Diffusion, osmosis and active transport defined
Diffusion: particles moving down a concentration gradient (high to low); passive. Osmosis: water moving from a dilute to a concentrated solution through a partially permeable membrane; passive. Active transport: movement against the gradient (low to high) using energy released by respiration (ATP). Each term is marked on its direction, plus the energy source for active transport. For 4SD0, name water and the partially permeable membrane for osmosis.
Four factors that speed up movement into cells
The rate of movement into and out of cells rises with: (1) a steeper concentration gradient (bigger difference between the regions); (2) a larger surface-area-to-volume ratio (more membrane per unit volume); (3) a shorter diffusion path / thinner surface; and (4) a higher temperature (particles gain kinetic energy and cross faster). Exchange surfaces are therefore thin, moist and folded, and small cells (large SA:V) exchange fast enough by diffusion alone.
Surface-area-to-volume ratio of a cube
A cube of side L has surface area = 6 × L² (six faces) and volume = L³, so SA:V = 6 ÷ L, written as a ratio "_ : 1". A 2 mm cube gives SA = 24 mm², V = 8 mm³, so SA:V = 3 : 1; a 4 mm cube gives SA = 96 mm², V = 64 mm³, so SA:V = 1.5 : 1. The bigger cube has the smaller ratio, so it exchanges substances (and heat) more slowly for its size. Show SA, then V, then the ratio — each stage earns a method mark even if the final ratio is wrong.
Drawn from real examiner reports.
Osmosis direction: dilute → concentrated
Osmosis moves water from a dilute to a concentrated solution across a partially permeable membrane. Two errors lose marks: reversing the direction, and the ambiguous "more/less concentrated" without saying WHAT is concentrated — water or solute? Say "water moves dilute → concentrated". Another slip is claiming the solute entered the cell; it is water that moves.
Active transport: against gradient + energy
An active-transport answer must state BOTH that particles move against the concentration gradient (low to high) AND that it uses energy released by respiration (ATP). Leaving out either half caps the mark. Do not confuse it with diffusion, which is passive and moves particles down the gradient (high to low) with no energy input.
Diffusion vs osmosis — not the same
Diffusion is the movement of any particle (a gas or a dissolved solute) down its concentration gradient. Osmosis is specifically the movement of WATER across a partially permeable membrane. If a question asks about osmosis, always name water and the partially permeable membrane; describing "particles moving" in general does not earn the osmosis marks.
Describe vs explain a rate change
When a rate question says explain, give the biological reason, not just the trend. "The rate increased" describes the graph and earns no explanation marks. Link the change to a mechanism: a steeper concentration gradient, a larger surface-area-to-volume ratio, a shorter diffusion path or a higher temperature (more kinetic energy) makes particles cross the membrane faster.
Bigger cell does NOT exchange faster
As a cell or block gets bigger, its surface-area-to-volume ratio falls, so it exchanges substances more slowly for its size — not faster. A common error is assuming the larger cube absorbs quickest. Also, when computing SA:V for a cube, use six faces (SA = 6 × side²), not side² alone, and simplify to "_ : 1".
Plasmolysis is plant cells only
Plasmolysis is a plant cell's membrane pulling away from its cell wall after water leaves by osmosis. A red blood cell has no cell wall, so it cannot plasmolyse: in concentrated salt it shrinks/crenates, and in distilled water it bursts (lysis). Writing "plasmolysis" for an animal cell scores nothing.
Use factor → mechanism → effect
For "explain" items, build the answer as factor (what changed) → mechanism (why movement changes) → effect on rate. Example: surface area doubled → more membrane per unit volume → faster diffusion. This gives the reason the mark scheme wants, not just the trend.
Show SA, then V, then the ratio
For SA:V, write three stages so method marks survive an arithmetic slip: (1) surface area (cube = 6 × side²), (2) volume (side³), (3) the ratio, simplified to "_ : 1" by dividing both sides by the volume. Keep units consistent — do not mix mm and cm.
Use % change in mass to compare
In an osmosis practical, use percentage change in mass = (final − initial) ÷ initial × 100 so samples of different size compare fairly. A negative value means the tissue lost water to a more concentrated solution; zero change matches the cell's own concentration.
Cells exchange substances with their surroundings across the cell membrane. There are three processes to know: diffusion, osmosis and active transport. Diffusion and osmosis are passive (no energy needed); active transport needs energy from respiration.
Factors that increase the rate of diffusion / osmosis:
| Factor | Effect on rate | Why |
|---|---|---|
| Concentration gradient steeper | Faster | Bigger difference means more net movement per second |
| Surface-area-to-volume ratio larger | Faster | More membrane area per unit of volume to cross |
| Distance / membrane thinner | Faster | Shorter diffusion path to travel |
| Temperature higher | Faster | Particles have more kinetic energy and move faster |
Define osmosis.
A root hair cell absorbs mineral ions from soil water even though the ions are at a higher concentration inside the cell than in the soil.
(a) Name the process the cell uses. (1) (b) Explain why this process, and not diffusion, is needed here. (2)
