Tasks of the small intestine sections

Most of the carbohydrate digestion takes place in the duodenum and jejunum. Enzymes in the brush border break down more complex carbohydrates, which are then absorbed as simple sugars (monosaccharides) via transporters into the small intestine cells. The digestion of fats (lipids) and the absorption of lipid cleavage products also take place here with the help of enzymes from the secretion of the pancreas.

Iron is also absorbed in the duodenum. The digestion of proteins also takes place mainly in the duodenum and empty intestine. First, certain enzymes (so-called oligopeptidases) split the proteins into their components, and then these (smaller proteins or peptides and amino acids) are absorbed into the cells of the mucosa (enterocytes).

In the ileum, the absorption of vitamin C and vitamin B12 takes place with the help of the so-called intrinsic factor, which is produced in the stomach. Among other things, vitamin B12 plays an important role in blood formation, which is why damage in the ileum is often accompanied by anaemia. Since the pH value in the duodenum is almost neutral, the enzyme pepsin, which is active in the acidic gastric juice, can no longer digest and break down proteins.

Thus the protein digestion in the duodenum is stopped for the time being. Now the pancreatic juice enters the duodenum. The enzymes trypsin and chymotrypsin from the pancreatic juice are activated in the alkaline environment of the duodenum and continue the protein digestion.

The peptides (crushed proteins) resulting from the cleavage are then again cleaved by other enzymes (peptidases) located at the microvilli of the small intestinal mucosa to smaller peptides (di and tripeptides). These small protein units can then be absorbed into the superficial intestinal cells (enterocytes) by various molecular absorption mechanisms. Different enzymes participate in the splitting of the different sugars (carbohydrates) that people consume.Carbohydrate digestion begins in the oral cavity, where ptyalin (an a-amylase) already breaks down the starch into maltose (maltose) and other polysaccharides (oligisaccharides).

In the small intestine, the enzymes lactase, sucrase and maltase then break down the sugars into their constituent parts glucose, galactose, mannose and fructose. These sugar components are then taken up by the small intestine cells (enterocytes) through various molecular mechanisms. Under the influence of the enzyme lipase from the pancreas, the triglycerides (neutral fats) are split into glycerol and free fatty acids.

The bile acids formed in the liver incorporate these components into a structure called the micelle. In the micelles, these fat-soluble substances can pass through the intestinal cells where they are incorporated into a protein-fat molecule (chylomicron). The vitamins, which are fat-soluble, are channelled through the intestinal wall together with other fats in the above-mentioned micelles. The water-soluble vitamins diffuse passively through the intestinal wall. A special exception is vitamin B-12, which forms a complex with the intrinsic factor formed in the stomach and can only be absorbed in the ileum through this compound.