Sciences

Active Transport: Moving Substances Uphill

Active transport moves substances against the concentration gradient using energy from respiration. Root hair cells and the small intestine, explained.

Active transport

Active transport is the movement of substances across a cell membrane against the concentration gradient, using energy released by respiration.

Où les élèves le rencontrent: Grade 10 science, immediately after diffusion and osmosis, and again wherever root hair cells or absorption in the small intestine are examined at IGCSE level.

La réponse en bref
Active transport moves substances across a cell membrane from a lower concentration to a higher one, against the concentration gradient, using carrier proteins and energy released by respiration. It is how root hair cells absorb mineral ions from very dilute soil water and how the small intestine absorbs the last of the glucose.

Un exemple

low concentration outside → high concentration inside (against the gradient)

Soil water contains mineral ions such as nitrate, but at a much lower concentration than the cell sap inside a root hair cell. Diffusion would therefore pull nitrate out of the root, not into it. The root hair cell moves the ions inwards anyway, using carrier proteins in its membrane and energy from respiration, which is why these cells are packed with mitochondria. Without nitrate the plant cannot make amino acids, and without amino acids it cannot make protein or grow.

Going the wrong way is what costs energy

Diffusion and osmosis run downhill. Particles are already moving, the gradient decides where they pile up, and the cell pays nothing. Active transport runs the other way, and moving something against the direction it would naturally go can never be free.

The energy comes from respiration, which is the reason cells that do a great deal of active transport contain unusually many mitochondria. The mechanism is a carrier protein embedded in the membrane: it binds the particular molecule or ion on one side, changes shape using the energy supplied, and releases it on the other. Each carrier handles specific substances, so a cell controls what it takes in by controlling which carriers it makes.

One consequence is worth remembering because it separates the processes cleanly in experiments. Deprive a root of oxygen, or poison its respiration, and active transport slows and stops while diffusion and osmosis carry on unaffected.

The two examples questions expect by name

Root hair cells absorb mineral ions from the soil. The soil solution is far more dilute in those ions than the cell is, so the gradient points the wrong way and only active transport gets them in. The long thin extension of a root hair cell is there to increase surface area, and the mitochondria inside it are there to supply the energy.

The small intestine absorbs glucose into the blood. Early in digestion the gut contains more glucose than the blood does and diffusion is enough. Later, once most of it has been absorbed, the gut concentration falls below the blood concentration and diffusion would send glucose back the wrong way. Active transport takes up what remains, which is why almost none of the sugar in a meal is lost.

Recognising it in a question

Questions rarely name the process; they describe a situation and expect you to. A handful of phrases give it away, and one of them is nearly always present.

The test also works backwards. If the substance ends up less concentrated where it arrived than where it started, no energy was required and the answer is diffusion — or osmosis, if what moved was water across a membrane.

"Against the concentration gradient", or "from a low to a high concentration"
"Requires energy", "uses ATP", or a mention of many mitochondria in the cell
"Carrier proteins" in the membrane
A description in which absorption continues even though the concentration outside is lower than inside
A result showing that absorption stops when oxygen is removed or respiration is inhibited

Questions fréquentes

What is the difference between diffusion and active transport?

Direction and cost. Diffusion moves particles down a concentration gradient and uses no energy from the cell. Active transport moves them up the gradient, from low concentration to high, and needs energy from respiration together with carrier proteins in the membrane. If the substance ends up more concentrated where it arrived, the process cannot have been diffusion.

Where does the energy for active transport come from?

From respiration, which releases energy from glucose and stores it in ATP for the carrier proteins to use. This is why cells specialised for absorption, such as root hair cells and the cells lining the small intestine, contain far more mitochondria than average. A photograph showing dense mitochondria is a strong hint that active transport is being tested.

Does active transport happen in plants?

Yes, and the root hair cell is the standard example: mineral ions are taken from a soil solution far more dilute than the root itself. Plants also use active transport to load sugars into the phloem for transport around the plant, though that detail goes beyond most syllabuses at this level.

What happens to active transport if a cell runs out of oxygen?

It slows and then stops. Without oxygen a cell can only respire anaerobically, which releases much less energy, so the carrier proteins run short of the ATP they depend on. Diffusion and osmosis continue regardless, because they never needed the cell's energy in the first place. Waterlogged soil harms roots for exactly this reason.

Dernière mise à jour 21 août 2026