Active transport is the movement of substances against their concentration gradient — from a region of lower concentration to higher concentration. Unlike diffusion, it requires energy in the form of ATP, supplied by mitochondria. Examples include the absorption of glucose in the gut and mineral ion uptake by plant roots.
What is active transport?
Active transport is a type of transport across a cell membrane that moves substances against their concentration gradient — from a region of lower concentration to one of higher concentration. This is the opposite direction to diffusion.
Because moving against a concentration gradient is not spontaneous, it requires energy. That energy comes from ATP (adenosine triphosphate), produced by cellular respiration in the mitochondria. Cells that carry out intensive active transport — such as root hair cells and intestinal epithelial cells — therefore contain large numbers of mitochondria.
How does active transport work at the molecular level?
Active transport uses specialised carrier proteins embedded in the cell membrane:
- The substance (e.g. a glucose molecule or mineral ion) binds to a carrier protein on one side of the membrane.
- ATP transfers a phosphate group to the carrier protein, causing it to change shape.
- This shape change moves the substance through the membrane and releases it on the other side.
- The carrier protein returns to its original shape, ready to transport another molecule.
Because the carrier protein must change shape using ATP, active transport is an energy-requiring (active) process — unlike diffusion and osmosis, which are passive and need no ATP.
What are the key biological examples of active transport?
| Example | Substance transported | Why active transport is needed |
|---|---|---|
| Small intestine | Glucose from gut lumen into blood | Glucose concentration in blood is already high after a meal |
| Root hair cells | Nitrate, potassium, and phosphate ions from soil | Mineral ions are far more concentrated inside the root than in the soil |
| Kidney tubules | Glucose from filtrate back into bloodstream | Prevents valuable glucose being lost in urine |
| Nerve cells | Sodium and potassium ions across membranes | Re-establishes resting potential after a nerve impulse |
The gut example is particularly important at GCSE: after a large meal, glucose concentration in the gut lumen may fall below that in the blood. Diffusion would then push glucose the wrong way — active transport overrides this and continues absorption regardless.
How does active transport compare to diffusion and osmosis?
| Feature | Diffusion | Osmosis | Active transport |
|---|---|---|---|
| Direction of movement | High → low concentration | High → low water potential | Low → high concentration |
| Energy (ATP) required | No | No | Yes |
| Carrier protein required | Not always | No (aquaporins) | Yes |
| Substance transported | Any small molecule | Water only | Specific ions or molecules |
Exam tip: if a question describes a cell with many mitochondria, it is signalling that active transport is occurring — mitochondria produce the ATP that powers the carrier proteins.
Why do root hair cells and gut epithelial cells have many mitochondria?
Both cell types carry out continuous and intensive active transport:
- Root hair cells absorb nitrate, potassium, and phosphate ions from the soil solution. These minerals are present at very low concentrations in the soil relative to the inside of the root. Without active transport, the plant could not obtain the mineral ions it needs to synthesise proteins, chlorophyll, and DNA.
- Intestinal epithelial cells absorb glucose and amino acids from digested food. Even when blood glucose is already relatively high, the gut must absorb as much as possible — active transport keeps the process running.
In both cases, many mitochondria provide the ATP needed to power the carrier proteins. This is a classic structure–function relationship: the cell's structural feature (high mitochondria count) directly enables its function (active transport).
How do you identify active transport in an exam question?
Look for these clues in the question:
- Movement is against the concentration gradient (from low to high concentration)
- The cell contains many mitochondria
- The process requires energy or uses ATP
- Adding a metabolic inhibitor (which blocks respiration and ATP production) stops the transport
- The rate of transport is higher than can be explained by diffusion alone
If a substance moves against its gradient — and continues even when diffusion would push it the other way — active transport must be involved.
Frequently asked questions
What is the difference between active transport and diffusion?
Diffusion moves substances from a region of high concentration to one of low concentration; it is passive and requires no energy. Active transport moves substances from low to high concentration (against the gradient), using ATP and specific carrier proteins embedded in the cell membrane. Both occur across membranes, but only active transport can work against a concentration gradient.
Why does active transport need ATP?
Moving a substance against its concentration gradient is not spontaneous — it requires an energy input, just as pushing a ball uphill requires effort. ATP provides that energy: when a phosphate group is transferred from ATP to a carrier protein, the protein changes shape and transports the substance across the membrane. Without ATP (for example, if cellular respiration is blocked by a metabolic poison), active transport stops completely.
What happens to active transport if you add a metabolic inhibitor?
A metabolic inhibitor such as cyanide blocks cellular respiration, cutting off ATP supply. Without ATP, the carrier proteins cannot change shape, so active transport ceases. Diffusion and osmosis, being passive, are unaffected by the inhibitor. This is the key experimental test that distinguishes active transport from passive processes and is regularly examined in GCSE required practical questions.
Why is active transport important in the kidneys?
During ultrafiltration in the kidneys, glucose passes from the blood into the tubule fluid (filtrate). Because glucose is valuable, virtually all of it must be reabsorbed. Tubule cells use active transport to move glucose from the filtrate back into the blood — even when glucose concentration in the filtrate is already low. People with diabetes may have blood glucose so high that their tubule cells cannot keep up, allowing glucose to appear in their urine.
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