Cambridge O Level Biology · Syllabus 5090 · Movement Into and Out of Cells
Active Transport
What is Active Transport?
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. It is the only one of the three transport processes that requires energy from the cell, and the only one that can move a substance uphill. Root hair cells use it to absorb mineral ions from soil that is more dilute than their own cytoplasm, and contain many mitochondria to release the energy required.
This definition is part of the Movement Into and Out of Cells chapter in Cambridge O Level Biology.
Active Transport 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.
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 Active Transport
- 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.)
- 10. “Mitochondria make energy for active transport.” Why wrongEnergy cannot be created. Respiration releases energy that was already stored in nutrient molecules such as glucose. CorrectMitochondria are the site of aerobic respiration, in which energy is released from glucose and made available to the cell. Say this“Active transport uses energy released during respiration, which takes place in the mitochondria.” CheckWhy do cells that carry out a lot of active transport contain many mitochondria? (To release the large amount of energy required.)
- 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 Active Transport
What is active transport and why does it need energy?
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. It needs energy because it moves substances uphill, the opposite of the direction that random movement alone would produce. Root hair cells use it to absorb mineral ions from a soil solution that is more dilute than their own cytoplasm, and they contain many mitochondria to release the energy required.
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.