The Digestive System, Enzymes and Absorption

Digestion is the hydrolysis of large, insoluble molecules into small, soluble ones that can be absorbed. Mechanical digestion (chewing, churning) increases the surface area; chemical digestion is done by enzymes.

• Food is pushed along by peristalsis: waves of smooth muscle contraction.
• The stomach absorbs very little: some water, alcohol and a few drugs. Most absorption is in the small intestine (duodenum, jejunum and ileum).
• The liver also stores glycogen, breaks down excess amino acids into urea (deamination), and removes toxins such as alcohol.
• Above the diaphragm (thorax): heart, lungs, oesophagus. Below it (abdomen): liver, stomach, pancreas, intestines, kidneys, spleen.

All digestive enzymes are hydrolases: they break bonds by adding water.

• Protein-digesting enzymes are made as inactive precursors (pepsinogen, trypsinogen), so they do not digest the cells that make them. Acid activates pepsinogen; the gut enzyme enterokinase activates trypsinogen.
• Starch is digested in two steps: amylase makes maltose, then maltase makes glucose.
• Brush-border enzymes are attached to the membranes of the cells lining the small intestine.
• People with lactose intolerance make too little lactase.

• Bile is made in the liver, stored and concentrated in the gall bladder, and released into the duodenum through the bile duct.
• Bile contains bile salts, hydrogencarbonate, cholesterol and the pigment bilirubin (from broken-down haemoglobin). It contains no enzymes.
• Emulsification: bile salts break large fat globules into many small droplets. This is physical, not chemical: no bonds are broken and the activation energy of lipase does not change, but the surface area for lipase becomes much larger.
• Pancreatic juice carries $\mathrm{HCO_3^-}$, which raises the pH, plus amylase, trypsinogen and lipase. It is the main juice that neutralises stomach acid.
• Bile also adds water, diluting the gut contents. Gastric juice ($\mathrm{HCl}$) lowers the pH; only bile and pancreatic juice raise it.
• Hormones: gastrin (stomach) increases acid secretion; secretin (duodenum) triggers hydrogencarbonate from the pancreas; CCK (duodenum) makes the gall bladder contract and the pancreas release enzymes.

• Villi each contain a network of blood capillaries and a central lymph vessel, the lacteal. Their cells have microvilli and many mitochondria for active transport.
• Glucose and amino acids enter the cells by co-transport with $\mathrm{Na^+}$ (secondary active transport) and go to the liver in the hepatic portal vein.
• Fatty acids and monoglycerides enter the cells, are rebuilt into triglycerides, packed into chylomicrons and enter the lacteals; the lymph later drains into the blood.
• Fat-soluble vitamins (A, D, E, K) are absorbed with the fats. Vitamin B12 is absorbed in the ileum with intrinsic factor from the stomach.
• Key nutrients: vitamin A (makes the retinal of rod cells; lack causes night blindness), vitamin C (collagen; lack causes scurvy), vitamin D (calcium absorption; lack causes rickets), vitamin K (clotting), iron (haemoglobin; lack causes anaemia), iodine (thyroxine).