Transport in Cells » Active Transport

What is active transport?

Sometimes a cell needs to take in a substance that is already in short supply outside the cell. Diffusion cannot help, because diffusion only moves particles from a high concentration to a low concentration. The cell needs a way to move them the other way.

That is active transport. Active transport moves substances from a more dilute solution to a more concentrated solution. This is against a concentration gradient, so it is a bit like pushing a ball uphill. It does not happen by itself. It needs energy, and the energy comes from respiration.

Key terms:

• Active transport: the movement of substances from a more dilute solution to a more concentrated solution (against a concentration gradient), using energy from respiration.
• Mineral ions: charged particles of minerals, such as those dissolved in soil water, which plants need for healthy growth.

Going with or against the gradient

The key idea is the direction of travel. With diffusion, particles go down the concentration gradient, so no energy is needed. With active transport, particles go up the gradient, so the cell has to spend energy.

The energy comes from respiration, which happens inside the cell. A cell that is short of energy cannot carry out active transport. If the cell's respiration stops, active transport stops too.

Example 1: mineral ions in plant roots

Plants need mineral ions for healthy growth. These ions are dissolved in the water in the soil, but the solution is usually very dilute. There are often more ions inside the root hair cells than in the soil around them.

If the plant relied on diffusion alone, ions would tend to drift out of the roots into the soil, not the other way. So root hair cells use active transport to absorb mineral ions from the very dilute soil solution into the cell, against the concentration gradient. This takes energy from respiration.

Example 2: sugar in the gut

Sugar molecules from food are absorbed from the gut into the blood. After a meal there is a high concentration of sugar in the gut, so diffusion can move it. But as the sugar in the gut runs low, its concentration drops below the concentration in the blood. Diffusion would then stop, or even move sugar the wrong way.

Active transport solves this. It allows sugar molecules to be absorbed from lower concentrations in the gut into the blood, which has a higher sugar concentration. Sugar molecules are used for cell respiration, which releases energy.

Comparing the three processes

You need to describe how substances move into and out of cells by diffusion, osmosis and active transport, and explain how they differ.

All three can move substances into cells and out of cells. There are three differences to explain:

• Energy: diffusion and osmosis need no energy from respiration. Active transport does.
• Direction: diffusion goes down the concentration gradient. Active transport goes against it.
• What moves: osmosis is only about water. Diffusion and active transport move other substances, such as ions and sugar.

Exam-style question

Plant root hair cells absorb mineral ions from the soil.

(a) Name the process that moves mineral ions into root hair cells when the concentration in the soil is lower than in the cell. [1 mark]

(b) Explain why this process needs energy. [2 marks]

(c) Give two differences between this process and diffusion. [2 marks]