The fluid mosaic model of the cell membrane, diffusion, osmosis and active transport, the role of water as a solvent and its properties, and the features of efficient exchange surfaces.

What this dot point is asking

Edexcel wants you to describe the fluid mosaic model of the cell membrane, explain diffusion, osmosis and active transport, describe the properties of water that make it a good solvent and transport medium, and explain the features that make a gas-exchange surface efficient. Osmosis calculations on potato or plant tissue are a recurring practical-based question.

The fluid mosaic model

Cholesterol fits between phospholipids and regulates fluidity; proteins act as channels, carriers, receptors and enzymes.

Movement across membranes

• Diffusion: the net movement of particles from a region of high to low concentration, down a gradient, passively. Small, non-polar molecules such as oxygen and carbon dioxide cross the bilayer directly.
• Facilitated diffusion: diffusion of polar or charged molecules through channel or carrier proteins, still down the gradient and without ATP.
• Osmosis: the net movement of water across a partially permeable membrane from a higher (less negative) to a lower (more negative) water potential.
• Active transport: movement against the concentration gradient using carrier proteins and ATP.

Properties of water

Water is a polar molecule that forms hydrogen bonds. This makes it a good solvent for ions and polar molecules (so it transports substances in blood and cytoplasm), gives it a high specific heat capacity (stabilising temperature) and high cohesion (helping transport in xylem).

Efficient exchange surfaces

Surfaces such as the alveoli are efficient because they have a large surface area, a thin barrier (short diffusion distance), and a steep concentration gradient maintained by ventilation and a good blood supply. Fick's law summarises this: $\text{rate of diffusion} \propto \frac{\text{surface area} \times \text{concentration difference}}{\text{diffusion distance}}$. So a bigger area, a steeper gradient and a thinner barrier all speed diffusion.

Small organisms exchange gases over their whole body surface because their surface area to volume ratio is large. As organisms get bigger this ratio falls, so they need specialised exchange surfaces (lungs, gills) and a transport system to keep gradients steep.

Examples in context

Example 1. The alveoli. Each lung has hundreds of millions of alveoli, giving a total surface area of around $70 \text{ m}^2$. The alveolar wall and capillary wall are each one cell thick, so the diffusion distance is under a micrometre. Breathing constantly replaces alveolar air and blood flow constantly removes oxygenated blood, keeping a steep oxygen gradient. Every term in Fick's law is maximised, which is why this is the standard exam example of an efficient surface.

Example 2. Active transport in the gut. Glucose is absorbed from the small intestine even when its concentration in the gut is lower than in the blood. Sodium ions are pumped out of the epithelial cells by active transport (using ATP), creating a sodium gradient that drags glucose in by co-transport. This shows active transport moving a substance against its gradient at the expense of ATP, a favourite contrast with passive diffusion.

Try this

Q1. Explain why oxygen can diffuse directly through the phospholipid bilayer but glucose cannot. [2 marks]

• Cue. Oxygen is small and non-polar so it crosses the hydrophobic tails; glucose is large and polar, so it needs a transport protein.

Q2. State two ways an exchange surface is adapted for rapid diffusion. [2 marks]

• Cue. Large surface area and thin barrier (short diffusion distance); a steep gradient maintained by blood flow.