The enzyme urease catalyses the breakdown of urea into carbonate ions and ammonium ions as shown in Fig. 1.1. urease urea + water --[urease]--> carbonate ion + ammonium ion CO(NH₂)₂ 2H₂O CO₃²⁻ 2NH₄⁺ Fig. 1.1 A solution of urea does not conduct electricity, but a solution of the two ions does conduct electricity. An increase in ions increases conductivity, so the rate of reaction is proportional to the conductivity. A meter measures conductivity in microsiemens per centimetre (µScm⁻¹). The conductivity meter also records temperature. K_m shows the affinity of an enzyme for its substrate. The lower the K_m the greater the affinity. A student carried out an investigation to find the K_m of the enzyme urease at different temperatures. The student: • made 500 cm³ of 0.2 mol dm⁻³ solution of urea, molar mass 60.1 g mol⁻¹ • used serial dilution to make a total of five urea solutions from 0.2 moldm⁻³ stock solution • used 1 g per 10 cm³ urease solution • used a conductivity meter to measure the initial rate of reaction at each temperature. Fig. 1.2 shows the experimental set-up during the course of a reaction. [Figure 1.2]