The human digestive system: Students should be able to recall the sites of production and the action of amylase, proteases and lipases.

The idea

Digestive enzymes are produced in specific organs and act on specific substrates. Amylase is produced in the salivary glands and the pancreas; it catalyses the breakdown of starch to maltose. Proteases are produced in the stomach, the pancreas and the small intestine; they catalyse the breakdown of proteins to amino acids. Lipases are produced in the pancreas and the small intestine; they catalyse the breakdown of lipids (fats) to fatty acids and glycerol. The products are small enough to be absorbed through the wall of the small intestine into the blood. For example, starch is digested by amylase in the mouth and small intestine, while proteins begin digestion in the stomach and continue in the small intestine. Bile, produced by the liver and stored in the gall bladder, emulsifies lipids to increase the surface area for lipase action.

What earns the marks

• Amylase is produced in the salivary glands and pancreas and breaks down starch to maltose.
• Proteases are produced in the stomach, pancreas and small intestine and break down proteins to amino acids.
• Lipases are produced in the pancreas and small intestine and break down lipids to fatty acids and glycerol.
• Each enzyme acts on a specific substrate because the active site and substrate shapes are complementary.
• The products of digestion are small soluble molecules absorbed through the small intestine wall.
• Bile from the liver emulsifies lipids, increasing surface area for lipase action.

Common mistakes and how to put them right

• Stating that lipase is produced in the stomach.

  Put it right by naming the pancreas and small intestine as the sites of lipase production.

• Saying proteases break down proteins to glucose.

  Put it right by stating the products are amino acids.

• Confusing the role of bile with that of lipase.

  Put it right by stating bile emulsifies lipids but does not chemically digest them.

Exam tips

• Use a table with columns for enzyme, site(s) of production, substrate and products to organise your recall.
• Write the products precisely: maltose from starch, amino acids from proteins, fatty acids and glycerol from lipids.
• Link each enzyme to its substrate by referring to complementary active site shape to show understanding of specificity.