Active transport

Biology · Movement into and out of cells. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Sometimes a cell needs to take in a substance that is already more concentrated inside the cell than outside. Diffusion cannot do this, because diffusion only goes from high to low concentration.

Active transport moves molecules or ions through the cell membrane from a lower concentration to a higher concentration, against the concentration gradient. It needs energy, which the cell releases in respiration. Carrier proteins in the membrane pick up the particle on one side and release it on the other.

The main example is the root hair cell. Soil water contains very few mineral ions, yet the cell keeps taking them in.

Rules to remember

Active transport: movement of molecules or ions through the cell membrane from lower to higher concentration (against a concentration gradient), using energy released during respiration.
Diffusion and osmosis need no energy from the cell; active transport does.
Carrier proteins in the cell membrane carry the particles across.
Root hair cells take up mineral ions such as nitrate ions from the soil by active transport.
Anything that slows respiration slows active transport: lack of oxygen (waterlogged soil), a respiratory poison, low temperature.
Cells that carry out a lot of active transport have many mitochondria.

Common mistake

Saying that water enters roots by active transport. Water always moves by osmosis. Only the mineral ions are taken up by active transport.

Worked example

Barley roots take up phosphate ions quickly when air is bubbled through the solution, but very slowly when nitrogen gas is bubbled through instead. What does this show?

Bubbling nitrogen removes oxygen, so the root cells cannot respire aerobically.
Less respiration means less energy is released.
Uptake fell when energy was short, so the uptake depends on energy.
Diffusion does not need energy from the cell, so the process must be active transport.

Answer: Phosphate ions are taken up by active transport, which needs energy from respiration.

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