How do substances move into and out of cells across the cell-surface membrane?
The fluid-mosaic model of membrane structure and how substances cross membranes by simple diffusion, facilitated diffusion, osmosis, active transport and co-transport; the role of carrier and channel proteins; the factors affecting the rate of transport across membranes.
A focused answer to the AQA 3.2 dot point on membrane transport. Covers the fluid-mosaic model and simple diffusion, facilitated diffusion, osmosis, active transport and co-transport, plus the factors affecting transport rate.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
AQA wants you to describe the fluid-mosaic model and explain five transport processes (simple diffusion, facilitated diffusion, osmosis, active transport, co-transport), the proteins involved, and what changes the rate of transport.
The answer
The fluid-mosaic model
Components:
- Phospholipid bilayer. Hydrophilic (water-loving) heads face outwards to the water; hydrophobic (water-hating) tails face inwards. This makes the membrane a barrier to large and charged molecules.
- Intrinsic (integral) proteins span the bilayer; these include channel and carrier proteins.
- Extrinsic (peripheral) proteins sit on one surface.
- Cholesterol sits between phospholipids, restricting movement and controlling fluidity and stability.
- Glycoproteins and glycolipids have carbohydrate chains used in cell recognition and as receptors.
Passive transport (no ATP)
Simple diffusion. The net movement of particles from a high to a low concentration, down a concentration gradient. Small, non-polar molecules (oxygen, carbon dioxide) cross the bilayer directly.
Facilitated diffusion. Diffusion of larger or charged molecules (glucose, ions) down their gradient through membrane proteins, because they cannot cross the lipid bilayer:
- Channel proteins form water-filled pores; specific ones open to let particular ions through.
- Carrier proteins change shape to move a specific molecule across.
Osmosis. The net movement of water molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane, through the bilayer and through aquaporin channel proteins.
Active processes (require ATP)
Active transport. The movement of molecules against their concentration gradient (low to high) using carrier proteins and ATP from respiration. The carrier binds the molecule, ATP is hydrolysed, and the protein changes shape to move it across.
Co-transport. Two substances are moved across together by one carrier protein, with the gradient of one driving the other. The classic example is glucose absorption in the ileum:
- A sodium-potassium pump actively transports sodium ions out of the epithelial cell into the blood (using ATP), keeping the cell's internal sodium concentration low.
- Sodium then diffuses from the gut lumen into the cell down its gradient through a co-transporter protein, and glucose is carried in with it, even though glucose is moving against its own gradient.
- Glucose leaves the cell into the blood by facilitated diffusion.
Factors affecting the rate of transport
- Concentration (or water-potential) gradient. A steeper gradient gives a faster rate for diffusion and osmosis. For active transport, rate is independent of the gradient because it can work against it.
- Temperature. Higher temperature increases the kinetic energy of molecules, speeding up movement (until proteins denature).
- Surface area of membrane. A larger area (for example microvilli) allows more transport at once.
- Number of channel or carrier proteins. Facilitated diffusion, osmosis (aquaporins), active transport and co-transport all level off (plateau) once all the available proteins are working at maximum, even if the gradient increases further.
- Diffusion distance. A thinner membrane gives a shorter path and faster diffusion.
Examples in context
Example 1. Epithelial cells of the ileum and microvilli. The cells lining the ileum are folded into microvilli, greatly increasing surface area for absorption. They are packed with co-transporter proteins and mitochondria, so the sodium-glucose co-transport system can run quickly to absorb the products of digestion. More carrier proteins and a larger area both raise the transport rate.
Example 2. Red blood cells in different solutions. A red blood cell placed in pure water (high water potential) gains water by osmosis and bursts (haemolysis) because it has no wall. In a concentrated salt solution (low water potential) it loses water and shrinks (crenation). This shows why intravenous fluids must be isotonic with blood plasma.
Try this
Q1. Distinguish between facilitated diffusion and active transport. [3 marks]
- Cue. Facilitated diffusion is passive, moves substances down the gradient, needs channel or carrier proteins, no ATP; active transport moves substances against the gradient using carrier proteins and ATP.
Q2. Explain why the rate of facilitated diffusion reaches a plateau as concentration gradient increases. [2 marks]
- Cue. There is a fixed number of channel or carrier proteins; once all are working at maximum (saturated), increasing the gradient cannot increase the rate further.
Q3. A cell of water potential -500 kPa is placed in a solution of water potential -800 kPa. Predict and explain the net movement of water. [2 marks]
- Cue. Water moves out of the cell, because the solution (-800 kPa) has a lower (more negative) water potential than the cell (-500 kPa), and water moves from higher to lower water potential.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2018 AQA4 marksDescribe how glucose is absorbed from the lumen of the ileum into the epithelial cells by co-transport.Show worked answer →
A 4-mark answer needs the sodium gradient set up first, then the coupled glucose movement.
- Sodium ions are actively transported out of the epithelial cell into the blood by a sodium-potassium pump, using ATP. This keeps the sodium concentration inside the cell low.
- This creates a sodium concentration gradient between the lumen (high) and the cell (low).
- Sodium ions then diffuse back into the cell down this gradient through a co-transporter protein, and glucose is carried in with them, against the glucose concentration gradient.
- Glucose then leaves the cell into the blood by facilitated diffusion.
Markers reward the active transport of sodium, the gradient it creates, and the coupled movement of glucose.
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