The fluid-mosaic model of membrane structure, and how substances cross membranes by diffusion, facilitated diffusion, osmosis, active transport and bulk transport.

What this dot point is asking

WJEC wants you to describe the fluid-mosaic model of the cell membrane, explain the roles of its components, and compare the ways substances cross it: simple diffusion, facilitated diffusion, osmosis, active transport, and bulk transport by endocytosis and exocytosis. You should also be able to interpret osmosis experiments and calculate percentage change in mass.

The fluid-mosaic model

The bilayer has hydrophilic phosphate heads facing the watery surroundings on each side and hydrophobic fatty acid tails facing inward, away from water. This arrangement forms a stable barrier that small non-polar molecules cross easily but large, polar or charged particles cannot. Cholesterol sits between the phospholipids and regulates fluidity: it stops the membrane becoming too fluid at high temperature and too rigid at low temperature.

Intrinsic (integral) proteins span the whole bilayer and include channel proteins and carrier proteins for transport. Extrinsic (peripheral) proteins sit on one surface. Glycoproteins and glycolipids carry carbohydrate chains on the outer surface and act as receptors for hormones and in cell-to-cell recognition, which is the basis of the immune system distinguishing self from non-self.

Passive transport: no ATP needed

• Simple diffusion: small, non-polar molecules such as oxygen ($\text{O}_2$) and carbon dioxide ($\text{CO}_2$) move down their concentration gradient straight through the bilayer.
• Facilitated diffusion: larger or charged particles (glucose, amino acids, ions) move down their gradient through channel proteins (water-filled pores, often gated) or carrier proteins (which change shape to ferry the solute across).
• Osmosis: water moves from a region of higher water potential to a region of lower (more negative) water potential across a partially permeable membrane. Pure water has a water potential of $0$ kPa; adding solute lowers (makes more negative) the water potential.

Active transport and bulk transport

Active transport uses carrier proteins and ATP to move substances against their concentration gradient. The ATP is hydrolysed at the carrier protein, which changes shape to push the solute across; the sodium-potassium pump in nerve cells is the classic example. Bulk transport moves large quantities or large particles too big for proteins: endocytosis brings material in by engulfing it in a membrane vesicle (phagocytosis for solids, pinocytosis for liquids), and exocytosis releases material such as digestive enzymes or neurotransmitters by fusing a vesicle with the membrane. Both require ATP.

Examples in context

Example 1. The sodium-potassium pump in neurones. A nerve cell's resting potential depends on active transport. The $\text{Na}^+/\text{K}^+$ pump uses ATP to move three $\text{Na}^+$ ions out and two $\text{K}^+$ ions in per cycle, both against their gradients. This maintains the ion gradients that make the membrane polarised and ready to fire, directly linking membrane transport to the nervous system.

Example 2. Glucose uptake in the small intestine. Epithelial cells lining the ileum absorb glucose by co-transport: a carrier protein moves $\text{Na}^+$ down its gradient and uses that energy to drag glucose in against its gradient. The sodium gradient is itself maintained by active transport, so glucose absorption ultimately depends on ATP, showing how passive and active mechanisms combine in real tissues.

Try this

Q1. State two factors that increase the rate of simple diffusion across a membrane. [2 marks]

• Cue. Steeper concentration gradient, larger surface area (or shorter distance, higher temperature).

Q2. Explain why active transport stops when a cell is treated with a respiratory inhibitor. [2 marks]

• Cue. Active transport needs ATP from respiration; the inhibitor stops ATP production, so no energy is available to move substances against the gradient.

Q3. A red blood cell is placed in pure water. Predict and explain what happens to it. [3 marks]

• Cue. Pure water has a higher water potential than the cytoplasm, so water enters by osmosis; the cell has no cell wall, so it swells and bursts (haemolysis).