Active transport: moving against the flow

A problem for a root

Soil water holds only a little nitrate, a mineral that plants need to make proteins. Yet a root hair cell can hold ten times as much nitrate as the soil water around it. If diffusion were the only way in, the nitrate would stop entering as soon as the inside matched the outside. So how does the cell keep taking more?

Down the slope and up the slope

Think of the difference in concentration as a slope. Diffusion and osmosis go down the slope, from more to fewer, by themselves, like a ball rolling downhill. Moving a substance up the slope, from fewer to more, is like pushing the ball uphill. It needs energy.

Active transport is the movement of a substance through a membrane from a lower concentration to a higher concentration, using energy from the cell.

Pumps in the membrane

The membrane holds special proteins called carriers. A carrier takes hold of a particle on one side, changes its shape, and lets the particle go on the other side. Each turn uses a little energy. This energy comes from respiration in the mitochondria. A cell that does a lot of active transport, such as a root hair cell, has many mitochondria.

You should find that the blue curve stops at the dashed outside level, however long you wait, while the red curve climbs past it. Set the pump to 0, as if there were no oxygen, and the two curves become the same.

The evidence

How do we know that root cells use energy for this? Give the roots no oxygen, or a poison that stops respiration. The uptake of minerals almost stops, though the minerals are still in the water. Diffusion alone is not enough; the pump needs the energy that respiration supplies.

Try it in Python

The program follows a root cell for one hour, minute by minute: once with diffusion only, and once with the pump as well.

Three ways across a membrane

Endocytosis, which you met in Course 1.1, also uses energy: the membrane has to fold and close around the food.

Practise with new numbers

Common mistakes

• Saying active transport goes from more to fewer. It goes the other way; that is why it needs energy.
• Forgetting where the energy comes from. It comes from respiration in the mitochondria.
• Thinking osmosis needs energy. Osmosis, like all diffusion, needs none.
• Thinking a root takes in minerals only by diffusion. That would stop at the outside level.

Remember these

• Active transport moves a substance from a lower to a higher concentration, using energy.
• Carrier proteins in the membrane act as pumps; the energy comes from respiration.
• Root hair cells take up minerals by active transport; without oxygen, the uptake stops.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why can diffusion alone never make a root cell hold more nitrate than the soil water?

2. Where does the energy for active transport come from?

3. Roots are kept in water with no oxygen. Why does their uptake of minerals almost stop?

4. Which of these needs energy from the cell: oxygen entering a cell, water entering a root, or nitrate entering a root hair?