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.

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:

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

However, ATP is used to maintain the concentration gradient of sodium ions between the lumen and the epithelial cells of the ileum.