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

The idea

Proteins are large polymers made of amino acid monomers joined by peptide bonds. They are too large and insoluble to be absorbed directly, so they must be hydrolysed during digestion. Proteases are enzymes that catalyse the breakdown of proteins into amino acids. Proteases are produced in the stomach, pancreas and small intestine. In the stomach, hydrochloric acid provides a low pH that is optimum for stomach protease; in the small intestine, pancreatic and intestinal proteases work at a higher pH. The amino acids produced are small, soluble molecules that can be absorbed through the wall of the small intestine into the blood and used to build new proteins in cells.

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.
• Amino acids are small, soluble molecules that can be absorbed across the wall of the small intestine.
• Proteases are produced in the stomach, pancreas and small intestine.
• Stomach protease works best at low pH provided by hydrochloric acid, while intestinal proteases work at a higher pH.
• The amino acids absorbed are used by cells to synthesise new proteins.

Common mistakes and how to put them right

• Writing that proteases break down proteins into glucose.

  Put it right: State that the products are amino acids.

• Confusing the optimum pH of stomach protease with that of intestinal protease.

  Put it right: Link stomach protease to acidic pH and intestinal protease to a higher pH.

• Stating that amino acids are further digested before absorption.

  Put it right: Amino acids are already small and soluble and are absorbed directly, without further breakdown.

Exam tips

• Always name the substrate and the product, for example proteins to amino acids.
• Link each named protease to its site of production and its working pH.
• Use the term 'absorbed' for amino acids crossing the small intestine wall, not 'digested'.