Enzymes as biological catalysts, the lock and key model and the active site, how temperature and pH affect enzyme activity including denaturing, the organs of the digestive system, the enzymes amylase, protease and lipase, the role of bile, and absorption in the villi.

What this dot point is asking

CCEA Double Award wants enzymes as biological catalysts, the lock and key model, how temperature and pH change enzyme activity, then the digestive system: its organs, the three enzyme groups, the role of bile, and how the small intestine absorbs the products. The lock and key model is the idea that ties the whole topic together.

Enzymes as catalysts

The lock and key model explains the specificity. The enzyme has a region called the active site whose shape matches one substrate. The substrate fits into the active site to form an enzyme-substrate complex, the reaction happens, and the products are released, leaving the enzyme free to work again.

Temperature and pH

As temperature rises, enzyme and substrate molecules move faster and collide more often, so the rate increases up to the optimum (about 37 degrees Celsius in humans). Above the optimum, the heat breaks the bonds holding the enzyme's shape, so the active site changes shape and the enzyme is denatured - the substrate no longer fits and the reaction stops.

Each enzyme also has an optimum pH. Pepsin in the stomach works best in acidic conditions (around pH 2), while enzymes in the small intestine work best in slightly alkaline conditions. Moving away from the optimum pH also denatures the enzyme.

The digestive system

Food passes through the mouth, oesophagus, stomach, small intestine and large intestine, with the pancreas and liver adding secretions. The three enzyme groups break large insoluble molecules into small soluble ones:

• Amylase (made in the salivary glands and pancreas) breaks starch into maltose.
• Protease (stomach, pancreas) breaks protein into amino acids.
• Lipase (pancreas) breaks fat into fatty acids and glycerol.

Bile and absorption

Bile is made by the liver and stored in the gall bladder. It is alkaline, so it neutralises the acid from the stomach to give the right pH for the intestinal enzymes, and it emulsifies fat - breaking it into small droplets to give a larger surface area for lipase. Bile is not an enzyme.

The small intestine is lined with millions of villi, giving a large surface area for absorption. Each villus has a thin wall (one cell thick), a good blood supply and a lacteal, keeping a steep concentration gradient so the soluble products diffuse quickly into the blood.

Examples in context

Example 1. Why we have a high body temperature. Human enzymes work fastest at about 37 degrees, so the body keeps that temperature to maximise the rate of digestion and respiration. A fever above this can begin to denature enzymes, which is one reason a very high fever is dangerous.

Example 2. Why fat needs bile. Without bile, fat stays in large globules with a small surface area, so lipase works slowly. Bile emulsifies the fat into tiny droplets, increasing the surface area, so lipase can digest it much faster. This is a clear surface-area-and-rate link.

Try this

Q1. What is meant by the term biological catalyst? [1 mark]

• Cue. A protein that speeds up a reaction without being used up.

Q2. Name the products when lipase digests fat. [2 marks]

• Cue. Fatty acids and glycerol.