Cambridge O Level Biology · Syllabus 5090 · Movement Into and Out of Cells
Diffusion
What is Diffusion?
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, as a result of random movement. It requires no energy from the cell: the driving energy is the kinetic energy of the moving particles themselves. Diffusion supplies oxygen to respiring cells, removes carbon dioxide, and moves dissolved food molecules across exchange surfaces such as the alveoli and the villi.
This definition is part of the Movement Into and Out of Cells chapter in Cambridge O Level Biology.
Diffusion in context
Every cell is surrounded by a membrane, and everything a cell needs must cross it. Three processes do the crossing. Diffusion is the net movement of molecules or ions from higher to lower concentration, down a concentration gradient, as a result of random movement. Osmosis is the same random movement applied to water molecules only, from higher to lower water potential, through a partially permeable membrane. Neither of those two costs the cell anything — the energy comes from the kinetic energy the particles already have. Active transport is the exception: it drags molecules or ions against a concentration gradient, from lower to higher concentration, through a cell membrane, using energy released during respiration. Read every question in this topic by asking three things in order: which particle is moving, which way relative to the gradient, and who is paying for it?
Water acts as a solvent in living organisms. Substances that dissolve in it can be transported in a liquid, can react in solution, and can be excreted dissolved in a liquid. Digested food must dissolve before it can be absorbed; blood plasma carries dissolved glucose, amino acids, ions, hormones, carbon dioxide and urea; urine removes dissolved urea and excess ions; and plant xylem carries water containing dissolved mineral ions. Every process in the rest of this chapter — diffusion, osmosis and active transport — assumes that the substance being moved is already in solution.
Molecules and ions possess kinetic energy and are in constant, random motion. Each particle travels until it collides, then moves off in a new direction; no particle knows where the concentration is lower and none is aiming for it. But where particles are crowded, more of them happen to wander out of that region than happen to wander in, so the population as a whole shows a net movement from higher to lower concentration. Because the driving energy is the particles’ own random motion, the cell supplies no energy for diffusion or for osmosis.
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, as a result of random movement. The word net tells you that particles travel both ways and that only the imbalance is diffusion. The phrase molecules or ions tells you that both uncharged particles such as oxygen and charged ones such as sodium ions diffuse. The phrase as a result of random movement tells you where the energy comes from — the particles themselves, never the cell.
Four changes make diffusion faster: a larger surface area, because more particles can cross at the same time; a higher temperature, because particles gain kinetic energy and move faster; a steeper concentration gradient, because the difference between the two regions is greater so the imbalance in crossings is greater; and a shorter diffusion distance, because each particle has a shorter route to travel. Learn each one as a three-part chain — change → molecular mechanism → effect on net rate — because that is exactly how the marks are allocated.
Active transport is the movement of molecules or ions through a cell membrane from a region of lower concentration to a region of higher concentration, against a concentration gradient, using energy released during respiration. Diffusion and osmosis can only take a substance downhill. When a cell needs something that is already more concentrated inside than outside — nitrate ions in a root hair, glucose in the last stretch of the small intestine, sodium ions in a kidney tubule — the only way to get it is to spend energy moving it uphill. That energy is released by respiration; the mitochondria transfer it, they do not manufacture it.
Common mistakes with Diffusion
- 2. “Osmosis is just water diffusing, so it does not need a membrane.” Why wrongWithout a partially permeable barrier the solute diffuses as freely as the water and nothing accumulates on either side. The membrane is what makes the water movement one-sided, and it is part of the definition. CorrectOsmosis is a special case of diffusion in which the particle is water and the barrier is a partially permeable membrane. Both conditions are required. Say this“Osmosis requires a partially permeable membrane, which allows water molecules through but not the solute.” CheckTwo sucrose solutions of different concentration are mixed in an open beaker. Is that osmosis? (No — there is no partially permeable membrane; the sucrose and the water simply diffuse.)
- 3. “Diffusion needs energy from the cell.” Why wrongDiffusion happens perfectly well in a beaker with no cells in it, and continues in dead tissue. Nothing is spending energy on it. CorrectThe energy is the kinetic energy the particles already have. Their random movement produces the net movement without any contribution from the cell. Say this“Diffusion results from the random movement of particles and requires no energy from the cell.” CheckCyanide stops respiration. Which of diffusion, osmosis and active transport does it stop? (Only active transport.)
- 9. “Active transport moves substances from high concentration to low concentration.” Why wrongThat is a description of diffusion, and it makes the energy requirement pointless — nothing needs to be paid for when a substance runs downhill. CorrectActive transport moves substances from lower to higher concentration, against the concentration gradient. Say this“Active transport moves ions against the concentration gradient, from a lower to a higher concentration.” CheckWhy can a root hair cell not absorb nitrate ions from dilute soil by diffusion? (The concentration inside is already higher, so diffusion would move them out.)
- 14. “The agar block is a model cell, so the acid enters the cell.” Why wrongAgar has no membrane, no cytoplasm and no respiration. It models the geometry of diffusion and nothing else. CorrectThe agar block models how surface-area-to-volume ratio and diffusion distance affect the time taken for a substance to reach the centre of an object. Say this“The agar block is a model of diffusion into an object; it does not model a membrane or any transport requiring energy.” CheckName one thing this model cannot demonstrate. (Active transport, selective permeability, or osmosis.)
Questions students ask about Diffusion
What is the difference between diffusion and osmosis?
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, as a result of random movement. Osmosis is the net movement of water molecules only, from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. Both are driven by the random movement of the particles themselves and need no energy from the cell; the restriction to water and the need for a partially permeable membrane are what make osmosis a special case.
Does diffusion need energy from the cell?
No. Diffusion happens in a beaker with no cells in it and continues in dead tissue. The energy comes from the kinetic energy that molecules and ions already possess, which makes them move randomly; where particles are crowded, more of them wander outwards than inwards, giving net movement down the concentration gradient. Only active transport uses energy released during respiration, which is why a respiratory inhibitor reduces mineral-ion uptake but does not stop diffusion or osmosis.