Co-transport of Glucose in the Ileum Revision notes | A-Level Biology AQA

Co-transport of Glucose in the Ileum

This lesson covers:

The process of co-transport
How co-transport is used in the absorption of glucose and amino acids

Co-transport

Some carrier proteins can bind to two molecules at once, these proteins are known as co-transporters. They use the concentration gradient of one molecule to move the other molecule against its own concentration gradient.

The absorption of glucose

During digestion, carbohydrates are broken down into glucose. This glucose is absorbed from the lumen of the small intestine into the bloodstream. However, diffusion can only result in equal glucose concentrations, so not all the glucose will be absorbed.

To overcome this, glucose is absorbed from the intestines into the blood using co-transport.

Diagram showing glucose absorption in the ileum, including sodium-glucose cotransporter protein, epithelial cell, glucose protein channel, sodium-potassium pump, and blood.

The diagram above shows the final part of the small intestine known as the ileum. As food passes through the ileum, the glucose passes from the lumen (the inside space) into the epithelial cells (lining the ileum), and finally into the blood.

The co-transport of glucose involves three different proteins:

Sodium-potassium pumps - These actively transport sodium (Na⁺) and potassium (K⁺) ions.
Sodium-glucose co-transporter proteins - These use facilitated diffusion to transport Na⁺ ions and glucose molecules.
Glucose protein channels - These use facilitated diffusion to transport glucose molecules.

Co-transport of sodium and glucose

Glucose is absorbed into the blood using the following process:

Na⁺ is actively transported out of epithelial cells into the blood by the sodium-potassium pump. This pump transports three Na⁺ out for every two K⁺ in. This creates a concentration gradient as there is now a higher concentration of Na⁺ in the lumen than in the epithelial cells.

Diagram showing the co-transport of sodium and glucose across a cell membrane, highlighting the sodium-glucose cotransporter, sodium ions, potassium ions, and glucose molecules.

2. Na⁺ diffuses from a high concentration in the lumen to a low concentration in the epithelial cells. It is transported via sodium-glucose co-transporter proteins, which also carry glucose molecules. This causes the concentration of glucose inside the epithelial cells to increase.

Diagram showing the co-transport of sodium and glucose across a cell membrane, with sodium and potassium ions and glucose molecules.

3. There is now a higher glucose concentration in the epithelial cells than in the blood. So, glucose diffuses out of epithelial cells and into the blood via facilitated diffusion.

Glucose molecules are moved against their concentration gradient due to the Na⁺ concentration gradient, rather than by using ATP. The same process is used to absorb amino acids in the small intestine (where amino acids take the place of glucose).