The human digestive system: Lipases break down lipids (fats) to glycerol and fatty acids.

The idea

Lipids are large, insoluble food molecules that must be chemically digested before absorption. Lipases are enzymes that catalyse the hydrolysis of lipids, splitting each triglyceride molecule into glycerol and three fatty acid molecules. In the human digestive system, lipase is produced by the pancreas and secreted into the small intestine, and also by the small intestine itself; bile from the liver emulsifies lipids into small droplets, increasing surface area so lipase acts faster. The products are small, soluble enough to cross the intestinal wall and be absorbed into the blood, then reassembled or used for energy and storage. A useful check is that lipase is specific to lipids, so it does not digest starch or protein.

What earns the marks

• Lipids are broken down by enzymes called lipases.
• The reaction is hydrolysis, in which water is used to split the lipid molecule.
• The products of lipid digestion are glycerol and fatty acids.
• Lipase is produced by the pancreas and small intestine and acts in the small intestine.
• Bile emulsifies lipids, increasing surface area for lipase action.
• The products are absorbed and can be reassembled into lipids or used in respiration.

Common mistakes and how to put them right

• Writing that lipase produces glucose and amino acids.

  Put it right by linking lipase only to lipids and its products glycerol and fatty acids.

• Confusing lipase with bile.

  Put it right: Bile is not an enzyme and does not chemically digest lipids; it emulsifies them.

• Stating that lipids are digested in the stomach.

  Put it right: Most lipid digestion occurs in the small intestine after emulsification.

Exam tips

• Name the enzyme, the substrate and both products in one sentence to secure the key marking point.
• If asked why bile helps, link emulsification to increased surface area and faster enzyme action.
• Use the term hydrolysis when explaining how the lipid molecule is split, and avoid saying it is 'melted' or 'dissolved'.