have a limited ability to cope with changes in osmolality.

During early fetal stages, injection of hypotonic or hypertonic solutions into the mother brings about equivalent changes in maternal and fetal plasma osmolality (Bruns Ramirez et al. 1964; Dancis et al. 2002). Once the fetal kidneys become functional, osmotic challenges can be dealt with by concentration or dilution of the fetal urine and excretion of this urine into the amniotic cavity. et al. 1957). The consequences of these changes are unknown, although there is some evidence from studies in pregnant sheep suggesting that prolonged fetal hyperosmolality (induced by maternal water deprivation) may imprint the hypothalamic-pituitary arginine vasopressin regulatory system (

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# data[].attributes.attachment_link.description
Osmosis is a biophysical phenomenon occurring commonly in biologic systems, in which cells of fluid compartments are separated by semipermeable membranes. Osmosis describes the diffusion of the solvent through a semipermeable membrane. The driving force of the solvent shift is the concentration difference of solutes in the solutions separated by the semipermeable membrane.

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Cell Membranes and Diffusion

In order to function, cells are required to move materials in and out of their cytoplasm via their cell membranes. These membranes are semipermeable, meaning that certain molecules are allowed to pass through, but not others. This movement of molecules is mediated by the phospholipid bilayer and its embedded proteins, some of which act as transport channels for molecules that otherwise would not be able to pass through the membrane, such as ions and carbohydrates.

Cell Size and the Surface-Area to Volume Ratio

One reason cells are so small is the need to transport molecules into, throughout, and out of the cell. There is a geometrical constraint on cells due to the relationship between surface area and volume that limits the ability to bring in enough nutrients to support a larger cell size. The ratio between surface area and volume (SA:V) decreases as the cell increases in size due to the different scaling factors of surface area and volume. This means that as the cell grows larger, there is less membrane area able to supply nutrients to a greater cell volume.

Some ions are brought into the cell by diffusion, which is the net movement of particles from an area of high concentration to an area of lower concentration. This is known as moving "down" a concentration gradient. Diffusion is net directional; while the net movement of particles is down the concentration gradient, they are constantly moving in both directions due to the random motion of particles. This means that particles in solutions at equilibrium are still moving, but at a constant exchange rate so the solution remains evenly mixed. In an aqueous environment such as the cell, this process involves dissolved ions, known as solutes, moving through water, the solvent. It can take place in an open environment, such as dye spreading through a beaker, or across a cell membrane, such as ions moving through a protein channel.

Osmosis and the Movement of Water

Water moves across cell membranes by diffusion, in a process known as osmosis. Osmosis refers specifically to the movement of water across a semipermeable membrane, with the solvent (water, for example) moving from an area of low solute (dissolved material) concentration to an area of high solute concentration. In this case, the semipermeable membrane does not allow the solute to pass through. This can be thought of as water moving down its own concentration gradient and involves the same random process as diffusion.

Solutions that are separated by semipermeable membranes can be described as hypertonic, hypotonic, or isotonic depending on the relative solute concentrations in each. A solution that is hypertonic ( hyper- meaning "above" in Greek) has a greater concentration of solutes than an adjacent solution, while a hypotonic ( hypo- meaning "below" in Greek) solution has a lower concentration of solutes. In this situation, water will move from the hypotonic solution to the hypertonic solution until the solute concentrations are equal. Solutions that are isotonic ( iso- meaning "equal" in Greek) have equal concentrations of solute, and therefore do not have a concentration gradient 1.

Osmosis and the Plant Cell

The capacity for water to move into cells is different between plant and animal cells due to the presence of a cell wall in plants. Cell walls are rigid and only permeable to very small molecules. As water moves into the cell, the membrane is pushed up against the cell wall, creating hydrostatic, or turgor, pressure. This pressure limits the rate and amount of water that can enter the cell. The likelihood of water moving into a cell is referred to as water potential, defined quantitatively as the pressure potential plus the solute potential. The pressure potential is dependent on the pressure inside the cell and the solute potential depends on the solute concentration in the cell.

Water potential can be observed in action in a living plant cell, such as Elodea, an aquatic plant. Under the microscope, a phenomenon called cytoplasmic streaming, or cyclosis, in which cytoplasm and organelles such as chloroplasts move throughout the cell, can be monitored. This process changes visibly when the cells are immersed in different solutions. Interestingly, this motion allows chloroplasts to function more efficiently in photosynthesis; they move in and out of the shadows, collecting photons when they re-enter the lighted regions of the cells ³.

The process of osmosis is essential for the mechanism whereby plants get water from their roots to their leaves, even dozens of feet above ground level. In brief, plants transport sugars and other solutes to their roots in order to generate a gradient between the inside and outside of the root; water from the soil then moves in to the root by osmosis. From that point, a process called transpiration results in the water being pulled up tubes inside the plant called the xylem and evaporating out the leaves. Ideally, once this water column is established, it remains intact throughout the life of the plant. ⁴

This naturally occurring phenomenon has been used to develop valuable technologies. One example is in water purification. Recently, NASA has begun to study using the process of forward osmosis to clean and reuse wastewater aboard the International Space Station, as well as for Earth-bound applications. 2 This process uses semi-permeable membranes to remove impurities from water, making it safe to drink. This technology was deployed recently to aid in relief efforts after a severe flood in Western Kenya ⁵.

References

1. Soult, Allison. LibreTexts, Chemistry. 8.4 Osmosis and Diffusion. [Online] October 19, 2017. https://chem.libretexts.org/LibreTexts/University_of_Kentucky/UK%3A_CHE_103_-_Chemistry_for_Allied_Health_(Soult)/Chapters/Chapter_8%3A_Properties_of_Solutions/8.4%3A_Osmosis_and_Diffusion.

2. Levine, Howard. NASA Forward Osmosis Bag. NASA Kennedy Space Center, Cape Canaveral, FL, United States. [Online] July 11, 2018. https://www.nasa.gov/mission_pages/station/research/experiments/846.html.

3. Dodonova SO, Bulychev AA (2011). 'Effect of Cytoplasmic Streaming on Photosynthetic Activity of Chloroplasts in Internodes of Chara Corallina'. Russian Journal of Plant Physiology. 59: 35-41. doi:10.1134/S1021443711050050.

4. Osmosis and Plant Nutrition. Hammer, Michael. 2000, The Rhododendron, Vol. 40.

5. Hydration Technology Innovations. Humanitarian Forward Osmosis Water Filtration. [Online] [Cited: August 21, 2018.] http://www.htiwater.com/divisions/humanitarian/lead_story.html.

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Cell Membranes and Diffusion In order to function, cells are required to move materials in and out of their cytoplasm via their cell membranes. These membranes are semipermeable, meaning that certain molecules are allowed to pass through, but not others.

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Active transport is the movement of dissolved molecules into or out of a cell through the cell membrane, from a region of lower concentration to a region of higher concentration. The particles move against the concentration gradient , using energy released during .

Sometimes dissolved molecules are at a higher concentration inside the cell than outside, but, because the organism needs these molecules, they still have to be absorbed. Carrier proteins pick up specific molecules and take them through the cell membrane against the concentration gradient.

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Active transport - Movement across cell membranes - GCSE Biology (Single Science) Revision - BBC Bitesize

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Active transport is the movement of dissolved molecules into or out of a cell through the cell membrane, from a region of lower concentration to a region of higher concentration. The particles move against the concentration gradient , using energy released during .

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Endoplasmic Reticulum (Rough and Smooth) | British Society for Cell Biology

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These dark, narrow, 100 meter-long streaks (called recurring slope lineae) are flowing downhill on Mars, and are inferred to have been formed by contemporary flowing water. Recently, planetary scientists detected hydrated salts on these slopes at Hale crater, corroborating their original hypothesis that the streaks are indeed formed by liquid water. The blue color seen upslope of the dark streaks are thought not to be related to their formation, but instead are from the presence of the mineral pyroxene.

Credit: NASA/JPL/University of Arizona

Water. It's found everywhere on Earth, from the polar ice caps to steamy geysers. And wherever water flows on this planet, you can be sure to find life.

"When we find water here on Earth - whether it be ice-covered lakes, whether it be deep-sea hydrothermal vents, whether it be arid deserts - if there's any water, we've found microbes that have found a way to make a living there," said Brian Glazer, an oceanographer at the University of Hawaii at Manoa, who has studied astrobiology.

That's why NASA's motto in the hunt for extraterrestrial life has been "follow the water."

Yesterday (Sept. 28), NASA scientists announced they'd found it on Mars: Dark streaks that scientists have spotted seasonally for more than a decade in images of the Red Planet are evidence of flowing water, new research suggests. While the briny flows may be too full of chlorine-based salts to support life, they do raise the odds that Mars could have life right now, the researchers said. [ In Photos: Is Water Flowing on Mars?]

But just why is water such a crucial molecule for life? And could there be other ingredients that also provide the perfect recipe for life on other planets?

It turns out that several chemical properties of water make it indispensable for living creatures. Not only can water dissolve nearly anything, but it is also one of only a few materials that can exist as solid, liquid and gas within a relatively narrow range of temperatures.

Flowing life

At heart, all life on Earth uses a membrane that separates the organism from its environment. To stay alive, the organism takes in important materials for making energy, while shuttling out toxic substances such as waste products.

In this regard, water is essential simply because it's a liquid at Earth-like temperatures. Because it flows, water provides an efficient way to transfer substances from a cell to the cell's environment. By contrast, deriving energy from a solid is a much tougher prospect (though there are microbes that eat rock), Glazer said.

But the other part of the equation - that water can carry things into and out of the cell - has to do with water's unique chemical configuration.

The humble water molecule is made up of two hydrogen atoms bonded to an oxygen atom.

"The way they're bonded together makes water this wonderful universal solvent," meaning that almost every substance can dissolve in water, Glazer told Live Science.

That's because the molecule has polarity, meaning the hydrogen atoms tend to bunch on one side of the molecule, creating a positive region, while the oxygen end has a negative charge. The positive hydrogen end tends to attract negative ions (or atoms with an extra electron in the outer shell), while the negative region lures in positive ions (which have had one of their electrons stripped off).

Water, with its amazing dissolving properties, is the perfect medium for transmitting substances, such as phosphates or calcium ions, into and out of a cell.

Phases of water

Another feature of water is that it can act as a solid, liquid and gas within the range of temperatures that occur on Earth. Other molecules that have been identified as good candidates for supporting life tend to be liquid at temperatures or pressures that would be inhospitable for most known life-forms, Glazer said. [ 5 Mars Myths and Misconceptions]

"Water really is at that sweet spot," Glazer said.

The fact that water can be in all three phases in a relatively tight range of pressures creates many opportunities for life to flourish, he added.

"All three [states of water] available on our planet creates this really neat variety of habitats and microclimates," Glazer said.

For instance, frozen ice can be found in glaciers that carve through mountains, whereas water vapor helps warm the atmosphere, Glazer said.

Watery cradle of life

Water may be more than a fluid to help facilitate life's essential processes - it may also have been the protective cradle that carried the building blocks of life to Earth, said Ralf Kaiser, a physical experimental chemist at the University of Hawaii at Manoa, who has research experience in astrochemistry.

One theory for how life on Earth emerged, called panspermia, posits that icy comets smashed into Earth, bearing tiny organic molecules that formed the precursors to life. But traveling through space is a harsh journey, with punishing levels of radiation that would normally degrade those delicate molecules, Kaiser said.

However, in its solid form, water could have provided a way to shield those molecules from radiation, Kasier speculated.

"One possibility is that because the building blocks are frozen within the water, it has this protective mantle around it that could be delivered," Kaiser told Live Science.

Accept some substitutes

Of course, while water is crucial to life on our home planet, there could be life-forms that don't conform to the Earthling playbook.

Scientists are also looking at other liquids that could play a similar role as universal solvent and transport medium. Some of the top contenders are ammonia and methane, said Chris McKay, an astrobiologist at the NASA Ames Research Center in Moffett Field, California. Ammonia, like water, is a polar molecule that is relatively abundant in the universe, but scientists haven't found any large bodies of ammonia anywhere in the solar system, McKay said.

Methane isn't polar, but it can dissolve many other substances. Unlike water, however, methane becomes liquid only at very cold temperatures - at a frigid minus 296 degrees Fahrenheit (minus 182 degrees Celsius).

"We know that there are large lakes of liquid methane and ethane on Titan," one of the moons of Saturn, McKay told Live Science in an email. "Thus there is keen interest is the question of whether life can use liquid methane/ethane."

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Water. It's found everywhere on Earth, from the polar ice caps to steamy geysers. And wherever water flows on this planet, you can be sure to find life.

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Learning Outcomes

Distinguish between monosaccharides, disaccharides, and polysaccharides

Identify several major functions of carbohydrates

Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we eat. To lose weight, some individuals adhere to "low-carb" diets. Athletes, in contrast, often "carb-load" before important competitions to ensure that they have enough energy to compete at a high level. Carbohydrates are, in fact, an essential part of our diet; grains, fruits, and vegetables are all natural sources of carbohydrates. Carbohydrates provide energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient in many staple foods. Carbohydrates also have other important functions in humans, animals, and plants.

Carbohydrates can be represented by the stoichiometric formula (CH ₂O) n, where n is the number of carbons in the molecule. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. This formula also explains the origin of the term "carbohydrate": the components are carbon ("carbo") and the components of water (hence, "hydrate"). Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.

Monosaccharides

Monosaccharides ( mono- = "one"; sacchar- = "sweet") are simple sugars, the most common of which is glucose. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide names end with the suffix - ose. If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is known as an aldose, and if it has a ketone group (the functional group with the structure RC(=O)R′), it is known as a ketose. Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons). See Figure 1 for an illustration of the monosaccharides.

The chemical formula for glucose is C ₆H ₁₂O ₆. In humans, glucose is an important source of energy. During cellular respiration, energy is released from glucose, and that energy is used to help make adenosine triphosphate (ATP). Plants synthesize glucose using carbon dioxide and water, and glucose in turn is used for energy requirements for the plant. Excess glucose is often stored as starch that is catabolized (the breakdown of larger molecules by cells) by humans and other animals that feed on plants.

Galactose and fructose are other common monosaccharides - galactose is found in milk sugars and fructose is found in fruit sugars. Although glucose, galactose, and fructose all have the same chemical formula (C ₆H ₁₂O ₆), they differ structurally and chemically (and are known as isomers) because of the different arrangement of functional groups around the asymmetric carbon; all of these monosaccharides have more than one asymmetric carbon (Figure 2).

Practice Question

What kind of sugars are these, aldose or ketose?

Monosaccharides can exist as a linear chain or as ring-shaped molecules; in aqueous solutions they are usually found in ring forms (Figure 3). Glucose in a ring form can have two different arrangements of the hydroxyl group (−OH) around the anomeric carbon (carbon 1 that becomes asymmetric in the process of ring formation). If the hydroxyl group is below carbon number 1 in the sugar, it is said to be in the alpha ( α) position, and if it is above the plane, it is said to be in the beta ( β) position.

Disaccharides

Disaccharides ( di- = "two") form when two monosaccharides undergo a dehydration reaction (also known as a condensation reaction or dehydration synthesis). During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond. A covalent bond formed between a carbohydrate molecule and another molecule (in this case, between two monosaccharides) is known as a glycosidic bond (Figure 4). Glycosidic bonds (also called glycosidic linkages) can be of the alpha or the beta type. An alpha bond is formed when the OH group on the carbon-1 of the first glucose is below the ring plane, and a beta bond is formed when the OH group on the carbon-1 is above the ring plane.

Common disaccharides include lactose, maltose, and sucrose (Figure 5). Lactose is a disaccharide consisting of the monomers glucose and galactose. It is found naturally in milk. Maltose, or malt sugar, is a disaccharide formed by a dehydration reaction between two glucose molecules. The most common disaccharide is sucrose, or table sugar, which is composed of the monomers glucose and fructose.

Polysaccharides

A long chain of monosaccharides linked by glycosidic bonds is known as a polysaccharide ( poly - = "many"). The chain may be branched or unbranched, and it may contain different types of monosaccharides. The molecular weight may be 100,000 daltons or more depending on the number of monomers joined. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides.

Starch is the stored form of sugars in plants and is made up of a mixture of amylose and amylopectin (both polymers of glucose). Plants are able to synthesize glucose, and the excess glucose, beyond the plant's immediate energy needs, is stored as starch in different plant parts, including roots and seeds. The starch in the seeds provides food for the embryo as it germinates and can also act as a source of food for humans and animals. The starch that is consumed by humans is broken down by enzymes, such as salivary amylases, into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose.

Starch is made up of glucose monomers that are joined by α 1-4 or α 1-6 glycosidic bonds. The numbers 1-4 and 1-6 refer to the carbon number of the two residues that have joined to form the bond. As illustrated in Figure 6, amylose is starch formed by unbranched chains of glucose monomers (only α 1-4 linkages), whereas amylopectin is a branched polysaccharide ( α 1-6 linkages at the branch points).

Glycogen is the storage form of glucose in humans and other vertebrates and is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis.

Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose; this provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Cellulose is made up of glucose monomers that are linked by β 1-4 glycosidic bonds (Figure 7).

As shown in Figure 7, every other glucose monomer in cellulose is flipped over, and the monomers are packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength-which is so important to plant cells. While the β 1-4 linkage cannot be broken down by human digestive enzymes, herbivores such as cows, koalas, buffalos, and horses are able, with the help of the specialized flora in their stomach, to digest plant material that is rich in cellulose and use it as a food source. In these animals, certain species of bacteria and protists reside in the rumen (part of the digestive system of herbivores) and secrete the enzyme cellulase. The appendix of grazing animals also contains bacteria that digest cellulose, giving it an important role in the digestive systems of ruminants. Cellulases can break down cellulose into glucose monomers that can be used as an energy source by the animal. Termites are also able to break down cellulose because of the presence of other organisms in their bodies that secrete cellulases.

Carbohydrates serve various functions in different animals. Arthropods (insects, crustaceans, and others) have an outer skeleton, called the exoskeleton, which protects their internal body parts (as seen in the bee in Figure 8).

This exoskeleton is made of the biological macromolecule chitin, which is a polysaccharide-containing nitrogen. It is made of repeating units of N-acetyl- β-d-glucosamine, a modified sugar. Chitin is also a major component of fungal cell walls; fungi are neither animals nor plants and form a kingdom of their own in the domain Eukarya.

In Summary: Structure and Function of Carbohydrates

Carbohydrates are a group of macromolecules that are a vital energy source for the cell and provide structural support to plant cells, fungi, and all of the arthropods that include lobsters, crabs, shrimp, insects, and spiders. Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides depending on the number of monomers in the molecule. Monosaccharides are linked by glycosidic bonds that are formed as a result of dehydration reactions, forming disaccharides and polysaccharides with the elimination of a water molecule for each bond formed. Glucose, galactose, and fructose are common monosaccharides, whereas common disaccharides include lactose, maltose, and sucrose. Starch and glycogen, examples of polysaccharides, are the storage forms of glucose in plants and animals, respectively. The long polysaccharide chains may be branched or unbranched. Cellulose is an example of an unbranched polysaccharide, whereas amylopectin, a constituent of starch, is a highly branched molecule. Storage of glucose, in the form of polymers like starch or glycogen, makes it slightly less accessible for metabolism; however, this prevents it from leaking out of the cell or creating a high osmotic pressure that could cause excessive water uptake by the cell.

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Distinguish between monosaccharides, disaccharides, and polysaccharides Identify several major functions of carbohydrates Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we eat. To lose weight, some individuals adhere to "low-carb" diets. Athletes, in contrast, often "carb-load" before important competitions to ensure that they have enough energy to compete at a high level.

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2.2.2 Secondary Structure

The secondary structure of a protein describes the spatial arrangement of the atoms in the protein backbone. The repeating pattern of α-carbon peptide bond can exist in a disorganized array (called a random coil) or in a distinctly well- defined manner, with the angles of the two planar peptide bonds attached to each α-carbon repeating in a regular fashion. The most common secondary structures are α-helices, β-pleated sheets, and β-turns. The α-helix is a right-handed helix with amino acid residues spaced at 3.6 residues per turn and a rise of 0.54 nm per turn. (A "right-handed helix" is wound such that if one were to curl the fingers of the right hand around the backbone of the helix with the fingers pointing in the N → C direction, the thumb would point along the helical axis in the direction of N → C helix progression.) Figure 2.7 illustrates approximate amino acid placement and rise for an α-helix. Notice that amino acid 0 is not directly below the third or fourth subsequent residue. If the coil were a number line, amino acid 0 would be beneath 3.6, which is where "3.6 residues per turn" comes from. Because of this, amino acid 0 can form hydrogen bonds with the third or fourth subsequent residue, as is shown in the second panel of the figure. The side groups of each amino acid would point out from the helix (not shown).

The β-pleated sheet is composed of multiple, relatively straight segments within one polypeptide, with the strands lining up adjacently (Figure 2.8). It is the zigzag nature of the N C α C backbone that gives the sheet its pleats. The sheet is stabilized by hydrogen bonding between peptide oxygens and amide hydrogens on a neighboring segment. The straight segments can be parallel, meaning that the N → C directions of all β-sheet segments point the same way or antiparallel, meaning that the segments alternate in direction. Antiparallel β-sheets have hydrogen bonds perpendicular to strand directions, while parallel sheets have hydrogen bonds occurring at alternating angles. Both forms of the β-pleated sheet have amino acid side groups occurring parallel and on the same side of the sheet. In the figure, take note of the N → C directions, hydrogen bonding, and alignment of the side groups for both parallel and antiparallel configurations.

A β-turn (also known as a β-bend) is a structure that allows for sharp turns in the middle of a polypeptide. One way in which β-turns appear in proteins is during the formation of β-sheets. A β-turn is made up of four amino acid residues. Residues 1 and 4 often interact as illustrated in Figure 2.9. The amino acids glycine and proline are often present in β-turns. Glycine works well in a beta turn because of its small side group ( H) that is sterically favorable for the compact structure, rendering the residue relatively flexible. (Keeping this in mind, one might also correctly predict that alanine would be a relatively common amino acid in β-turns.) Proline works well because of some interesting properties, which merit deeper discussion.

Proline is a unique amino acid in that its side group is attached to its backbone atoms in two places. The fact that the side group attaches to both the chiral carbon and the amino nitrogen of this amino acid renders proline a rigid amino acid. As such, it is not often found in α-helices. Prolines are found in β-turns, though. There are two different conformations for proline (Figure 2.10). In the trans conformation, the polypeptide backbone will be somewhat straight. In the cis conformation, the backbone makes a sharp turn. Because the β-turn itself is a sharp turn, the cis conformation is overwhelmingly favored for prolines involved in β-turns.

In summary, for β-turns, amino acids 1 and 4 interact with each other, and glycine and proline are commonly present-glycine because it is small and proline because it is rigid and can naturally form this sharp turn.

2.2.2.1 Supersecondary Structure

Supersecondary structure is simply a combination of secondary structures. For instance, consider a piece of paper. If the piece of paper were rolled into a tube, it would still be a piece of paper, but it would have been modified into a more complex structure. The same can be done to a β-sheet, and in this case, the resulting structure would be a β-barrel. Another form of a supersecondary structure is the helix-loop-helix, where two α-helices are connected by a β-turn. Any combination of secondary structures can be used to produce a supersecondary structure.

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The secondary structure of a protein describes the spatial arrangement of the atoms in the protein backbone. The repeating pattern of α-carbon peptide bond can exist in a disorganized array (called a random coil) or in a distinctly well- defined manner, with the angles of the two planar peptide bonds attached to each α-carbon repeating in a regular fashion.

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Fats and Proteins | Biology | Visionlearning

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Fats and proteins are two of the major nutrient groups that our bodies need. This module provides an introduction to these two macronutrients. The basic chemical structure of fats as triglycerides is presented along with the purposes and types of fat.

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Food Tests - Reducing Sugars, Starch, Proteins and Fats

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Food Tests - Reducing Sugars, Starch, Proteins and Fats

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Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

This page will help you answer questions such as ...

How do you carry out the test for a reducing sugar in food?

How do you carry out the test for starch in food?

How do you carry out the test for protein in food?

How do you carry out the test for lipids in food?

So, this page is all about testing for biological molecules that might be present in various foods.

The food tests described below are qualitative only, they area NOT quantitative analysis, you simple get a 'YES or NO' result for the presence of particular type of food molecule in your prepared sample.

Introduction - preparing the starting material (need picture of pestle and mortar)

Take the food sample and mash it up using a pestle and mortar.

Mix it with some pure water (distilled/deionised) and mix it all up.

Try to have a good 25cm ³ of food mixture solution from various types of food, i.e. enough to carry out a variety of tests on each food sample.

AND, remember that negative results are just as important as positive results!

AND, make sure you were safety goggles throughout the experiments.

With a filter funnel and paper, filter the mixture to remove the 'waste' bits of solid food.

The filtrate can now be tested for specific types of food with various chemical reagents.

In chemistry, the word 'reagent' just means a chemical you use in carrying out a test.

e.g. like silver nitrate or sodium hydroxide in chemical tests for ions in your GCSE chemistry

Benedict's test for reducing sugars

(a) A positive direct test for a non-reducing sugar

Sugars are an important ingredient in biscuits, bread, breakfast cereals, cakes and sweets.

There are two types of sugar - reducing and non-reducing sugars.

Glucose and fructose are reducing sugars. Sucrose is a non-reducing sugar.

You can use Benedict's reagent to test for reducing sugars.

Put a few cm ³ of your previously prepared filtered food sample mixture into a test tube.

Set up a water bath and set the thermostat to ~75 o C and wait for it to warm up.

With a teat pipette, add at least half a dozen drops of Benedict's solution (blue) to the food sample solution and carefully shake the mixture gently.

(Benedict's solution is based on a complex copper salt mixture, hence the blue colour)

Place the test tube in the water bath using a test tube holder and leave it for 5 minutes - make sure the test tube points away from you in case of any accident.

If the food sample contains a reducing sugar, the solution in the test tube will change from the initial blue colour through a series of colours from a green solution ==> yellow solution ==> orange ==> brick-red precipitate.

The more reducing sugar in the food, the more likely you are to see the full colour sequence changes to give the final brick-red precipitate - the best confirming positive result that a reducing sugar was in the food.

If no reducing sugars are present, you should not see any significant colour change.

However, this does not mean that no sugars are present - there might be non-reducing sugars in the food ... read on in section (b) below

(b) Using Benedict's solution to test for a non-reducing sugar

BUT, follow the logic carefully, its an indirect test!

Put a few cm ³ of your previously prepared filtered food sample mixture into a test tube and add a few cm ³ of dilute hydrochloric acid and gently and carefully shake the mixture.

Set up a water bath and set the thermostat to ~75 o C and wait for it to warm up.

Place the test tube in the water bath using a test tube holder and leave it for at least 10 minutes - make sure the test tube points away from you in case of any accident.

At this point you need to appreciate a bit of chemistry!

The hydrochloric acid catalyses and hydrolyses non-reducing sugars like sucrose into reducing sugars like glucose or fructose.

e.g. sucrose ====> glucose + fructose

C ₁₂H ₂₂O ₁₁ + H2O ===> C ₆H ₁₂O ₆ + C ₆H ₁₂O ₆

Therefore, you can now test the solution for reducing sugars derived from non-reducing sugars!

Remove the test tube from the water bath and place it in a test tube rack.

You then add sodium carbonate to neutralise the acid.

Then test the neutral solution with Benedict's reagent.

If reducing sugars have been formed, the solution in the test tube will change from the initial blue colour through a series of colours from a green solution ==> yellow solution ==> orange ==> brick-red precipitate.

If the test with Benedict's solution is negative for both tests (a) and (b), the food probably didn't contain any sugar at all.

Testing for the carbohydrate starch with iodine solution

The carbohydrate starch is present in many foods such as beans, bread, cereal grains, corn, pasta, peas, potatoes and rice.

Put a few cm ³ of your previously prepared filtered food sample mixture into a test tube.

Add a few drops of a dilute iodine solution (pale orange-brown) and carefully shake the mixture gently.

If the food sample contains starch the mixture will turn from pale orange to a dark blue-black colour.

(If the solutions are very dilute, you might see a nice blue-purple colour.)

If no starch is present, you should not see any significant colour change - the solution remains a pale orange-brown.

(In chemistry you might test for iodine using starch solution - its the same test and result!)

The Biuret test for proteins

Foods such as meat, fish and cheese are rich in protein.

Put a few cm ³ of your previously prepared filtered food sample mixture into a test tube.

Add 2 cm ³ of Biuret solution (made of blue copper salt compounds) to the food sample and carefully gently shake the mixture.

If protein is present in the food sample the solution should turn from blue to pink or purple.

Biuret reagent solution is made from copper sulfate, sodium/potassium hydroxide and sodium tartrate.

You can carry out the test by just adding a few cm ³ of sodium/potassium hydroxide to the food sample to make it alkaline and then add a cm ³ of dilute bright blue copper sulfate solution, but using the 'proper' Biuret reagent is better in my opinion.

If there is no protein in the food sample the mixture just stays the characteristic blue of a copper salt solution.

Two tests for lipids (fats)

Foods such as margarine, milk, vegetable oils like olive oil or sunflower oil contain lipids (chemically they are often fatty acid esters).

(a) The emulsion test for lipids

For this test you do not use the mixture prepared for the other tests.

You mash up a sample of food, as before, with a pestle and mortar and mix with a few cm ³ of ethanol and mix up in a test tube (there is no need to filter it).

The ethanol dissolves any lipid (fatty/oily) molecules present - lipids are NOT soluble in water.

You then pour contents of the 1st test tube into a 2nd test tube containing a few cm ³ of pure water and carefully shake the mixture.

Since lipids are not soluble in water, any lipids dissolved in the ethanol are precipitated out as fine globules of oil, so you see a milky emulsion.

The more lipids in the food sample, the more milky the mixture becomes, but this is not a quantitative test!

If no fatty/oily lipid molecules are present, you don't see a milky emulsion.

(I wonder if you could add the Sudan III stain at this point to get the red colour described below?)

(b) The Sudan III test for lipids

For this test you do not have to use the mixture prepared for the other tests.

You mash up a sample of food, as before, with a pestle and mortar and mix with a few cm ³ of water and there is no need to filter it.

With a teat pipette add three drops of Sudan III stain solution and carefully and gently shake the mixture. Sudan III stain is red-brown in colour

If the food sample contains lipids, the mixture will separate into two layers.

The 'thinner' top layer contains the lipids (insoluble oil) which becomes stained a bright red by the Sudan III stain if lipids are present.

If there are no lipids in the food sample, you will not get an upper separate red layer forming.

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Food tests Benedicts reducing sugars iodine for starch Sudan III test for lipids methods results observations instructions on how to carry out food tests igcse/O level/gcse 9-1 biology revision notes examinations

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Doc Brown's Biology Revision Notes Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent This page will help you answer questions such as ... How do you carry out the test for a reducing sugar in food? How do you carry out the test for starch in food?

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Enzymes help speed up chemical reactions in the human body. They bind to molecules and alter them in specific ways. They are essential for respiration, digesting food, muscle and nerve function, among thousands of other roles.

In this article, we will explain what an enzyme is, how it works, and give some common examples of enzymes in the human body.

Enzymes are built of proteins folded into complicated shapes; they are present throughout the body.

The chemical reactions that keep us alive - our metabolism - rely on the work that enzymes carry out.

Enzymes speed up (catalyze) chemical reactions; in some cases, enzymes can make a chemical reaction millions of times faster than it would have been without it.

A substrate binds to the of an enzyme and is converted into products. Once the products leave the active site, the enzyme is ready to attach to a new substrate and repeat the process.

The digestive system - enzymes help the body break down larger complex molecules into smaller molecules, such as glucose, so that the body can use them as fuel.

DNA replication - each cell in your body contains DNA. Each time a cell divides, that DNA needs to be copied. Enzymes help in this process by unwinding the DNA coils and copying the information.

Liver enzymes - the liver breaks down toxins in the body. To do this, it uses a range of enzymes.

The "lock and key" model was first proposed in 1894. In this model, an enzyme's active site is a specific shape, and only the substrate will fit into it, like a lock and key.

This model has now been updated and is called the induced-fit model.

In this model, the active site changes shape as it interacts with the substrate. Once the substrate is fully locked in and in the exact position, the catalysis can begin.

Enzymes can only work in certain conditions. Most enzymes in the human body work best at around 37°C - body temperature. At lower temperatures, they will still work but much more slowly.

Similarly, enzymes can only function in a certain pH range (acidic/alkaline). Their preference depends on where they are found in the body. For instance, enzymes in the intestines work best at 7.5 pH, whereas enzymes in the stomach work best at pH 2 because the stomach is much more acidic.

If the temperature is too high or if the environment is too acidic or alkaline, the enzyme changes shape; this alters the shape of the active site so that substrates cannot bind to it - the enzyme has become denatured.

Some enzymes cannot function unless they have a specific non-protein molecule attached to them. These are called cofactors. For instance, carbonic anhydrase, an enzyme that helps maintain the pH of the body, cannot function unless it is attached to a zinc ion.

To ensure that the body's systems work correctly, sometimes enzymes need to be slowed down. For instance, if an enzyme is making too much of a product, there needs to be a way to reduce or stop production.

Enzymes' activity can be inhibited in a number of ways:

Competitive inhibitors - a molecule blocks the active site so that the substrate has to compete with the inhibitor to attach to the enzyme.

Non-competitive inhibitors - a molecule binds to an enzyme somewhere other than the active site and reduces how effectively it works.

Uncompetitive inhibitors - the inhibitor binds to the enzyme and substrate after they have bound to each other. The products leave the active site less easily, and the reaction is slowed down.

Irreversible inhibitors - an irreversible inhibitor binds to an enzyme and permanently inactivates it.

There are thousands of enzymes in the human body, here are just a few examples:

Lipases - a group of enzymes that help digest fats in the gut.

Amylase - helps change starches into sugars. Amylase is found in saliva.

Maltase - also found in saliva; breaks the sugar maltose into glucose. Maltose is found in foods such as potatoes, pasta, and beer.

Trypsin - found in the small intestine, breaks proteins down into amino acids.

Lactase - also found in the small intestine, breaks lactose, the sugar in milk, into glucose and galactose.

Acetylcholinesterase - breaks down the neurotransmitter acetylcholine in nerves and muscles.

Helicase - unravels DNA.

DNA polymerase - synthesize DNA from deoxyribonucleotides.

Enzymes play a huge part in the day-to-day running of the human body. By binding to and altering compounds, they are vital for the proper functioning of the digestive system, the nervous system, muscles, and much, much more.

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Enzymes: Function, definition, and examples

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Enzymes help speed up chemical reactions in the body. They affect every function, from breathing to digestion. Lipases, for example, help digest fat. Enzymes only work in certain conditions. If the body is too hot or too acidic, they will not work properly. Understanding them can contribute to new drug therapies.

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The Golgi apparatus or Golgi complex is found in most cells. It is another packaging organelle like the endoplasmic reticulum (ER). It was named after Camillo Golgi, an Italian biologist. It is pronounced GOL-JI in the same way you would say squee-gie, as soft a "G" sound. While layers of membranes may look like the rough ER, they have a very different function.

The Golgi apparatus gathers simple molecules and combines them to make molecules that are more complex. It then takes those big molecules, packages them in vesicles, and either stores them for later use or sends them out of the cell. It is also the organelle that builds lysosomes (cell digestion machines). Golgi complexes in the plant may also create complex sugars and send them off in secretory vesicles. The vesicles are created in the same way the ER does it. The vesicles are pinched off the membranes and float through the cell.

The Golgi apparatus is a series of membranes shaped like pancakes. The single membrane is similar to the cell membrane in that it has two layers. The membrane surrounds an area of fluid where the complex molecules (proteins, sugars, enzymes) are stored and changed. Because the Golgi complex absorbs vesicles from the rough ER, you will also find ribosomes in those pancake stacks.

The Golgi complex works closely with the rough ER. When a protein is made in the ER, something called a transition vesicle is made. This vesicle or sac floats through the cytoplasm to the Golgi apparatus and is absorbed. After the Golgi does its work on the molecules inside the sac, a secretory vesicle is created and released into the cytoplasm. From there, the vesicle moves to the cell membrane and the molecules are released out of the cell.

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The Golgi apparatus or Golgi complex is found in most cells. It is another packaging organelle like the endoplasmic reticulum (ER). It was named after Camillo Golgi, an Italian biologist. It is pronounced GOL-JI in the same way you would say squee-gie, as soft a "G" sound.