Carbohydrate digestion

Carbohydrate digestion breaks larger carbohydrates into monosaccharides, which can be absorbed. The bonds joining sugar units are called glycosidic bonds. Digestion breaks these bonds by hydrolysis: water is used to split a bond between sugar units.

Amylases hydrolyse starch, a carbohydrate made of many glucose units, into smaller carbohydrates. These include maltose, a disaccharide made of two glucose units.

Disaccharides are carbohydrates made of two sugar units. In the small intestine, enzymes called disaccharidases are bound to the membranes of epithelial cells lining the intestine. They hydrolyse disaccharides into monosaccharides:

• Maltase breaks maltose into two glucose molecules.
• Sucrase breaks sucrose into glucose and fructose.
• Lactase breaks lactose into glucose and galactose.

The resulting glucose, fructose and galactose are monosaccharides that can be absorbed.

Lipid digestion

Lipid digestion breaks large lipid molecules into smaller molecules. The main lipids digested are triglycerides, each made of glycerol joined to three fatty acids.

Lipase is an enzyme that breaks bonds in triglycerides using water. This reaction is called hydrolysis. Lipase produces mainly monoglycerides, which consist of glycerol joined to one fatty acid, and free fatty acids.

Emulsification and lipase action

Lipids can form large droplets in the watery contents of the small intestine. Bile salts break these large droplets into smaller droplets, a process called emulsification. The smaller droplets have a greater total surface area, giving lipase more surface on which to act.

Bile salts are not enzymes. They do not hydrolyse triglycerides: they make the lipid more accessible to lipase, which carries out the hydrolysis.

Protein digestion

Proteins are made of amino acids joined into chains by peptide bonds. Protein digestion breaks these bonds by hydrolysis: water is used to split a bond. The products of digestion are individual amino acids.

Endopeptidases hydrolyse peptide bonds within a polypeptide chain. This divides the chain into shorter peptides, creating more chain ends.

Exopeptidases hydrolyse peptide bonds at the ends of peptide chains, removing amino acids one at a time.

Some peptides are reduced to dipeptides, each consisting of two amino acids joined by a peptide bond. Dipeptidases bound to the membranes of small-intestinal epithelial cells hydrolyse this bond, releasing the two amino acids.

Sodium co-transport in ileum absorption

Cells lining the ileum absorb glucose, galactose and amino acids from the intestinal lumen. They can take up these solutes even when their concentration is higher inside the cell than in the lumen. The energy for this movement comes indirectly from a sodium ion concentration gradient.

Co-transport into ileum cells

The membrane facing the lumen contains sodium-dependent co-transport proteins. Each protein binds sodium ions, $$\ce{Na+}$$, together with a particular sugar or amino acid. Sodium moves into the cell down its concentration gradient. This drives the simultaneous movement of the other solute into the cell, potentially against its own concentration gradient.

Different co-transport proteins carry different solutes. Glucose, galactose and amino acids are absorbed using these sodium-dependent proteins.

Maintaining the sodium gradient

The membrane on the opposite side of the cell contains sodium–potassium pumps. These use ATP to move sodium out of the cell, keeping its intracellular sodium concentration low. Sodium can therefore continue to move into the cell from the lumen, driving co-transport.

Once absorbed, the sugars and amino acids can leave the epithelial cell across this opposite membrane and enter the blood.

Not all monosaccharides use sodium co-transport. Fructose enters ileum cells by facilitated diffusion, moving down its own concentration gradient through a membrane transport protein.

Micelle-mediated lipid absorption

Fatty acids and monoglycerides are products of lipid digestion. They do not dissolve well in water, so they need a way to travel through the watery contents of the small intestine to the cells that absorb them.

Bile salts associate with fatty acids and monoglycerides to form tiny groups called micelles. Micelles carry these lipid digestion products through the intestinal contents to the surface of epithelial cells lining the ileum.

At the cell surface, fatty acids and monoglycerides leave the micelles and enter the epithelial cells across their membranes. The bile salts do not enter the cells with them: they remain in the intestinal lumen and can be reused to form more micelles.