Digestion of food and role of digestive enzymes

Digestion turns solid food into monomers small enough to absorb, using both mechanical breakdown and enzyme-driven chemical breakdown at each stop along the canal. In the buccal cavity, the sight, smell and mechanical presence of food trigger reflex salivation; chewing (mastication) physically grinds the food, while saliva — a mix of water, electrolytes, the antibacterial enzyme lysozyme, lubricating mucus and the enzyme salivary amylase (ptyalin) — begins the chemical breakdown of starch into the disaccharide maltose (only a modest fraction of the total starch is converted at this stage) and binds the chewed food into a bolus for swallowing. In the stomach, food is held for several hours while peristaltic churning mixes it with gastric juice into a semi-liquid mass called chyme. Gastric juice supplies hydrochloric acid and inactive proenzymes: HCl converts pepsinogen into active pepsin, which breaks proteins down into proteoses and peptones, and the acidic environment (around pH 1.8) also kills ingested bacteria; mucus and bicarbonate in the same juice protect the stomach's own lining from that acid. In infants, a second enzyme, rennin, converts the milk protein caseinogen into casein in the presence of calcium ions, though its secretion fades with age. In the small intestine, bile, pancreatic juice and intestinal juice (succus entericus, from Brunner's glands and the intestinal glands) all arrive together and are thoroughly mixed with the food by the muscularis layer's movements. Pancreatic juice supplies trypsinogen (activated to trypsin by the intestinal enzyme enterokinase, with active trypsin in turn activating chymotrypsinogen), carboxypeptidase, pancreatic amylase (which converts starch and glycogen to maltose), pancreatic lipase (which acts on emulsified fat) and nucleases. Bile itself contains no enzymes, but its bile salts emulsify fat — breaking large fat droplets into many small globules and so increasing the surface area lipase can act on — and also activate lipase directly. Finally, the intestinal juice's own enzymes (maltase, lactase and sucrase for the sugars; dipeptidases for short pept …