IGCSE Biology 0610: Digestive Enzymes and the Role of Bile

A stew contains rice (rich in starch), diced chicken (rich in protein) and a spoonful of groundnut (peanut) oil (a fat). After the stew is swallowed, it is digested as it passes along the alimentary canal.

(a) For each of the following nutrients in the stew, name the type of enzyme that digests it and state the small, soluble product(s) that are formed.

(i) starch in the rice [1]

(ii) protein in the chicken [1]

(iii) fat in the groundnut oil [1]

(b) Explain how bile helps in the digestion of the fat in this stew, and explain why bile itself is not classified as a digestive enzyme. [3]

(c) The partly digested food (chyme) leaving the stomach is strongly acidic, but the enzymes acting on the stew in the small intestine work best in alkaline conditions. Explain how the alkaline mixture released into the small intestine solves this problem, and suggest one reason why a strongly acidic stomach is still useful earlier in digestion. [3]

Part (a): Enzymes and their products

(i) Starch in the rice

Starch is digested by amylase, a type of carbohydrase. Amylase breaks starch down into maltose, a simple reducing sugar.

(ii) Protein in the chicken

Protein is digested by protease. Protease breaks protein down into amino acids.

(iii) Fat in the groundnut oil

Fat is digested by lipase. Lipase breaks fat down into fatty acids and glycerol.

Part (b): The role of bile

Fat and oil do not mix with the watery fluids of the alimentary canal, so they naturally form large droplets. Bile emulsifies these large fat droplets, breaking them up into many much smaller droplets. This greatly increases the total surface area of fat exposed to the surrounding fluid, allowing lipase to come into contact with far more fat molecules at once and digest the fat much faster.

Bile itself is not classified as an enzyme because it does not chemically break any bonds within the fat molecules. Emulsification is a physical process. It changes the size of the fat droplets, not their chemical structure. The actual chemical digestion of fat into fatty acids and glycerol is carried out by lipase, not by bile.

Part (c): pH change between the stomach and small intestine

As food leaves the stomach, it has been mixed with hydrochloric acid from the gastric juice and is strongly acidic. However, the enzymes that continue digesting the stew in the small intestine, including protease and lipase, work best in alkaline conditions. Bile is an alkaline mixture, so when it is released into the small intestine it neutralises the acidic chyme arriving from the stomach, raising the pH to a level at which these enzymes have their optimum activity.

A strongly acidic stomach is still useful earlier in digestion because the hydrochloric acid kills many of the harmful microorganisms that may be present in food, and it also provides the acidic pH at which the stomach’s own protease enzyme (pepsin) works best.

Final answers

(a)(i) Amylase (a carbohydrase); breaks starch down into maltose, a simple reducing sugar.
(a)(ii) Protease; breaks protein down into amino acids.
(a)(iii) Lipase; breaks fat down into fatty acids and glycerol.
(b) Bile emulsifies the fat into smaller droplets, increasing the surface area for lipase to act on; it is not an enzyme because it does not chemically break down the fat itself. Lipase does.
(c) Bile neutralises the acidic chyme, raising the pH to the optimum for the enzymes in the small intestine; the acidic stomach is still useful because it kills microorganisms and allows the stomach’s own protease (pepsin) to work.