The human digestive system: Proteases break down proteins to amino acids.

The idea

Proteases are enzymes that catalyse the hydrolysis of proteins into amino acids. Proteins are polymers of amino acids joined by peptide bonds. Hydrolysis uses water to break these bonds, releasing smaller peptides and eventually individual amino acids, which are small enough to be absorbed across the wall of the small intestine into the blood. Proteases are produced in the stomach, pancreas and small intestine. Stomach protease works in acidic conditions, while pancreatic and intestinal proteases work in alkaline conditions.

What earns the marks

• Proteases are enzymes that catalyse the breakdown of proteins into amino acids.
• Proteins are polymers of amino acids joined by peptide bonds.
• Digestion is hydrolysis: water is used to break peptide bonds.
• Proteases are produced in the stomach, pancreas and small intestine.
• Amino acids are small and soluble, so they can be absorbed across the small intestine wall.
• Enzyme specificity means a protease active site is complementary to its protein substrate.

Common mistakes and how to put them right

• Saying proteases break proteins into glycerol and fatty acids.

  Put it right: Proteases produce amino acids, while lipases produce glycerol and fatty acids.

• Writing that proteins are broken down into glucose.

  Put it right: Proteins are broken down into amino acids.

• Ignoring the role of water in hydrolysis.

  Put it right: Hydrolysis uses water to split peptide bonds.

Exam tips

• State the substrate and product clearly: proteins to amino acids.
• Mention the sites of protease production, such as the stomach, pancreas and small intestine.
• Use the term peptide bonds when explaining how proteins are broken down.