Active transport
Moving substances against a concentration gradient using energy.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 3.2.1Define active transport and explain its importance, including ion uptake by root hairs.Supplement
4 minute read
Active transport
Diffusion and osmosis only ever move substances down a gradient. But cells often need the opposite — to hoard something that is already scarce outside and plentiful inside.
The definition
Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration — that is, against a concentration gradient — using energy from respiration and carrier proteins in the membrane. Three things distinguish it from diffusion: the direction is against the gradient, it requires energy, and it needs specific protein carriers.
Why organisms need it
Root hair cells absorb mineral ions such as nitrate from the soil, where the concentration is very low, into the cell where it is already higher. Diffusion cannot do this, so the root hair cell uses active transport — and this is exactly why root cells contain many mitochondria. The same applies to glucose absorption in the small intestine, ensuring none is wasted even when the concentration in the gut falls below that in the blood.
The consequence of needing energy
Because active transport depends on respiration, anything that stops respiration stops it. A plant in waterlogged soil cannot get oxygen to its roots, aerobic respiration slows, less energy is available, and mineral ion uptake falls — so the plant becomes deficient even though the minerals are present.
Think of it like this
Diffusion is water flowing downhill on its own; active transport is pumping water uphill. The pump needs a power supply, and if you cut the power the water stops moving — no matter how much water there is.
Worked examples
Method, step by step
Root hair cells contain many mitochondria. Explain how this relates to mineral ion uptake.
- 1Mineral ions in soil are at a lower concentration than inside the root hair cell.
- 2They must therefore be absorbed by active transport, against the concentration gradient.
- 3Active transport requires energy released by aerobic respiration, which occurs in mitochondria.
Mineral ions are absorbed by active transport against a concentration gradient, which needs energy from aerobic respiration — so many mitochondria are present to supply it.
Common misconceptions
- Thinking active transport is just "fast diffusion". The defining feature is direction — against the gradient — not speed.
- Forgetting to mention carrier proteins, which is usually a separate marking point from the energy requirement.
- Not connecting it to respiration. Questions about waterlogged soil or a respiratory inhibitor are really testing whether you know active transport needs energy from respiration.
In the exam
- A full-mark definition needs three elements: against the concentration gradient, using energy from respiration, via carrier proteins in the membrane.
- If a question mentions cyanide, lack of oxygen, or low temperature reducing uptake, the expected answer is that respiration is reduced, so less energy is available for active transport.