The human digestive system: Students should be able to understand simple word equations but no chemical symbol equations are required.

The idea

In digestion, enzymes break large insoluble food molecules into smaller soluble products. A word equation names the reactant on the left and the product or products on the right, joined by an arrow, for example starch → glucose, or protein → amino acids. The arrow means 'is converted into'. You should be able to read, complete and write such equations for the three main nutrient groups, naming the substrate, the enzyme type and the products. For example, lipase acts on lipids to produce fatty acids and glycerol, written as lipids → fatty acids + glycerol. You do not need to write or balance chemical symbol equations such as C₆H₁₂O₆, and no formulae are required. Always check that every substance named in the statement appears in your equation and that the arrow points from substrate to product.

What earns the marks

• A word equation names the substrate on the left of the arrow and the product or products on the right, with the arrow read as 'is converted into'.
• For carbohydrates, the substrate is starch or other carbohydrate and the product is simple sugars such as glucose.
• For proteins, the substrate is protein and the products are amino acids.
• For lipids, the substrate is lipid or fat and the products are fatty acids and glycerol.
• Each equation should name the relevant enzyme, such as amylase, protease or lipase, when the question asks for the enzyme involved.
• The equation must be consistent with the idea that large insoluble molecules are converted into small soluble molecules that can be absorbed.
• No chemical symbols or formulae are needed; a correct answer can be written entirely in words.

Common mistakes and how to put them right

• Writing the arrow in the wrong direction, for example glucose → starch.

  Put it right: The arrow always points from the large substrate to the small products, so write starch → glucose.

• Naming only one product for lipid digestion, for example lipids → fatty acids.

  Put it right: Lipids are converted into fatty acids and glycerol, so both products must be named.

• Using chemical formulae such as C₆H₁₂O₆ instead of words.

  Put it right: This specification only requires word equations, so write glucose rather than a formula.

• Confusing the enzyme with the substrate, for example writing amylase → starch.

  Put it right: Name the substrate first and the product after the arrow, and mention the enzyme separately if required.

Exam tips

• Underline the substrate and the products in the question before writing your equation so you do not omit a product.
• Practise the three key equations until you can write them from memory: starch → glucose, protein → amino acids, and lipids → fatty acids + glycerol.
• If a question asks for the enzyme as well, write it before the arrow, for example starch --amylase--> glucose, and keep the arrow pointing to the products.
• Check that every word you use is a named substance, not a description such as 'broken down food'.