Co-transport

Co-transport is the movement of two different substances across a membrane at the same time, through the same carrier protein, called a co-transporter. One substance, usually sodium ions, moves down its concentration gradient, and this pulls the other, such as glucose or amino acids, across against its concentration gradient. ATP is used to set up the sodium ion gradient, so co-transport is a form of active transport, sometimes called indirect or secondary active transport.

What is co-transport?

Some substances need to be moved into a cell where there is already more of them than outside. That needs energy. In ordinary active transport, the carrier protein uses ATP directly. In co-transport, the carrier protein uses no ATP itself. Instead it couples two movements:

• Sodium ions diffuse into the cell, from a high concentration to a low one. That movement releases energy.
• The co-transporter will only move sodium ions if it carries a glucose (or amino acid) molecule at the same time. So glucose is carried in too, even against its own gradient.

The catch is that the sodium ion gradient would soon run down. A separate pump keeps it going, and that pump does use ATP.

Co-transport of glucose and sodium in the ileum

AQA A-level Biology uses the absorption of sodium ions and glucose by the cells lining the ileum, the last part of the small intestine, as its example. Follow the numbers on the diagram:

1. The sodium-potassium pump. On the side of the cell next to the blood, a carrier protein uses ATP to pump sodium ions across into the blood and potassium ions back in. As a result, only a few sodium ions are left in the cell.
2. Co-transport. The sodium ion concentration in the gut lumen is now higher than inside the cell, so sodium ions diffuse in through a co-transporter protein in the membrane facing the lumen. Each one brings a glucose molecule with it, even though glucose is already more concentrated inside the cell.
3. Facilitated diffusion. Glucose builds up inside the cell, so it diffuses out through a carrier protein into the blood, down its concentration gradient. See facilitated diffusion.

Amino acids are absorbed by co-transport with sodium ions in the same way; AQA names co-transport for both monosaccharides and amino acids in the ileum.

Is co-transport active or passive?

Co-transport is active, because it moves glucose against its concentration gradient and depends on energy from ATP. But the ATP is used indirectly: it powers the sodium-potassium pump, which builds the sodium ion gradient, and the co-transporter itself does not hydrolyse ATP. That is why it is called indirect active transport or secondary active transport.

When two substances travel the same way through a co-transporter, as sodium ions and glucose do, it is sometimes called a symport. The sodium-potassium pump moves its two ions in opposite directions, which is an antiport. You do not need these words for AQA, but some textbooks use them. Sort examples into the right kind of transport in this question from the Cell Biology course.

Other examples of co-transport

• Glucose reabsorption in the kidney. Cells of the proximal convoluted tubule take glucose back from the filtrate into the blood by co-transport with sodium ions, so normally no glucose is left in urine.
• Loading sucrose into phloem. Companion cells pump hydrogen ions out using ATP; the hydrogen ions diffuse back in through co-transporters, bringing sucrose with them. Some A-level courses include this when explaining mass flow in the phloem.

Exam questions with answers

Explain how glucose is absorbed into the blood from the ileum.

1. The sodium-potassium pump, on the blood side of the epithelial cell, uses ATP to move sodium ions into the blood.
2. So the cell holds fewer sodium ions than the lumen does.
3. Sodium ions diffuse into the cell down their concentration gradient through a co-transporter protein.
4. Glucose enters with them, against its concentration gradient.
5. Glucose then moves from the cell into the blood by facilitated diffusion.

A respiratory inhibitor is added to ileum cells. Why does glucose absorption stop?

Without respiration the cells make no ATP, so the sodium-potassium pump stops. Sodium ions are no longer removed from the cell, the lumen soon has no more sodium ions than the cell, and the co-transporter has nothing to drive it.

Check your understanding.

Common mistakes

• Saying the co-transporter uses ATP. It does not; the sodium-potassium pump does.
• Saying glucose moves down its gradient into the cell. It moves against it; sodium ions move down theirs.
• Putting the pump on the wrong side. The pump is on the membrane next to the blood; the co-transporter faces the lumen.
• Missing the last step. Glucose leaves the cell into the blood by facilitated diffusion.
• Writing "sodium" for "sodium ions". Examiners expect ions, Na⁺.
• Calling it diffusion because sodium diffuses. Overall it is active, because glucose goes against its gradient and ATP is needed.

Practise with a short timed paper

Transport across membranes and absorption in the ileum are in the first year of A-level Biology. The daily A-level Biology: Year 1 paper is a short timed paper of 8 questions, with a new one every day. For how membranes are built, see what the cell membrane does.

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