The gut breaks down food. It turns large molecules into small ones. Small molecules can pass into the blood. This breakdown happens in two ways. The teeth a...

This question follows a bread-and-egg meal through the gut and tests digestion: which enzyme acts on each food, where it is made, and why chemical digestion is needed at all. Digestion converts large, insoluble molecules into small, soluble ones that can be absorbed into the blood.

(a) The starch in bread is a carbohydrate, and the enzyme that begins its breakdown is amylase [1]. Amylase is produced in the salivary glands (and also in the pancreas), so digestion of starch starts in the mouth [1]. It works by catalysing the hydrolysis of the long starch chains.

(b) When starch is completely digested the small molecules produced are glucose molecules [1]. (Amylase first breaks starch into maltose, and maltase then completes the job to glucose, so the final product is glucose.)

(c) Protein in the boiled egg is digested in the stomach by the protease pepsin [1], which works best in the acidic conditions the stomach provides.

(d) Three linked ideas earn the marks. The digested food must be absorbed into the blood to be carried to the cells [1]. Large molecules such as starch and protein are too big to pass through the wall of the small intestine into the blood [1]. Only the small, soluble products (glucose, amino acids) can be absorbed and then used by the cells for energy and growth [1]. So digestion is essentially the step that makes food small enough to cross the gut wall.

(e) Bile is made by the liver and released into the small intestine [1]. It does not contain a digestive enzyme; instead it emulsifies the fat, breaking large fat globules into many small droplets [1]. This greatly increases the surface area of fat exposed to lipase, so the fat is digested faster [1]. Bile is also alkaline and neutralises the acid from the stomach, giving lipase its correct pH.