Biology · Nutrition. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Human digestion breaks large food molecules into small soluble ones in stages. Each region of the gut has its own pH and enzymes. Starch digestion begins in the mouth, protein digestion begins in the acid stomach, and all digestion is completed in the alkaline small intestine, where most absorption also happens.
Protein-digesting enzymes are released as inactive forms (zymogens) so that they do not digest the cells that make them. They are activated in the gut cavity.
Hormones control the juices. Gastrin switches on gastric juice and secretin switches on pancreatic juice and bile. Villi absorb the products: sugars and amino acids enter blood capillaries, fats enter lacteals. The large intestine absorbs water and salts.
Students write that bile digests fat. Bile contains no enzyme: it only emulsifies fat into small droplets, giving lipase a larger surface area. Lipase does the chemical digestion.
Answer: Fats, because they are not emulsified, so lipase works slowly and much fat passes out undigested.
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