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Diffusion and osmosis both move substances down a concentration gradient, from where there is more to where there is less, and neither needs energy. But cells often need to do the opposite — to absorb a substance that is already more concentrated inside the cell than outside. Moving a substance "uphill" like this requires energy and a special process: active transport. This lesson, part of Topic B2 of OCR Gateway Science A, explains active transport, gives its two key examples, and pulls together the three transport processes — diffusion, osmosis and active transport — in one comparison you must know.
By the end of this lesson you should be able to define active transport, explain why it needs energy from respiration, give examples in root hair cells and the gut, and compare diffusion, osmosis and active transport.
Active transport is the movement of substances across a membrane against a concentration gradient — that is, from a lower concentration to a higher concentration — using energy released by respiration.
Two ideas are at the centre of this definition:
flowchart LR
A["Low concentration<br/>outside the cell"] -->|"energy from respiration"| B["Membrane"]
B --> C["High concentration<br/>inside the cell"]
Exam Tip: The two banker marks for defining active transport are against the concentration gradient (low to high) and using energy from respiration. If you only say "substances move into the cell", you have not distinguished it from diffusion — the against the gradient and energy points are essential.
It helps to think about why active transport must cost energy. Particles naturally spread out from high to low concentration — that is diffusion, and it happens by itself with no energy input. To move particles the other way, gathering them from a low concentration and concentrating them where there is already plenty, goes against this natural tendency, rather like rolling a ball uphill. Work has to be done, and that work needs energy.
The energy is supplied by respiration, the process that transfers energy from glucose in every living cell. This explains a structural feature you have already met: cells specialised for active transport, such as root hair cells and the cells lining the small intestine, contain large numbers of mitochondria (the site of aerobic respiration) to provide the energy their active transport needs. Spotting "many mitochondria" in a cell is therefore a strong clue that the cell does a lot of active transport.
This also gives a way to test whether a substance is being moved by active transport rather than diffusion. If you slow down or stop respiration — for example by lowering the temperature or by depriving the cells of oxygen — then active transport slows or stops too, because its energy supply has been cut off. Diffusion and osmosis, which need no energy, are not stopped in this way (though they may slow a little with temperature). So an experiment showing that a cell stops taking up a substance when respiration is blocked is good evidence that the uptake was by active transport. This dependence on respiration is the single most important feature that sets active transport apart from the two passive processes.
Exam Tip: A useful way to remember the link is: active transport depends on respiration; diffusion and osmosis do not. If anything that stops respiration (no oxygen, very low temperature, a poison) also stops the uptake of a substance, that substance was being moved by active transport.
Plants need mineral ions (such as nitrate ions for making proteins) from the soil. The problem is that these ions are often more concentrated inside the root hair cell than in the surrounding dilute soil water. Diffusion would move them the wrong way — out of the cell. So the root hair cell uses active transport to absorb mineral ions against the concentration gradient, from the dilute soil into the more concentrated cell, using energy from respiration. This is exactly why root hair cells contain many mitochondria, as you saw when studying specialised cells.
(Note the contrast with water, which the root hair cell absorbs by osmosis, not active transport, because water moves down its concentration gradient into the cell.)
After a meal, digested food is absorbed from the small intestine into the blood. Much of the glucose is absorbed by diffusion, but a problem arises when most of the glucose has already been absorbed: the concentration of glucose in the gut can fall below the concentration in the blood. At that point diffusion would carry glucose the wrong way, back into the gut, and valuable glucose would be lost. To prevent this, the cells lining the small intestine use active transport to absorb the remaining glucose against the concentration gradient, from the gut (low) into the blood (high), using energy from respiration. This ensures that all the glucose is absorbed and none is wasted.
Exam Tip: Both classic examples follow the same logic: a useful substance (mineral ions, or glucose) needs to be absorbed even when it is more concentrated inside than outside, so active transport moves it against the gradient using energy from respiration. Learn one example thoroughly and you can adapt it to the other.
This comparison is the single most exam-important part of the lesson. Learn it so you can pull out any row.
| Diffusion | Osmosis | Active transport | |
|---|---|---|---|
| What moves | Any small particles (e.g. oxygen, CO₂, glucose) | Water only | Dissolved substances (e.g. mineral ions, glucose) |
| Direction | Down the gradient (high → low) | Down the water gradient (dilute → concentrated) | Against the gradient (low → high) |
| Needs a membrane? | Not necessarily | Yes — a partially permeable membrane | Yes |
| Energy needed? | No (passive) | No (passive) | Yes — from respiration |
| Example | Oxygen into a cell | Water into a root hair cell | Mineral ions into a root hair cell; glucose in the gut |
The pattern to remember: diffusion and osmosis are both passive and go down a gradient; active transport is the odd one out — it goes against the gradient and needs energy from respiration. Osmosis is really just a special case of diffusion (diffusion of water through a partially permeable membrane).
Exam Tip: A very common question gives a situation and asks which process is at work. Decide using two questions: (1) Which way is the substance moving — down the gradient (diffusion/osmosis) or against it (active transport)? (2) Is it water (osmosis) or something else? Those two checks identify the process every time.
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