The human digestive system: The products of digestion are used to build new carbohydrates, lipids and proteins. Some glucose is used in respiration.

The idea

Digestion converts large food molecules into small soluble products that are absorbed into the blood. These products are not simply excreted; they are used by cells to synthesise new biological molecules. Amino acids are joined by condensation reactions to build new proteins, including enzymes and structural proteins. Fatty acids and glycerol are reassembled into new lipids for membranes and energy storage. Glucose and other simple sugars are used to build new carbohydrates such as glycogen, and some glucose is oxidised in respiration to release energy for cell processes. This links digestion to metabolism, growth and repair. A useful example is a muscle cell using amino acids to build contractile proteins and glucose in respiration to power contraction.

What earns the marks

• Amino acids are used to build new proteins.
• Fatty acids and glycerol are used to build new lipids.
• Glucose and other sugars are used to build new carbohydrates.
• Some glucose is used in respiration to release energy.
• The products of digestion are absorbed into the blood and transported to cells.
• Building new molecules from digestion products is part of metabolism.

Common mistakes and how to put them right

• Saying all glucose is used only for respiration.

  Put it right by noting that some glucose is used to build new carbohydrates such as glycogen.

• Writing that amino acids are used to build carbohydrates.

  Put it right: Amino acids are used to build proteins.

• Confusing digestion with respiration.

  Put it right: Digestion breaks food down, while respiration releases energy from glucose.

Exam tips

• Match each digestion product to the molecule it builds: amino acids to proteins, fatty acids and glycerol to lipids, sugars to carbohydrates.
• Include respiration as a use of glucose, and state that it releases energy for the cell.
• Use the phrase 'new' carbohydrates, lipids and proteins to show synthesis rather than simply reusing food molecules.