Digestion and Absorbtion

• Digestion is the hydrolysis of large biological molecules into smaller molecules which can be absorbed across cell membranes.
• Carbohydrates are digested by many different enzymes including amylase in the mouth which digests large polymers, maltase in the ileum which breaks down monosaccharides, and sucrase and lactase which break down sucrose and lactose respectively.
• Lipids are digested by lipase which hydrolyse the ester bond between the monolycerides and fatty acid.
• Before being broken down, lipids are emulsified into micelles, an process that increases the surface area and speeds up the chemical reaction.
• Proteins are digested by enzymes called peptidases, which they are divided into 3 main groups: endopeptidases which hydrolyse peptide bonds between specific amino acids in the middle of a polypeptide, exopeptidases which hydrolyse bonds at ends of a polypeptides, and dipeptidases which break dipeptides into individual amino acids.
• The products of digestion are then absorbed by cells lining the ileum of mammals, for example, amino acids are absorbed by facilitated diffusion through specific carrier molecules in the surface membrane of epithelial cells.
• Each amino acid, one Na+ molecule is also taken up through co-transport where a diffusion gradient is maintained.
• Monoglycerides and fatty acids are polar so they can easily diffuse across the cell membrane into the epithelial cells lining the epithelium.
• Once inside, monoglycerides and fatty acids are transported to the endoplasmic reticulum where they are reformed into triglycerides again.