Diffusion in biology for A-Level
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of the random motion of those particles. It is a passive process, meaning it does not require ATP, and it continues until equilibrium is reached.
This guide covers the A-Level definition AQA expects in mark schemes, Fick's law and the factors affecting rate, the difference between simple and facilitated diffusion, and the worked-example calculations examiners reward.
Passive and random
Diffusion does not require energy. It happens because particles are constantly moving in random directions.
Fick's law for rate
Rate of diffusion is proportional to (surface area × concentration gradient) divided by diffusion distance.
Two types in cells
Simple diffusion (through the bilayer) and facilitated diffusion (through protein channels or carriers).
Defining diffusion at A-Level
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. It is driven by the random kinetic energy of the particles, requires no metabolic energy, and continues until the concentrations are equal (dynamic equilibrium).
The word net is important. Particles continue to move in both directions even after equilibrium, but there is no overall change in concentration because the rates in each direction are equal. Examiners deduct marks for definitions that leave out net or random.
Why diffusion is passive Diffusion does not require ATP because particles are already moving as a consequence of their kinetic energy. The energy comes from the particles themselves, not from the cell. Active transport, in contrast, uses ATP to move particles against a concentration gradient.
Fick's law: The factors affecting rate
Fick's law gives the rate of diffusion across a membrane as a proportion of three variables: Surface area, concentration gradient, and diffusion distance. The relationship is summarised in the equation below, which AQA expects you to be able to apply qualitatively even though you do not have to calculate exact values.
| Factor | Effect on rate of diffusion | Biological example |
|---|---|---|
| Surface area | Larger surface area increases rate | Villi in the small intestine; alveoli in the lungs |
| Concentration gradient | Steeper gradient increases rate | Counter-current flow in fish gills maintaining a gradient |
| Diffusion distance | Shorter distance increases rate | Alveolar walls are one cell thick |
| Temperature | Higher temperature increases rate | Warmer body environments speed up diffusion |
| Particle size | Smaller particles diffuse faster | Oxygen diffuses faster than glucose |
Reading Fick's law in the exam If a question asks why a tissue is well adapted for diffusion, look for the three Fick's law factors. Large surface area, short diffusion distance, and a maintained concentration gradient (often through good blood supply). Three points, three marks, every time.
Simple diffusion through the phospholipid bilayer
Simple diffusion is the movement of small, non-polar or uncharged molecules straight through the phospholipid bilayer. Examples include oxygen (O₂), carbon dioxide (CO₂), nitrogen (N₂) and small lipid-soluble molecules like steroid hormones.
These molecules can cross because the hydrophobic core of the bilayer (the fatty acid tails) does not repel them. Polar and charged molecules cannot pass through because the hydrophobic tails block them. This is why ions and large polar molecules need protein channels.
Facilitated diffusion through membrane proteins
Facilitated diffusion is the movement of polar molecules and ions across a membrane through specific transport proteins, down a concentration gradient. It is still passive (no ATP) but uses channel proteins for ions and small polar molecules, and carrier proteins for larger polar molecules like glucose.
Channel proteins form water-filled pores that ions pass through. Carrier proteins bind the molecule on one side of the membrane, change shape, and release it on the other side. Both are specific: A glucose carrier will only carry glucose.
| Feature | Simple diffusion | Facilitated diffusion |
|---|---|---|
| Energy required | None (passive) | None (passive) |
| Route across membrane | Through phospholipid bilayer | Through channel or carrier proteins |
| Particles moved | Small, non-polar (O₂, CO₂) | Polar molecules and ions (glucose, Na⁺) |
| Direction | Down concentration gradient | Down concentration gradient |
| Saturates? | No, increases with concentration gradient | Yes, limited by number of proteins |
Adaptations for fast diffusion in living organisms
Exchange surfaces in animals and plants are adapted to maximise the Fick's law variables. AQA wants you to apply this principle to specific examples in the exam. The three classic examples are alveoli in the lungs, villi in the small intestine, and gills in fish.
Diffusion adaptations: Three named examples
Memorise one example per organ and link it explicitly to a Fick's law factor.
- Alveoli: Millions of tiny sacs give a huge surface area
- Alveoli: Walls one cell thick, plus a one-cell-thick capillary wall, minimise diffusion distance
- Alveoli: Good blood supply removes oxygenated blood and maintains the concentration gradient
- Villi: Folded surface and microvilli increase surface area in the small intestine
- Villi: Rich blood supply moves absorbed glucose away, maintaining the gradient
- Fish gills: Lamellae give a large surface area
- Fish gills: Counter-current flow keeps a steep concentration gradient along the entire gill
- Fish gills: Thin gill epithelium reduces diffusion distance
Why size matters: Surface area to volume ratio
Small organisms (like amoebae and flatworms) can rely on diffusion across their body surface alone, because their surface area to volume ratio is very high. As organisms get larger, the ratio falls, and diffusion across the outer surface is no longer fast enough to supply every cell.
This is why large multicellular organisms have specialised exchange surfaces (lungs, gills, gut) and transport systems (blood, xylem, phloem). Linking surface area to volume ratio to the need for exchange surfaces is a classic 6-mark question theme.
Surface area to volume ratio: A common pitfall Students often state that the ratio decreases as size increases, but stop there. To pick up full marks you must explain the consequence: A lower ratio means diffusion across the body surface alone cannot meet the metabolic demand of every cell, so specialised exchange surfaces and mass transport systems are required.
Worked example: Calculating rate change
Question: A cell increases its surface area by a factor of 3 and the concentration gradient is doubled. The diffusion distance stays the same. By what factor does the rate of diffusion change?
Step 1: Apply Fick's law. Rate is proportional to (surface area × concentration gradient) divided by distance.
Step 2: Substitute the changes. Rate change = (3 × 2) / 1 = 6.
The rate of diffusion increases by a factor of 6. This kind of proportional reasoning question turns up in synoptic papers and is worth practising with different combinations.
Key facts to memorise for the exam
- Definition: Net movement of particles from higher to lower concentration, down a gradient, as a result of random motion
- Diffusion is passive (no ATP) and continues until dynamic equilibrium
- Fick's law: Rate is proportional to (surface area × concentration gradient) ÷ diffusion distance
- Simple diffusion: Small, non-polar molecules (O₂, CO₂) through the phospholipid bilayer
- Facilitated diffusion: Polar molecules and ions via channel or carrier proteins
- Facilitated diffusion saturates because protein numbers are limited
- Alveoli, villi and gills are the three classic exchange-surface examples
- Surface area to volume ratio falls with size, so large organisms need specialised exchange surfaces
