Enzymes and Digestion

Large food molecules such as carbohydrates, proteins and lipids can be found in our food. Digestive enzymes break down these large insoluble molecules into smaller soluble molecules. This allows them to be absorbed into the bloodstream.

Digestive Enzymes

The three main types of digestive enzymes are:

  • Carbohydrases
  • Proteases
  • Lipases

Carbohydrases

Carbohydrases are a group of enzymes which break down carbohydrates into simple sugars. Let’s look at the digestion of starch, which is a carbohydrate.

Starch is made up of a chain of glucose molecules. The enzyme amylase breaks it down into maltose, and then the enzyme maltase further breaks down maltose into glucose.

Diagram showing the breakdown of starch into chains of glucose molecules by the enzyme amylase

Amylase is mainly produced by the salivary glands and the pancreas.

Proteases

Proteases are a group of enzymes that break down proteins into amino acids, which are then absorbed into the bloodstream.

Diagram showing the breakdown of proteins into amino acids by protease enzymes

Proteases are produced in the stomach, pancreas and small intestine. Then they digest proteins in the stomach and small intestine.

Lipases

Lipases are a group of enzymes that break down lipids (fats) into fatty acids and glycerol.

Diagram illustrating the action of the enzyme Lipase on a lipid molecule. On the left, a vertical blue glycerol molecule is aligned with three horizontal red fatty acid molecules. On the right, following the action of Lipase, the structures are broken apart, indicating the separation of glycerol and fatty acids.

Lipase enzymes are produced in the pancreas and secreted into the small intestine (duodenum).

Summary

EnzymeWhere it is producedSubstrateProductWhere they work
Carbohydrase (Amylase)Salivary glands (in saliva)
Pancreas (in pancreatic juices)
CarbohydratesSimple SugarsSaliva
Small intestine
Protease Stomach
Pancreas (in pancreatic juices)
Small intestine
ProteinsAmino AcidsStomach
Small intestine
LipasePancreas (in pancreatic juices) Lipids (Fats) Fatty Acids and GlycerolSmall intestine (duodenum)

The Role of Bile

Bile is an alkaline substance that is produced by the liver, stored in the gall bladder and secreted into the small intestine.

Although bile does not digest lipids, it helps in two main ways:

  • By emulsifying fats – Breaking them down into smaller droplets, so there is a larger surface area for lipase enzymes to work. This is also known as emulsification.
  • By neutralising acid – By neutralising the hydrochloric acid from the stomach, bile provides the alkaline conditions required in the small intestine. This is because the enzymes in the small intestine work best when the pH is slightly alkaline.

Thanks to bile, the alkaline conditions and larger surface area allow lipase to break down lipids into fatty acids and glycerol at a faster rate.

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