Active transport — the two markable clauses
Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient), using energy from respiration. The two clauses score separately: the direction (low to high, the opposite of diffusion) and the named energy source. It is therefore an active process, unlike passive diffusion and osmosis, and cells that do a lot of it carry many mitochondria.
Only active transport works against the gradient
Diffusion and osmosis are passive: they move particles only down a gradient and stop once concentrations are equal, so neither can raise a substance above its outside concentration. Active transport is the only one of the three that moves particles against the gradient, letting a cell accumulate a substance above the concentration of its surroundings. Energy from respiration powers transport (carrier) proteins that move the particles across.
(Extended) Root hair ion uptake
(Extended) Soil solution is more dilute in mineral ions — nitrate (for amino acids), magnesium (for chlorophyll), potassium — than root hair cytoplasm, so diffusion cannot supply them. Root hair cells use active transport instead: the long thin hair gives a large surface area, and many mitochondria release the energy by respiration. The same principle absorbs glucose and amino acids in the small intestine once gut concentration falls below blood.
Drawn from real examiner reports.
Dropping the gradient-direction clause
Omitting "against a concentration gradient", or writing the direction the wrong way round as "higher to lower", describes diffusion, not active transport. The direction clause (from lower to higher) is credited separately from the energy clause, so a paraphrase such as "moving particles into the cell through the membrane" scores nothing for direction.
Flagged Nov 2022 P32 Q11a · Nov 2023 P32 Q10c
"Using energy" without naming respiration
The energy source must be named as energy from respiration. "Uses energy", "energy from the cell", "energy from ATP" without mentioning respiration, and "energy from the sun" are all insufficient. Respiration is the chemical release of energy from glucose inside the cell — that is the phrase mark schemes look for, and it is what links active transport to mitochondria.
Xylem water rise is not active transport
Water rising up the xylem is driven by transpiration pull (a water-potential gradient set up by evaporation from the leaves) with the molecules held together by cohesion. It is passive and needs no energy from respiration, yet candidates routinely name active transport, osmosis, adhesion or "suction" instead.
Flagged Jun 2022 P42 Q7bi/bii · Jun 2023 P43 Q4bi · Nov 2023 P42 Q4bi
Water by osmosis, ions by active transport
Both happen at the same root hair membrane but by different mechanisms. Water enters by osmosis — down a water-potential gradient, across the partially permeable membrane, no energy needed. Mineral ions enter by active transport — against their own gradient, using energy from respiration. Do not call ion uptake osmosis, or water uptake active transport.
Cell membrane, not cell wall
Active transport takes place at the cell membrane, which controls what enters and leaves the cell and holds the transport proteins. The cell wall outside it is freely permeable and has no transport role, so it cannot select or move ions. Naming the wall as the site of transport is the single most repeated cell error on this specification.
Flagged Nov 2022 P32 Q1ai, P43 Q4ci · Jun 2023 P32 Q10aii
Root hair cell, not "root cell"
The Extended example is worked entirely through the root hair cell — a single surface cell of a root drawn out into one long, thin hair. "Root cell" is too general (most cells in a root have no hair) and "hair cell" is an unrelated cell type; neither is credited as the precise structure name.
Flagged Jun 2022 P41 Q1ai/aii
The four-step "why active transport?" scaffold
Compare the two concentrations given → state that the gradient runs the wrong way for diffusion → name active transport as the process that can work against it → give the energy source, respiration, linked to many mitochondria if a structure is named.
Two-question test before you name it
Ask (1) is the particle moving from lower to higher concentration? and (2) does the process need energy from respiration? If either answer is no, the process is diffusion, osmosis or transpiration pull — not active transport.
Match the command word
Describe = the definition only (direction plus energy source). Explain = the definition AND why it is needed here, which means the concentration comparison. Suggest, on unfamiliar data, = apply the same gradient argument to the new context.
"The cell needs it" earns nothing
Never write that active transport happens "because the cell needs the substance". Needing something is not a mechanism. The creditable idea is that the outside concentration is already lower than inside, so diffusion cannot move more of it in.
Cambridge 0654 spec reference: Section B3 "Movement into and out of cells", sub-topic B3.3 (Core + Extended). This leaf covers the definition of active transport, why it is needed (unlike diffusion, it can move particles against a concentration gradient), and (Extended) its importance for moving molecules and ions across membranes, including ion uptake by root hairs.
Out of scope for B3.3 (covered elsewhere in B3): the full definitions of diffusion and osmosis -- named only briefly below to sharpen the boundary of the active-transport definition.
| Term | Mark-scheme-precise definition |
|---|---|
| Active transport | The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient), using energy from respiration |
| Concentration gradient | The difference in the concentration of a substance between two regions (see B3.1) |
| Respiration | The chemical release of energy from glucose in living cells -- the energy source that active transport depends on |
Active transport is an active process -- unlike diffusion and osmosis (both passive), it requires the cell to spend energy, because it moves particles the "wrong way" relative to their concentration gradient.
(Extended) Why active transport matters -- ion uptake by root hairs:
Define active transport.
(a) Define active transport. (3 marks)
(b) State the source of the energy used for active transport. (1 mark)
