How Digestion and Absorption Work in the Small Intestine
Learn how enzymes digest carbohydrates, proteins and lipids in the small intestine, and how glucose, amino acids and lipids are absorbed through the ileum.
In the small intestine, enzymes break large food molecules into smaller molecules, which are then absorbed through the lining of the ileum. Digestion is the breakdown; absorption is the movement of the products into the body.
How digestion works
- Carbohydrates: Pancreatic amylase breaks starch into maltose. Membrane-bound disaccharidases on the ileum’s epithelial cells then break disaccharides into monosaccharides, such as glucose.
- Proteins: Endopeptidases break peptide bonds within proteins. Exopeptidases remove amino acids from the ends of peptides, and membrane-bound dipeptidases break dipeptides into amino acids.
- Lipids: Bile salts emulsify large lipid droplets into smaller ones, giving lipase more surface area to act on. Lipase breaks triglycerides into fatty acids and monoglycerides. Bile salts then help form tiny structures called micelles, which carry these products to the epithelial cell surface.
How absorption works
The ileum has villi, which increase the surface area for absorption. Each villus has a thin epithelial lining, blood capillaries and a lymph vessel called a lacteal.
Glucose and amino acids enter epithelial cells by co-transport with sodium ions: the ions moving into the cell help carry these nutrients in. The sodium ion gradient is maintained by active transport, which requires ATP. Glucose and amino acids then pass into the blood.
Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There, they are re-formed into triglycerides, packaged into chylomicrons and passed into the lacteals. Chylomicrons are particles that transport lipids through the lymph.
Why the ileum is efficient
Villi and microvilli provide a large surface area. The epithelium is one cell thick, creating a short diffusion pathway. A good blood supply carries absorbed glucose and amino acids away, helping maintain concentration gradients for rapid absorption.
Exam mistake to avoid: Bile salts do not digest lipids; lipase does. Micelles deliver lipid digestion products to epithelial cells but do not enter the cells intact.
