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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 takes energy and a special process: active transport. This lesson, part of Topic B2 of your OCR Gateway Combined Science course, 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.
This lesson builds AO1 (understanding of active transport and why it needs energy) and AO2 (applying it to the root-hair and gut examples and comparing the three transport processes).
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 sit 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 packing them in 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.
So far we have said that active transport moves substances against the gradient using energy, but how does the membrane actually do it? The job is done by special carrier proteins built into the cell membrane. Each carrier protein is shaped to fit a particular substance — for example a particular mineral ion. The substance binds to the carrier protein on one side of the membrane; energy from respiration is then used to change the shape of the protein, which flips the substance across the membrane and releases it on the other side. The protein then returns to its original shape, ready to carry another particle. Because each "flip" is powered by respiration, the cell can keep moving the substance across even when it is moving from a low concentration to a high one — something that could never happen by diffusion alone.
This mechanism explains two things you already know. First, it is why active transport needs energy: changing the carrier protein's shape against the gradient is work, and work needs energy. Second, it is why active transport can be selective — because each carrier protein only fits certain substances, the cell can choose to take up exactly the ions or molecules it needs (such as nitrate ions for making proteins) and leave others behind. Diffusion, by contrast, cannot pick and choose in this way; anything small enough simply moves down its own gradient.
Exam Tip: At GCSE you do not need fine detail of the carrier proteins, but stating that active transport uses carrier proteins in the membrane and energy from respiration to move a substance against its concentration gradient is a strong, complete answer. The word "selective" — the cell takes up only the substances it needs — is a useful bonus point.
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 makes sure 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.
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