Digestion of proteins begins in stomach. The gastric juice contains HCl, which causes denaturation of proteins, and pepsin, that breaks down proteins into smaller peptides. Then, in the small intestine the enzymes of pancreatic and intestinal juice, such as trypsin, elastase, aminopeptidase, catalyze the hydrolysis of peptide bonds. The product of digestion are amino acids. Later, amino acids are absorbed through intestinal epithelium into the blood stream. It is an active process and uses energy of ATP.
Digestion of some carbohydrates, such as starch, begins in the mouth where they are partially hydrolyzed by salivary alpha-amylase. When the food mass reaches upper small intestine, pancreatic amylase breaks down long-chained carbohydrates into disaccharides. Then disaccharides are hydrolyzed into monosaccharides by enzymes of the brush border of the small intestine (sucrase, malatase, lactase). Monosaccharides are transported through the intestinal epithelium into the blood stream. This process requires energy.
Digestion of fats (triacylglycerol) starts in the small intestine. Lipids are hydrophobic, therefore the water soluble lipase can interact only with external part of fat globules. That is when emulsification takes place. Bile salts and phospholipids of the bile break up fat globules into emulsion droplets. After that, lipase hydrolyzes fats into fatty acids and monoglycerides. Then monoglycerides and fatty acids connect with bile salts and form micelles and are transported to the surface of enterocytes, where they are absorbed. In the enterocytes, fatty acids and monoglycerides are re-synthesized into triacylglycerol. Triacylglycerol binds to proteins and form lipoproteins, which deliver fats to the cells.
Vitamin digestion occurs in the small intestine. There are two types of vitamins: water-soluble and fat-soluble. Water-soluble vitamins (C and the B vitamins) are absorbed in jejunum and upper ileum mostly by diffusion and go straight into the blood stream. Fat-soluble vitamins (A, D, E and K) are absorbed along with triglycerides and are carried within lipoproteins through the lymphatic system into the blood stream.
Absorption of minerals mainly occurs in the small intestine. Sodium, potassium, calcium, iron are absorbed in small intestine by either passive diffusion or active transport.