What you'll learn
Membrane structure and transport explains how a cell controls what enters and leaves it, and how that control follows directly from the architecture of the membrane itself. The fluid mosaic model is the organising idea: a phospholipid bilayer that behaves as a fluid, studded with a mosaic of proteins that carry out most of the membrane's specific functions. Once the model is secure, every transport mechanism follows from asking a single question — can the substance cross the hydrophobic core unaided, and if not, what is needed? By the end of this topic you should be able to describe the fluid mosaic model and the role of each component, distinguish the five transport mechanisms, explain water potential and predict the behaviour of plant and animal cells, and describe endocytosis and exocytosis.
Key terms and definitions
Fluid mosaic model — the accepted model of membrane structure, a fluid phospholipid bilayer containing a mosaic of proteins
Phospholipid bilayer — the double layer of phospholipids with hydrophilic heads outwards and hydrophobic tails inwards
Amphipathic — having both hydrophilic and hydrophobic regions, as phospholipids do
Intrinsic protein — a protein spanning the whole bilayer, also called an integral protein
Extrinsic protein — a protein on one surface of the bilayer only, also called a peripheral protein
Diffusion — the net movement of particles from a region of higher to lower concentration, down a gradient
Facilitated diffusion — passive movement down a gradient through a channel or carrier protein
Active transport — movement against a concentration gradient using ATP and a carrier protein
Osmosis — the net movement of water from a higher to a lower water potential through a partially permeable membrane
Water potential — the tendency of water to move out of a system, measured in kilopascals, with pure water at zero
Plasmolysis — the pulling away of the cell surface membrane from the cell wall as a plant cell loses water
Turgid — a plant cell that is full of water and pressing against its wall
Core concepts
The fluid mosaic model
The membrane is described as fluid because the phospholipids are not fixed: they move laterally within their layer, giving the membrane flexibility and the ability to self-seal. It is described as a mosaic because the proteins are scattered irregularly through the bilayer like tiles in a mosaic.
Each component has a specific role, and questions routinely ask for them individually.
Phospholipids form the basic bilayer. Because each molecule has a hydrophilic phosphate head and two hydrophobic hydrocarbon tails, in an aqueous environment they arrange spontaneously with heads facing the water on both sides and tails pointing inwards. The hydrophobic core is what makes the membrane a barrier to water-soluble substances.
Cholesterol sits between the phospholipids in animal cell membranes and regulates fluidity. At higher temperatures it restrains the phospholipids and reduces fluidity; at lower temperatures it prevents them packing too closely and so stops the membrane becoming rigid. It also adds mechanical strength and reduces permeability to water-soluble substances.
Channel proteins are intrinsic proteins forming hydrophilic pores through which specific water-soluble ions and molecules diffuse. Many are gated, opening and closing in response to a signal.
Carrier proteins are intrinsic proteins that bind a specific molecule and change shape to move it across. They are used in both facilitated diffusion and active transport, the difference being whether ATP is required.
Glycoproteins are proteins with carbohydrate chains attached, acting as receptors for hormones and neurotransmitters, and as antigens allowing cells to be recognised as self or non-self. They also help cells adhere to one another.
Glycolipids are lipids with carbohydrate attached, acting as recognition sites and contributing to membrane stability.
Factors affecting membrane permeability
Temperature affects permeability in two phases and is a favourite practical investigation using beetroot, where the red pigment betalain leaks from the vacuole and its concentration is measured with a colorimeter.
Below the optimum, increasing temperature increases the kinetic energy of the phospholipids, so they move more and the membrane becomes more permeable.
Above about 40 to 50 degrees Celsius, permeability rises sharply because the membrane proteins begin to denature. Their tertiary structure is lost, channels and carriers no longer function correctly, and gaps appear in the bilayer.
Organic solvents such as ethanol dissolve the lipids of the bilayer, disrupting its structure and greatly increasing permeability, which is why alcohol is used as a disinfectant.
Simple diffusion
Simple diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, resulting from their random motion. It is passive, requiring no ATP.
Only small, non-polar molecules cross the bilayer directly, because they can pass through the hydrophobic core. Oxygen, carbon dioxide and steroid hormones do so. Water, though polar, is small enough to cross slowly, and lipid-soluble substances cross readily.
The rate of diffusion is increased by a steeper concentration gradient, a higher temperature, a larger surface area, and a shorter diffusion distance. Fick's law expresses this: the rate is proportional to surface area multiplied by concentration difference, divided by the thickness of the exchange surface.
Facilitated diffusion
Large or charged particles cannot cross the hydrophobic core, so they pass through proteins instead.
Channel proteins provide hydrophilic pores for specific ions such as sodium and potassium. Carrier proteins bind a specific molecule such as glucose or an amino acid, change shape, and release it on the other side.
Facilitated diffusion is still passive: movement is down the concentration gradient and no ATP is used. What distinguishes it from simple diffusion is that it requires a protein, is specific to particular substances, and can be saturated — once every available protein is occupied, the rate cannot increase further however steep the gradient becomes.
That saturation effect is how the two are distinguished experimentally, and it is a standard graph interpretation question: simple diffusion gives a straight line against concentration, while facilitated diffusion levels off.
Active transport
Active transport moves substances against the concentration gradient, from a lower to a higher concentration. It therefore requires energy, supplied by ATP, and a carrier protein.
The carrier binds the specific molecule on one side, ATP is hydrolysed to ADP and phosphate, and the phosphate group binding to the carrier causes it to change shape and release the molecule on the other side. The carrier then reverts.
Because it needs ATP, active transport is affected by anything altering respiration: it stops in the absence of oxygen, is reduced by respiratory inhibitors such as cyanide, and increases with temperature up to the optimum for respiratory enzymes. Cells carrying out much active transport contain many mitochondria, as in the epithelium of the small intestine and the proximal convoluted tubule of the nephron.
Co-transport is an important variant. In the ileum, sodium ions are actively pumped out of the epithelial cell into the blood, creating a low sodium concentration inside. Sodium ions then diffuse in from the gut lumen through a co-transporter protein, carrying glucose with them against its own concentration gradient. The glucose then leaves into the blood by facilitated diffusion. This allows glucose to be absorbed even when its concentration in the gut is lower than in the cell.
Osmosis and water potential
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 the tendency of water to move out of a system. Pure water has a water potential of zero, and adding any solute lowers it, so all solutions have negative water potentials. A more concentrated solution therefore has a more negative water potential.
Water always moves from a less negative to a more negative water potential — that is, from a more dilute to a more concentrated solution.
In plant cells, water potential has two components: solute potential, which is always negative and depends on the concentration of dissolved solutes, and pressure potential, which is usually positive and is generated by the cell wall pushing back on the contents. The water potential of the cell equals the sum of the two.
Cells in different solutions
An animal cell in a solution of higher water potential, such as pure water, takes in water by osmosis. Having no cell wall, it swells and eventually bursts, a process called cytolysis or haemolysis in red blood cells. In a solution of lower water potential it loses water, shrinks and becomes crenated.
A plant cell in a solution of higher water potential takes in water and its protoplast presses against the cell wall. The wall, being strong and relatively inelastic, pushes back, generating pressure potential, and the cell becomes turgid. It does not burst, and turgidity is what provides support to non-woody plant tissue.
A plant cell in a solution of lower water potential loses water. It first becomes flaccid, and with further loss the protoplast pulls away from the cell wall, which is plasmolysis. The point at which the protoplast just begins to pull away is called incipient plasmolysis, and at this point the pressure potential is zero, so the water potential of the cell equals its solute potential. This is the basis of the standard practical for determining the solute potential of plant tissue.
Bulk transport
Substances too large to cross even through proteins are moved by bulk transport, which requires ATP.
Endocytosis brings material in. The cell surface membrane invaginates around the material and pinches off to form a vesicle inside the cell. Phagocytosis takes in solid material, as when a macrophage engulfs a pathogen and the resulting phagosome fuses with a lysosome. Pinocytosis takes in liquid.
Exocytosis expels material. A vesicle, typically from the Golgi apparatus, moves to the cell surface membrane and fuses with it, releasing the contents outside. This is how digestive enzymes, hormones and neurotransmitters are secreted.
Worked examples
Example 1: Distinguishing transport mechanisms from data (5 marks)
An investigation measures the rate of uptake of two substances by a cell as external concentration increases. Substance X shows a rate that increases in direct proportion to concentration. Substance Y shows a rate that increases and then levels off. Adding a respiratory inhibitor has no effect on either. Identify the mechanisms and justify your answer.
Substance X enters by simple diffusion. The rate is directly proportional to the concentration gradient, which is characteristic of simple diffusion, and because no proteins are involved there is no limit to the rate other than the gradient itself.
Substance Y enters by facilitated diffusion. The levelling off shows saturation: once all the available channel or carrier proteins are occupied, further increases in concentration cannot increase the rate.
Neither is active transport, because the respiratory inhibitor has no effect. Active transport requires ATP from respiration, so inhibiting respiration would reduce or stop it.
Example 2: Predicting cell behaviour (5 marks)
A plant cell with a water potential of minus 800 kilopascals is placed in a solution of water potential minus 500 kilopascals. Describe and explain what happens.
Water moves from a region of higher, that is less negative, water potential to a region of lower, more negative water potential. The solution at minus 500 kilopascals is less negative than the cell at minus 800 kilopascals, so water moves into the cell by osmosis through the partially permeable cell surface membrane.
As water enters, the protoplast expands and presses against the cell wall. The wall is strong and resists, generating an increasing pressure potential, which makes the cell's water potential less negative.
Water continues to enter until the water potential of the cell equals that of the external solution at minus 500 kilopascals, at which point there is no net movement and the cell is turgid. The cell does not burst because the cellulose cell wall prevents further expansion.
Example 3: Explaining co-transport (5 marks)
Explain how glucose is absorbed from the ileum even when its concentration in the lumen is lower than inside the epithelial cell.
Sodium ions are actively transported out of the epithelial cell across the basal membrane into the blood, using ATP. This lowers the sodium ion concentration inside the cell relative to the lumen of the ileum.
A concentration gradient for sodium ions is therefore established from the lumen into the cell. Sodium ions diffuse down this gradient through a co-transporter protein in the cell surface membrane.
The co-transporter carries a glucose molecule at the same time, so the glucose is moved into the cell against its own concentration gradient, powered indirectly by the sodium gradient rather than by ATP directly.
The glucose then passes out of the cell into the blood by facilitated diffusion, down its concentration gradient, through a carrier protein.
Common mistakes and how to avoid them
The most frequent error is stating that water moves from high to low concentration in osmosis. Water moves from a higher to a lower water potential, and since all solutions have negative water potentials, students should work with less negative and more negative rather than high and low.
Students often describe facilitated diffusion as requiring energy. It is passive; only the requirement for a protein distinguishes it from simple diffusion.
Another common slip is saying that plant cells burst in pure water. The cell wall prevents this; they become turgid.
Many candidates confuse the effects of cholesterol, stating simply that it makes the membrane more or less fluid. It regulates fluidity in both directions depending on temperature.
Finally, in co-transport questions, answers frequently omit that the sodium gradient is established by active transport elsewhere in the cell. Without that step the mechanism has no energy source.
Exam technique for "Membrane structure and transport"
When identifying a transport mechanism from data, apply two tests: does the rate saturate, and does a respiratory inhibitor affect it. Saturation indicates a protein; inhibitor sensitivity indicates ATP.
In osmosis questions, always state water potential values with their signs and compare them explicitly before predicting the direction of movement.
Name the specific membrane component in structure questions — channel protein, carrier protein, glycoprotein, cholesterol — rather than referring generally to proteins.
For plant cell questions, mention the cell wall and pressure potential. The wall is what makes plant behaviour different from animal behaviour, and it is usually worth a mark.
When describing active transport, include the carrier protein, the hydrolysis of ATP, and the resulting shape change, since marks are typically distributed across all three.
Quick revision summary
The fluid mosaic model describes a fluid phospholipid bilayer with a mosaic of proteins. Phospholipids are amphipathic, forming a hydrophobic core that blocks water-soluble substances; cholesterol regulates fluidity in both directions and reduces permeability; channel proteins form hydrophilic pores; carrier proteins change shape to move specific molecules; glycoproteins and glycolipids act as receptors and antigens. Permeability rises with temperature and sharply above about 40 degrees as proteins denature, and organic solvents dissolve the bilayer. Simple diffusion moves small non-polar molecules passively down a gradient; facilitated diffusion is also passive but needs proteins and shows saturation; active transport moves substances against the gradient using ATP and carrier proteins, and is stopped by respiratory inhibitors. Co-transport in the ileum uses a sodium gradient created by active transport to draw glucose in against its gradient. Osmosis moves water from a less negative to a more negative water potential, with plant cells becoming turgid rather than bursting because the wall generates pressure potential, and plasmolysing when water is lost. Endocytosis and exocytosis move bulk material using ATP.