The enzyme lactase hydrolyses the disaccharide lactose to glucose and galactose. This enzyme is produced in the small intestine of mammals and by some fungi and bacteria. Fig. 1.1 shows this reaction. [Figure 1.1] The Michaelis-Menten constant, Km, shows the affinity of an enzyme for its substrate. The lower the Km the greater the affinity. A student carried out an investigation to find the Km of the enzyme lactase at different pH values commonly found in organisms. The student: • made 1 dm³ of 0.15moldm⁻³ solution of lactose, molar mass 342.3 g mol⁻¹ • used proportional dilution to make a total of five lactose solutions from 0.15moldm⁻³ stock solution, each of 500cm³ • used 1 g per 100 cm³ lactase solution • used a glucose biosensor to measure the initial rate of reaction at each pH. A glucose biosensor measures glucose in mmoldm⁻³. Fig. 1.2 shows the experimental set-up. [Figure 1.2]