Diffusion, osmosis and active transport
Cells must constantly exchange substances with their surroundings — oxygen in, carbon dioxide out, water balanced, nutrients absorbed. Three processes do the work, and you must know exactly when each applies.
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is passive — no energy from respiration is needed; it just relies on the random motion of particles.
In cells, diffusion supplies oxygen and glucose to respiring tissues and removes CO₂ and urea.
The rate of diffusion depends on:
- the concentration gradient (steeper → faster)
- the temperature (higher → particles have more KE → faster)
- the surface area of the membrane (bigger → more places for particles to cross)
Fick's-law-style examination questions reward stating all three factors.
Osmosis
Osmosis is the diffusion of water through a partially permeable membrane from a dilute solution (high water concentration) to a more concentrated solution (low water concentration).
It is also passive. Wording is precise — talking about "water moving from low to high concentration" generally loses marks; instead refer to dilute → concentrated, or use water potential (HT-friendly: water moves from high water potential to low water potential).
If a plant cell is placed in pure water, water enters by osmosis until the cell becomes turgid (vacuole pushes the cytoplasm against the cell wall). Animal cells in pure water have no wall and can lyse. In a concentrated solution, water leaves by osmosis: plant cells become flaccid then plasmolysed; animal cells shrivel (crenate).
Active transport
Active transport moves substances against a concentration gradient, from a low to a high concentration. This needs energy from respiration (transferred by ATP). Carrier proteins in the membrane act as molecular pumps.
Two GCSE examples to memorise:
- Root hair cells absorb mineral ions from soil where the ion concentration is much lower than in the cell.
- Cells lining the small intestine absorb glucose from the gut into the blood when blood glucose is already higher than gut glucose (after most of a meal has been absorbed).
Required practical — osmosis with potato
Cylinders of potato (cut to equal length and dried) are placed in salt or sucrose solutions of different concentration. After ~30 min, dry and remeasure mass. Percentage change in mass = ((final − initial) / initial) × 100. Plot a graph; the concentration where % change = 0 is the concentration where the water potential of the solution equals that of the potato.
Surface area to volume ratio
Single-celled organisms (e.g. amoeba) have a high SA:V — they exchange enough by simple diffusion across the membrane. As organisms grow larger, SA:V falls (volume grows faster than surface area), so they evolve specialised exchange surfaces (alveoli, villi, gills) — see B2.
⚠Common mistakes— Common mistakes / exam traps
- Calling osmosis "diffusion of water from low to high concentration" — must say dilute to concentrated (or solute terms).
- Saying active transport doesn't need energy because the carrier protein is "just a channel". Active transport always requires energy from respiration.
- Confusing turgid with plasmolysed — turgid = full of water, firm; plasmolysed = membrane pulled away from wall.
- Forgetting units in % change calculations — final mass − initial mass divided by INITIAL.
Links
Surface area to volume connects to B2.2 / B2.3 (specialised exchange surfaces) and B4.1 (gas exchange in plants).
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