The human digestive system: Digestive enzymes convert food into small soluble molecules that can be absorbed into the bloodstream.

The idea

Digestion uses enzymes to break large insoluble food molecules into small soluble molecules. Carbohydrases such as amylase convert starch into sugars such as glucose; proteases convert proteins into amino acids; lipases convert lipids into fatty acids and glycerol. These products are small enough to pass through the wall of the small intestine into the blood. The small intestine has a large surface area, a thin wall and a good blood supply, which makes absorption efficient. The blood then carries the soluble products to cells for respiration, growth and repair. Without digestion, large molecules such as starch could not cross the gut wall, so absorption would not occur. You should be able to name the enzyme, its substrate and its products, and explain why the products must be soluble and small.

What earns the marks

• Digestive enzymes are biological catalysts that break large insoluble food molecules into smaller soluble molecules.
• Carbohydrases such as amylase convert carbohydrates, including starch, into sugars such as glucose.
• Proteases convert proteins into amino acids.
• Lipases convert lipids into fatty acids and glycerol.
• The products are small and soluble, so they can pass through the wall of the small intestine into the bloodstream.
• Absorption happens mainly in the small intestine, which has adaptations such as a large surface area, a thin wall and a rich blood supply.
• The blood transports the absorbed products to body cells, where they are used for processes such as respiration, growth and repair.

Common mistakes and how to put them right

• Saying that enzymes 'kill' or 'melt' food.

  Put it right: Enzymes are biological catalysts that break large molecules into smaller soluble ones by chemical digestion.

• Claiming that large molecules such as starch are absorbed directly.

  Put it right: Starch must first be digested to glucose, which is small and soluble and can be absorbed.

• Naming the stomach as the main site of absorption.

  Put it right: Most absorption occurs in the small intestine, which has a large surface area and a good blood supply.

• Writing that lipase produces only fatty acids.

  Put it right: Lipase produces fatty acids and glycerol.

Exam tips

• Use the pattern 'enzyme, substrate, products' when explaining digestion, for example amylase converts starch into glucose.
• When explaining absorption, link the small size and solubility of the products to their movement into the blood.
• Include at least one adaptation of the small intestine, such as its large surface area or thin wall, when a question asks how absorption is made efficient.
• Avoid saying 'food is absorbed'; name the soluble products such as glucose, amino acids, fatty acids and glycerol.