Brent Cornell

    
Food can be digested by a combination of two methods – mechanical digestion and chemical digestion

• In chemical digestion, food is broken down by the action of chemical agents (such as enzymes, acids and bile)

Stomach Acids

• The stomach contains gastric glands which release digestive acids to create a low pH environment (pH ~2)
• The acidic environment functions to denature proteins and other macromolecules, aiding in their overall digestion
• The stomach epithelium contains a mucous membrane which prevents the acids from damaging the gastric lining
• The pancreas releases alkaline compounds (e.g. bicarbonate ions), which neutralise the acids as they enter the intestine 

Bile

• The liver produces a fluid called bile which is stored and concentrated within the gall bladder prior to release into the intestine
• Bile contains bile salts which interact with fat globules and divide them into smaller droplets (emulsification)
• The emulsification of fats increases the total surface area available for enzyme activity (lipase)

Enzymes

• Enzymes are biological catalysts which speed up the rate of a chemical reaction (i.e. digestion) by lowering activation energy
• Enzymes allow digestive processes to therefore occur at body temperatures and at sufficient speeds for survival requirements
• Enzymes are specific for a substrate and so can allow digestion of certain molecules to occur independently in distinct locations

Examples of Digestive Enzymes

    
Digestive enzymes are secreted predominantly by the pancreas, although other organs also contribute (salivary gland, stomach)

• The type of enzyme secreted and location of secretion depends on the specific macromolecule required for hydrolysis

Carbohydrates

• Carbohydrate digestion begins in the mouth with the release of amylase from the salivary glands (amylase = starch digestion)
• Amylase is also secreted by the pancreas in order to continue carbohydrate digestion within the small intestine
• Enzymes for disaccharide hydrolysis are often immobilised on the epithelial lining of the small intestine, near channel proteins
• Humans do not possess an enzyme capable of digesting cellulose (cellulase) and hence it passes through the body undigested

Proteins

• Protein digestion begins in the stomach with the release of proteases that function optimally in an acidic pH (e.g. pepsin = pH 2)
• Smaller polypeptide chains enter the small intestine where they are broken down by endopeptidases released by the pancreas
• These endopeptidases work optimally in neutral environments (pH ~ 7) as the pancreas neutralises the acids in the intestine

Lipids

• Lipid breakdown occurs in the intestines, beginning with emulsification of fat globules by bile released from the gall bladder
• The smaller fat droplets are then digested by lipases released from the pancreas

Nucleic Acids

• The pancreas also releases nucleases which digest nucleic acids (DNA, RNA) into smaller nucleosides

Locations of Enzymatic Digestion