Digestion of Carbohydrates, Proteins and Fats

Carbohydrate digestion

The amylase in saliva and pancreatic juices hydrolyse polysaccharides (starch and glycogen) into disaccharides such as maltose, sucrose and lactose as they are coated with saliva in the mouth and then move through the stomach to the small intestine where digestion resumes again (stomach acid halts carbohydrate digestion).  Enzymes (disaccharidases; maltase, sucrase and lactase) in the brush border of the lining of the small intestine promote further hydrolysis of the disaccharides into monosaccharides; glucose, fructose and galactose.

Protein Digestion

Protein digestion starts in the stomach with pepsin, a protease in gastric juice.  The pancreas contribute trypsin and chymotrypsin, proteases in pancreatic juice which hydrolyse proteins  into peptides.  Protein digestion is then completed by intestinal peptidases which hydrolyse dipeptides and tripeptides into amino acids.

The broken down amino acids are absorbed into the blood and are transported to the liver.  The liver regulates the distribution of amino acids to the rest of the body.


Fat digestion

Fats have to be made water soluble through the mechanical process of emulsification.  In the body bile salts and lecithin break down fats into water soluble micelles.

Micelles are spherical shaped shells formed by the orientation of lecithin and bile salt molecules according to their solubility in water and in lipids.

Hydrolysis of these micelles then takes place by lipases releases by the pancreas.  This breaks down the micelles into fatty acids and monoglycerides.  The fatty acids and monoglycerides diffuse into the epithelial cells of the small intestine.

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