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Diet, drugs and health

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Digestion and enzymes

Our teeth break food down into small pieces when we chew. This is only a start to the process of digestion, as chewed pieces of food are still too large to be absorbed by the body. Food has to be broken down chemically into really small particles before it can be absorbed. Enzymes are needed so that this break-down happens quickly enough to be useful.

Enzymes

Take care - enzymes are not living things. They are just special proteins that can break large molecules into small molecules. Different types of enzymes can break down different nutrients:

  • carbohydrase or amylase enzymes break down starch into sugar

  • protease enzymes break down proteins into amino acids

  • lipase enzymes break down fats into fatty acids and glycerol.

Carbohydrates

Carbohydrates are digested in the mouth, stomach and small intestine. Carbohydrase enzymes break down starch into sugars.

The saliva in your mouth contains amylase, which is another starch digesting enzyme. If you chew a piece of bread for long enough, the starch it contains is digested to sugar, and it begins to taste sweet.

Diagram shows how carbohydrase molecules break down starch into sugar during digestion

Proteins

Proteins are digested in the stomach and small intestine. Protease enzymes break down proteins into amino acids. Digestion of proteins in the stomach is helped by stomach acid, which is strong hydrochloric acid. This also kills harmful micro-organisms that may be in the food.

Diagram shows protease molecules breaking down proteins into amino acids

Fats

Lipase enzymes break down fat into fatty acids and glycerol. Digestion of fat in the small intestine is helped by bile, made in the liver. Bile breaks the fat into small droplets that are easier for the lipase enzymes to work on.

Digestion breaks down fat molecules into fatty acid molecules and glycerol molecules

Things that are not digested

Minerals, vitamins and water are already small enough to be absorbed by the body without being broken down, so they are not digested.

Digestive enzymes cannot break down fibre, which is why it cannot be absorbed by the body.

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