Active transport – GCSE Combined Science Biology

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Active transport is how cells move useful substances into themselves, even when it looks like the wrong direction. The idea is simple. We will take it one small step at a time.

It matters because plants need minerals from the soil and your gut needs to take in glucose. Without active transport, a plant could not get the minerals it needs to grow properly.

By the end you will be able to say what active transport is and why it needs energy. You will give examples from plants and animals. You will also compare it with diffusion and osmosis.

Uphill, not downhill

Particles in a liquid spread out. In diffusion, they move from where there are lots of them to where there are fewer. This direction is called moving down a concentration gradient. A gradient is just a difference in concentration.

Active transport does the opposite. It moves particles from a low concentration to a high concentration. We say this is against the gradient. It is like pushing a bike up a hill. You have to use energy.

Rolling down the hill is easy and free. Pushing up the hill costs effort. That is the key difference.

Where the energy comes from

Cells get energy from respiration. Respiration is a chemical reaction that releases energy from glucose. Most of it happens in the mitochondria.

Mitochondria are tiny parts inside a cell. Think of them as the cell’s power stations. They release the energy that the cell then uses to do jobs.

Active transport is one of those jobs. If a cell cannot respire, it has no energy to spare for it. That is why a lack of oxygen, which respiration needs, can stop active transport.

How the carrier works

The cell membrane has carrier proteins built into it. A carrier protein is a protein that picks up one kind of particle. A particle sticks to the carrier on one side of the membrane.

The cell then uses energy to change the shape of the carrier. This pushes the particle through to the other side. The particle is released there, even though it was already crowded.

A cell that does a lot of active transport needs a lot of energy. So it has many mitochondria.

Active transport in plants

Soil water has only a small amount of mineral ions dissolved in it. Mineral ions are tiny charged particles that plants need, such as nitrate. A plant uses nitrate to make proteins.

Root hair cells are the cells on a root that reach out into the soil. The ions are more concentrated inside these cells than in the soil. So diffusion would move them the wrong way. The cells use active transport instead.

Waterlogged soil has little oxygen. Roots cannot respire well, so less active transport happens and the plant struggles to take in ions.

Active transport in animals

In your small intestine, digested food is absorbed into the blood. Right after a meal there is lots of glucose in the gut, so glucose moves into the blood by diffusion.

Later the glucose in the gut becomes lower than in the blood. Diffusion stops working. Active transport takes over and moves the glucose uphill, so that none is wasted.

Comparing the three processes

Diffusion moves any particles down a concentration gradient. It needs no energy. It is passive, which means the cell does no work.

Osmosis is the diffusion of water only, through a partially permeable membrane. It is also passive. Water moves from a dilute solution to a more concentrated solution.

Active transport moves particles against the gradient. It is the only one of the three that is active, because it uses energy from respiration. A good clue in a question is the direction. Low to high means active transport.

Recap

Active transport moves particles from a low concentration to a high concentration. This is against the gradient.

It uses carrier proteins and energy from respiration, released in mitochondria.

Root hair cells use it for mineral ions. The gut uses it for glucose.

Diffusion and osmosis are passive. Active transport is the only one that needs energy. If uptake stops without oxygen, think active transport.