Absorption of digestion productsAQA A-Level Biology: Revision notes

After digestion, the small molecules produced (monosaccharides such as glucose, amino acids, monoglycerides and fatty acids) must cross the epithelial cells lining the ileum to enter the blood or lymph. The cell-surface membrane is a phospholipid bilayer: lipid-soluble molecules cross it easily, but polar or charged molecules need carrier or channel proteins. Absorption of glucose and amino acids is therefore very different from absorption of lipid products.

The key idea is that sodium ions carry glucose (and amino acids) into the cell. The steps are:

1. The sodium-potassium pump in the membrane facing the blood actively transports sodium ions out of the epithelial cell, using ATP. Sodium ion concentration inside the cell is kept low.
2. This creates a concentration gradient for sodium ions between the lumen (high) and the cell (low).
3. Sodium ions diffuse (facilitated diffusion) into the cell through a co-transporter protein in the cell-surface membrane. The protein will only carry sodium ions together with glucose or an amino acid.
4. Glucose is therefore carried into the cell against its own concentration gradient. The energy comes from the sodium ion gradient, which is maintained by active transport.
5. Glucose then moves from the cell into the blood by facilitated diffusion through a different carrier protein, down its concentration gradient.

Amino acids are absorbed in the same way, using their own co-transporter proteins.

After a meal, glucose is absorbed until its concentration in the lumen falls. Diffusion alone would stop once the concentrations in the lumen and the cell were equal. Because glucose is carried in by the sodium ion gradient, absorption continues even when the glucose concentration in the lumen is lower than in the cell, so more of the glucose is absorbed.

Lipase hydrolyses triglycerides to monoglycerides and fatty acids. These are not very soluble in water, so they would not easily reach the cell surface. Bile salts (made by the liver) mix with them to form tiny structures called micelles.

• Micelles carry monoglycerides and fatty acids to the surface of the epithelial cells.
• They bring the products into close contact with the cell-surface membrane and maintain a high concentration next to it, so the concentration gradient stays steep.
• Monoglycerides and fatty acids are lipid-soluble (non-polar), so they leave the micelles and diffuse through the phospholipid bilayer directly. No carrier protein and no ATP are needed.

If bile salts are missing (for example a blocked bile duct), fewer micelles form, less lipid is absorbed and fatty faeces result.

Glucose and amino acids are polar and water-soluble. They cross the membrane by co-transport with sodium ions through a carrier protein. ATP is used indirectly, by the sodium-potassium pump. They can be absorbed against their concentration gradient.

Monoglycerides and fatty acids are non-polar and lipid-soluble. They cross by simple diffusion through the bilayer, with no carrier protein and no ATP. They need micelles to keep the concentration gradient steep and can only move down a gradient.