Syllabus coverage
- Describe the fluid mosaic model of membrane structure with reference to the hydrophobic and hydrophilic interactions that account for the formation of the phospholipid bilayer and the arrangement of proteins
- Describe the arrangement of cholesterol, glycolipids and glycoproteins in cell surface membranes
- Describe the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins in cell surface membranes, with reference to stability, fluidity, permeability, transport (carrier proteins and channel proteins), cell signalling (cell surface receptors) and cell recognition (cell surface antigens)
- Outline the main stages in the process of cell signalling leading to specific responses: secretion of specific chemicals (ligands) from cells, transport of ligands to target cells, and binding of ligands to cell surface receptors on target cells
- Describe and explain the processes of simple diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis
- Investigate simple diffusion and osmosis using plant tissue and non-living materials, including dialysis (Visking) tubing and agar
- Illustrate the principle that surface area to volume ratios decrease with increasing size by calculating surface areas and volumes of simple 3-D shapes
- Investigate the effect of changing surface area to volume ratio on diffusion using agar blocks of different sizes
- Investigate the effects of immersing plant tissues in solutions of different water potentials, using the results to estimate the water potential of the tissues
- Explain the movement of water between cells and solutions in terms of water potential, and explain the different effects of the movement of water on plant cells and animal cells
Part of the AS & A Level Biology syllabus 9700.
The fluid mosaic model: a membrane that builds itself
The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes a cell surface membrane as a phospholipid bilayer with proteins scattered through it, like icebergs floating in a sea of lipid. A phospholipid has a hydrophilic phosphate head that mixes readily with water and two hydrophobic fatty acid tails that do not. In water these molecules arrange themselves so that the tails are hidden from water and the heads face it, and the arrangement that hides the most tail from water is a bilayer: two sheets of phospholipids with their tails pointing inwards towards each other and their heads outwards towards the cytoplasm and the tissue fluid.
The bilayer is held together almost entirely by these hydrophobic and hydrophilic interactions, not by covalent bonds, and that is why the membrane is fluid. Individual phospholipids slide sideways within their own sheet, so the membrane has the consistency of olive oil rather than butter, and it can bend, seal a puncture and pinch off a vesicle. It is called a mosaic because the proteins are dotted through the lipid in an irregular pattern rather than forming a continuous layer.
The proteins come in two main positions. Intrinsic (integral) proteins have hydrophobic regions that sit inside the bilayer, and most of them span it completely; those that form a water-filled pore through the membrane are channel proteins, and those that change shape to move a substance across are carrier proteins. Extrinsic (peripheral) proteins sit on the inner or the outer surface, held there by attachment to the bilayer or to an intrinsic protein, and many of them are enzymes or are anchored to the cytoskeleton.
The model has been modified as understanding has improved, and that is itself part of the syllabus. It is now known that the bilayer is not a uniform sea: regions richer in cholesterol and in saturated phospholipids are thicker and more rigid, and particular receptors and signalling proteins gather there in lipid rafts. The fluid mosaic model also provides the basis for understanding passive and active movement between cells and their surroundings, cell-to-cell interaction and long-distance cell signalling.
Cholesterol, glycolipids and glycoproteins in a cell surface membrane
Cholesterol is a lipid with a small hydrophilic -OH group at one end and a rigid hydrophobic ring system at the other. In an animal cell surface membrane it wedges between the phospholipid tails, its -OH group sitting near the heads, and it does two jobs. It restricts the movement of the phospholipids, so it makes the membrane less fluid at body temperature, and it makes the membrane less permeable to small water-soluble molecules such as ions and glucose. It is therefore a barrier-strengthening molecule, not a fluidity booster.
Glycolipids are phospholipids with a carbohydrate chain attached, and glycoproteins are proteins with a carbohydrate chain attached. Both are found only in the outer layer of the membrane, with their carbohydrate chains projecting outwards into the tissue fluid to form a fuzzy coat called the glycocalyx. The chains act as cell surface antigens for cell recognition, which is how the immune system tells self from non-self and how the ABO blood group antigens work; the protein part of a glycoprotein can also be a receptor for a hormone, so the same molecule can be both a label and a signal detector.
The lipid-soluble molecules that need no protein at all are the small non-polar ones. Oxygen and carbon dioxide cross by dissolving straight through the bilayer, and so do steroid hormones such as oestrogen and cortisol, because they are lipid-soluble. Water is the special case: it diffuses through the bilayer slowly, and much faster through narrow channel proteins called aquaporins.
Learn which component does which job, because exam questions ask for the link between structure and function rather than a list. Stability and fluidity come from phospholipids and cholesterol; permeability depends on the bilayer plus cholesterol; transport needs channel and carrier proteins; signalling needs receptors, which are usually glycoproteins; and recognition needs the carbohydrate chains of glycoproteins and glycolipids.
| Membrane component | Where it sits | What it does |
|---|---|---|
| Phospholipid | Forms the bilayer, hydrophilic heads outwards and hydrophobic tails inwards | Gives the membrane its basic structure and its barrier to large or polar water-soluble molecules |
| Cholesterol | Wedged between the phospholipid tails, with its -OH group near the heads | Reduces permeability to ions and small water-soluble molecules and stabilises the membrane by stopping the phospholipids from packing together too tightly |
| Glycolipid | In the outer layer, with the carbohydrate chain projecting outwards | Cell recognition: the carbohydrate acts as a cell surface antigen, and glycolipids help cells to attach to one another |
| Glycoprotein | Spanning the membrane or on the outer surface, with the carbohydrate chain projecting outwards | Cell recognition and cell signalling: the carbohydrate forms antigens such as the ABO blood group antigens, and the protein part can be a receptor that binds one specific ligand |
| Channel protein | Spanning the bilayer and forming a water-filled pore | Transport: lets specific small ions or water-soluble molecules through by facilitated diffusion, down their concentration gradient |
| Carrier protein | Spanning the bilayer, with a shape that changes | Transport: moves a specific solute across by facilitated diffusion, or, using ATP, by active transport |
| Cell surface receptor | A glycoprotein or protein on the outer surface | Cell signalling: binds one specific ligand, so the cell responds only to that signal |
Cell signalling and cell recognition
A ligand is a molecule that binds to a specific site on a protein. In cell signalling the ligand is the chemical signal, and the process has three main stages: the signal molecule is secreted by the signalling cell, it is transported to the target cell, usually dissolved in the blood plasma, and it binds to a cell surface receptor on the target cell, which triggers a specific response inside that cell.
Insulin is the clearest example. The beta cells of the islets of Langerhans in the pancreas secrete insulin into the blood when the blood glucose concentration rises, and the insulin travels in the plasma to muscle, liver and adipose cells. There it binds to a glycoprotein receptor in the cell surface membrane, and the response is that glucose transport proteins are moved into the membrane so that glucose enters the cells, and liver cells convert glucose to glycogen. Adrenaline from the adrenal medulla binds to a different receptor on liver cells and triggers the breakdown of glycogen instead. The ligand is the same shape of signal; the response depends on which receptor the target cell carries.
This is why signalling is specific. A receptor has a shape complementary to one ligand, in the same way that an enzyme has a shape complementary to its substrate, so only cells carrying that receptor respond. The same ligand can produce different responses in different target cells because the events triggered inside the cell differ, and a cell with no receptor for a ligand completely ignores it.
Cell recognition is the other side of the same coin. The carbohydrate chains of glycoproteins and glycolipids act as cell surface antigens, and the pattern of these chains is unique to the individual. That is why a blood transfusion with the wrong ABO group causes a reaction: the recipient's antibodies recognise the donor red blood cell antigens as foreign. The same antigens are what the immune system uses to identify its own cells, and they are why a transplanted kidney has to be tissue-typed before the operation, and why a phagocyte can tell a bacterium from a body cell on contact.
Simple diffusion, facilitated diffusion and osmosis
Simple diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient, and it is passive: no ATP is used. The rate depends on the steepness of the gradient, the distance to be crossed, the surface area available and the properties of the substance. Small, non-polar, lipid-soluble molecules cross the bilayer most easily, which is why oxygen moves from the alveolar air into the blood and carbon dioxide moves the other way through the same membranes and through every cell membrane in the body.
Facilitated diffusion is the same passive movement down a concentration gradient, but it uses a protein because the substance is too large or too polar to dissolve through the bilayer. Ions such as sodium and potassium cross through channel proteins, which are narrow water-filled pores with a charged lining that selects the ion; water crosses quickly through the aquaporins. Larger polar molecules such as glucose cross on a carrier protein, which has a binding site complementary to the molecule, changes shape to carry it to the other side and then returns to its original shape. Because the proteins are used, facilitated diffusion is specific and it saturates: once every carrier protein is occupied, raising the concentration further no longer increases the rate.
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. Water potential is measured in kPa, and pure water at atmospheric pressure has a water potential of zero. Adding any solute makes the water potential negative, and the more concentrated the solution the more negative it becomes. Water therefore moves down a water potential gradient, which usually means from the dilute solution into the concentrated one, but the driving force is water potential and not solute concentration, and writing it the other way round is the single most common error in this topic.
Diffusion sets the limits on cell size and cell shape. Because the rate of diffusion is proportional to the surface area available and inversely related to the distance over which the substance must travel, a cell that relies on diffusion alone stays small and often has a large, folded or branched surface, such as the microvilli of an epithelial cell of the small intestine or the root hair cell of a plant.
| Simple diffusion | Facilitated diffusion | |
|---|---|---|
| Route across the membrane | Directly through the phospholipid bilayer | Through a channel protein or on a carrier protein |
| Substances | Small, non-polar, lipid-soluble molecules such as oxygen, carbon dioxide and steroid hormones | Polar molecules and ions such as glucose, amino acids, sodium and potassium, and water through aquaporins |
| Energy from ATP | None, it is passive | None, it is passive |
| Effect of raising the concentration gradient | The rate keeps rising with the gradient | The rate rises until every protein is occupied, then levels off because the process saturates |
| Specificity | Not specific: anything lipid-soluble and small enough will cross | Specific: the protein selects the substance by shape or by charge |
Active transport, endocytosis and exocytosis
Active transport moves a substance across a membrane against its concentration gradient, and therefore from a region of lower concentration to a region of higher concentration, using energy from ATP. The ATP is hydrolysed by the carrier protein itself, which is why these carriers are called pumps. The best known is the sodium-potassium pump in the cell surface membrane of a neurone: for each ATP hydrolysed, three sodium ions are moved out of the cell and two potassium ions are moved in. Both ions are being pushed against their gradients, and the steady difference in ion concentrations that results is what the resting potential of the axon depends on.
Plants use active transport in the same way. A root hair cell absorbs nitrate and other mineral ions from the soil solution, where their concentration may be far lower than inside the cytoplasm, so the ions are taken in by active transport against their concentration gradient. If the root is treated with a respiratory inhibitor, or the soil becomes waterlogged so that the roots cannot respire, mineral uptake stops even though the ions are still there, and that is the classic evidence that the process needs ATP.
Co-transport couples the two ideas together. In the epithelial cells of the ileum, the sodium-potassium pump in the membrane facing the blood keeps the sodium ion concentration inside the cell low. Sodium ions therefore diffuse back into the cell from the lumen down their concentration gradient, and they do it through a co-transport protein that carries a glucose molecule with them. Glucose is dragged in against its own concentration gradient, and it then leaves the cell into the blood by facilitated diffusion. This is why an oral rehydration solution contains both sodium chloride and glucose: the sodium is needed for the glucose to be absorbed, and the glucose is needed for the sodium to be absorbed with it.
Endocytosis and exocytosis move material in bulk. In endocytosis the cell surface membrane invaginates around a particle or a droplet, closes behind it and pinches off a membrane-bound vesicle inside the cell; it needs ATP and the fluidity of the bilayer. Phagocytosis is the version used by macrophages and neutrophils, which engulf bacteria so that they can be digested by lysosomal enzymes. In exocytosis a vesicle moves to the cell surface membrane, fuses with it and releases its contents outside. Insulin is released from the beta cells of the islets of Langerhans this way, as is acetylcholine at a synapse and digestive enzymes from the acinar cells of the pancreas. Both processes are used when the substance could never cross the bilayer directly: a bacterium is far too large, and a protein such as insulin is too large and too polar.
Surface area to volume ratio: why small cells exchange faster
As a shape gets bigger, its surface area grows as the square of its length while its volume grows as the cube, so the surface area to volume ratio falls. A cube of side 1 cm has a surface area of 6 x 1 x 1 = 6 cm2 and a volume of 1 x 1 x 1 = 1 cm3, giving a ratio of 6:1. A cube of side 2 cm has a surface area of 6 x 2 x 2 = 24 cm2 and a volume of 8 cm3, giving 3:1. A cube of side 3 cm has a surface area of 6 x 3 x 3 = 54 cm2 and a volume of 27 cm3, giving 2:1. For a cube the ratio is always 6 divided by the length of the side, so doubling the side halves the ratio.
The agar block experiment shows what that means for diffusion. Blocks of agar of different sizes are made up containing an indicator, and they are placed in a solution that changes the indicator as it diffuses inwards. The time taken for the colour change to reach the centre of each block is recorded, and the depth the solution has penetrated is measured. The largest block has the largest surface area, but it also has by far the largest volume of tissue to supply, and the diffusing substance has to travel twice as far to reach its centre. The small block changes colour first because it has the largest surface area to volume ratio, so every part of it is close to the surface.
This is why exchange surfaces are built the way they are. Alveoli are tiny and there are hundreds of millions of them, each with a wall one cell thick, so the surface area for gas exchange is enormous compared with the volume of the lungs. The villi of the ileum and the microvilli on the surface of their epithelial cells multiply the surface area for absorption, and a root hair cell pushes out a long narrow extension that gives a large surface area for absorbing water and mineral ions.
It is also why organisms cannot simply grow as a solid sphere. A single-celled organism such as an Amoeba can rely on diffusion across its surface because every part of the cytoplasm is close to the outside, and a flatworm solves the problem by being flattened and thin. Once an animal is large, diffusion alone would take far too long, so it needs a mass transport system: a circulatory system to carry substances to and from a large, well-supplied exchange surface.
Water potential and the different effects on plant and animal cells
Water potential tells you which way water will move. Pure water at atmospheric pressure has a water potential of 0 kPa, which is the highest value possible, and any solution has a negative water potential because the solute molecules take up space and reduce the freedom of the water molecules to move. The more concentrated the solution, the more negative its water potential, so water always moves from the solution of higher (less negative) water potential to the solution of lower (more negative) water potential. Inside a living cell the water potential also depends on the pressure the cell wall or the membrane is putting on the contents, but at AS Level you are not required to split water potential into solute and pressure potential.
In a plant cell, the cellulose cell wall makes all the difference. Placed in pure water or in a dilute solution, the cell takes in water by osmosis because the contents have a lower water potential than the outside solution. The vacuole swells, pushing the cytoplasm against the cell wall, and the cell becomes turgid. The wall is strong and slightly elastic, so it resists further stretching and it pushes back; once that inward pressure balances the tendency of water to enter, no more net movement takes place and the cell does not burst. Turgor is what holds a herbaceous plant upright and why a leaf goes limp when the cells lose water.
A plant cell placed in a concentrated solution loses water instead, and the cytoplasm and vacuole shrink away from the cell wall. The stage at which the protoplast has just begun to pull away from the wall is incipient plasmolysis, and once it is fully pulled away the cell is plasmolysed. A cell that has lost its turgor but is not yet plasmolysed is flaccid. Note that a plant cell does not shrivel: the cell wall keeps its shape, and only the contents shrink away from it.
An animal cell behaves differently, because it has no cell wall. A red blood cell placed in distilled water takes in water until the cell surface membrane can no longer stretch, and then it bursts; this is haemolysis. In a concentrated salt solution the same cell loses water, shrinks and develops a wrinkled outline, and is described as crenated. This is why fluids given by a drip into a vein are made isotonic with blood plasma, using a solution of about 0.9 per cent sodium chloride, so that the red blood cells keep the same water content as they had in the circulation.
Investigating diffusion and osmosis in the laboratory
The standard method for estimating the water potential of a plant tissue uses potato cylinders. Cylinders of equal length and diameter are cut, blotted dry with paper towel to remove surface water, weighed, and each one is placed in a labelled boiling tube containing a different concentration of sucrose solution, from distilled water up to about 1.0 mol/dm3. After a fixed time, typically 20 to 30 minutes, the cylinders are removed, blotted dry in the same way and reweighed. The change in mass is worked out as a percentage of the starting mass, because the cylinders differ slightly in size, and the percentage change is plotted against the sucrose concentration.
The plot is what gives the answer. Where the line crosses the zero line, the cylinders neither gained nor lost mass, which means the water potential of the sucrose solution equals the water potential of the potato tissue, so reading the concentration at that crossing and converting it gives an estimate of the tissue water potential. Above that concentration the tissue loses mass because water leaves the cells, and below it the tissue gains mass because water enters the cells. The control variables matter: the same volume of solution, the same temperature, the same time, and blotting before both weighings so that the only mass change is the water that has moved into or out of the cells.
Visking tubing is a partially permeable membrane made of cellulose, and it is the non-living material of choice for showing the difference that molecular size makes. The tubing is soaked to soften it, one end is knotted and it is filled with a solution of glucose and starch. The other end is knotted, the outside is rinsed to remove any spilled solution, and the tubing is placed in a beaker of distilled water. Samples of the water outside are tested at intervals: a brick-red precipitate with Benedict's solution shows that glucose has diffused out, and a negative iodine test shows that the starch has not, because the starch molecules are far too large to pass through the pores. The same idea explains haemodialysis, where a partially permeable membrane removes small waste molecules from a patient's blood but keeps the proteins and cells in it.
Membrane permeability can be measured with beetroot. Discs of beetroot tissue are cut to the same size and rinsed thoroughly, because cutting damages cells and releases pigment, and each disc is placed in water at a different temperature. The red pigment betalain leaks out of the cells, and it leaks out much faster from tissue that has been heated, because heat melts the phospholipid bilayer and denatures the membrane proteins. The intensity of the red colour in the water is compared with a colour standard or measured with a colorimeter, and the same method is used to test the effect of alcohol or of a detergent on membrane permeability.
Key terms you must be able to define
- Fluid mosaic model
- The description of a cell surface membrane as a phospholipid bilayer with proteins of many shapes and sizes scattered through it, held together by hydrophobic and hydrophilic interactions, so that the whole structure is free to move sideways like a fluid.
- Phospholipid bilayer
- The double sheet of phospholipid molecules that forms the basic structure of a cell membrane, with the hydrophilic phosphate heads facing the watery solutions on each side and the hydrophobic fatty acid tails pointing inwards towards each other.
- Cholesterol
- A lipid in animal cell surface membranes that sits between the phospholipid tails and reduces the movement of the phospholipids, making the membrane less fluid and less permeable to ions and small water-soluble molecules.
- Glycolipid
- A phospholipid with a carbohydrate chain attached, found in the outer layer of the membrane with the chain projecting outwards, acting as a cell surface antigen for cell recognition.
- Glycoprotein
- A protein with a carbohydrate chain attached; the chain acts as a cell surface antigen for cell recognition, and the protein part can be a receptor that binds one specific ligand in cell signalling.
- Channel protein
- An intrinsic protein that spans the membrane and forms a water-filled pore lined with charged groups, allowing specific ions or water molecules to pass through by facilitated diffusion, down their concentration gradient.
- Carrier protein
- An intrinsic protein with a binding site complementary to one substance, which changes shape to move that substance across the membrane, either by facilitated diffusion or, when ATP is used, by active transport.
- Osmosis
- The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane, without the use of ATP.
- Water potential
- A measure, in kPa, of the tendency of water to move from one place to another; pure water at atmospheric pressure has a water potential of zero and any solution has a negative value, becoming more negative as the solute concentration rises.
Applied to the real world
Cambridge sets 9700 in unfamiliar contexts - a hospital, a research laboratory, a farm, a nature reserve. Use these as models for how to apply the biology, then write your own.
A patient whose kidneys have failed is treated by haemodialysis. Blood is pumped through thousands of fine tubes made of a partially permeable membrane, and the tubes are surrounded by dialysis fluid. Urea and excess mineral ions are small enough to diffuse out of the blood down their concentration gradients into the fluid, but the dialysis fluid already contains glucose and mineral ions at the concentrations found in healthy blood plasma, so those substances do not diffuse out. The blood cells and plasma proteins are far too large to cross the membrane and stay in the blood. The same principle of a large surface area and a short diffusion distance is why the tubes are so fine and so numerous.
Oral rehydration solutions used to treat the dehydration of severe diarrhoea, for example in cholera, contain both glucose and sodium chloride. The glucose is absorbed in the ileum by co-transport with sodium ions: the sodium-potassium pump keeps the sodium ion concentration inside the epithelial cell low, so sodium diffuses in from the lumen down its concentration gradient and drags glucose in with it. Because sodium and glucose each help the other to be absorbed, the mixture is far more effective than either the salt or the sugar on its own, and it has saved very large numbers of lives in countries where intravenous fluids are not available.
To estimate the water potential of plant tissue, cylinders of potato are blotted dry, weighed and left for 30 minutes in sucrose solutions of different concentrations. The cylinders in distilled water gain mass because water enters the cells by osmosis, and the cylinders in strong sucrose solution lose mass because water leaves them. Plotting the percentage change in mass against the sucrose concentration and finding the concentration at which the line crosses zero gives the value at which the solution and the tissue have the same water potential, so that concentration is converted into an estimate of the tissue water potential.
If a farmer spreads more fertiliser than the crop can use, the soil solution around the roots becomes more concentrated than the cytoplasm of the root hair cells. Water then leaves the root hair cells by osmosis instead of entering them, because the soil solution has the lower water potential, and the plant wilts even though the soil is damp. The damage is called fertiliser burn or salt scorch, and it is avoided by applying fertiliser at the recommended rate and watering it in, so that the salts are diluted before they reach the roots.
An Atlantic salmon is born in fresh water and later migrates to the sea, and it has to cope with an almost complete reversal of the water potential gradient. In the river the body fluids have the lower water potential, so water enters the gills by osmosis and the fish produces a large volume of dilute urine. In the sea the water around the fish has the lower water potential, so the fish drinks seawater and uses active transport in the chloride cells of its gills to pump the excess sodium and chloride ions out of the body against their concentration gradients, using ATP from respiration.
Fruit juice is concentrated by reverse osmosis. The juice is pumped at high pressure against a partially permeable membrane that water can pass through but sugar cannot, so water is forced out of the juice and the sugar solution left behind becomes more concentrated without any heating, which keeps the flavour compounds intact. The same membrane process is used to desalinate seawater for drinking and to remove alcohol from beer, because in each case the water that passes through the membrane is the useful product or the thing that has to be removed.
Flashcards
Click a card to flip it. Mark honestly — the tracker uses this.
Exam-style questions
8 questions, 26 marks. Write your answer, then reveal the mark scheme and tick each point you actually made. Answering honestly here is what makes the rest of the site useful.
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Exam technique for this topic
- Write osmosis in terms of water potential, never in terms of the concentration of water. Water moves from a region of higher water potential to a region of lower water potential, and the partially permeable membrane must be named. A solution of 0.5 mol/dm3 sucrose has a lower water potential than a solution of 0.1 mol/dm3, so water moves towards the more concentrated solution.
- Never say that facilitated diffusion uses ATP. It is passive. If a question expects energy from ATP, it is asking about active transport, endocytosis or exocytosis, and the mark is for naming ATP or the hydrolysis of ATP as the source of energy.
- For a plant cell in a concentrated solution, use the precise terms: flaccid means no turgor, incipient plasmolysis means the protoplast has just begun to pull away from the cell wall, and plasmolysis means it has pulled away. Writing that the plant cell shrivels loses the mark, because the cell wall keeps its shape.
- In a surface area to volume question, write the surface area and the volume out separately with their units before you divide them, then give the answer as a ratio such as 3:1. Marks are lost for an unexplained number, and a ratio is not the same thing as a decimal.
- For cell signalling, give the three stages in order: secretion of the ligand from the signalling cell, transport of the ligand to the target cell, and binding of the ligand to a cell surface receptor. Then explain the specificity, because the target cell has a receptor with a shape complementary to that ligand, so cells without the receptor do not respond.
- In a practical question, name a control variable and say what it controls. Blotting the potato cylinders before each weighing removes surface water, so the change in mass is caused only by water moving into or out of the cells, and keeping the temperature constant keeps the water potential of the solutions and the rate of diffusion the same in every tube.
Why students lose marks here
Where this fits in AS & A Level Biology
This page is one topic of Cambridge AS & A Level Biology 9700. AS & A Level Biology is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the AS & A Level Biology syllabus page.