Enzyme Reaction Rates: Skills (SL IB Biology)

• Enzyme catalysed reactions can be affected by changes in pH, temperature or substrate concentration
• The rate of reaction can be determined by measuring the rate of disappearance of a substrate or the rate of product accumulated in a given time period
• This may be shown as a change in quantity (usually volume or mass) of substrate or product over a measured time period:

• Or, if we cannot collect quantitative data on the amount of substrate or product, we can calculate the rate of reaction based on the time measured using the following equation: 

• • 1 ÷ time taken (seconds) and should include the units s⁻¹
• A high rate of reaction is when the reaction happens in less time i.e. it is faster
• A low rate of reaction is when the reaction happens in more time i.e. it is slower
• The rate of a reaction is likely to change throughout a reaction as the substrate concentration will decrease as the reaction proceeds
  • This leads to a graph that starts out as a directly proportional straight line (the value on the X increases at the same rate as the value on the Y) but then plateaus as the reaction slows down
• The steeper the line the faster the rate of reaction
• The rate of reaction can be calculated from a graph plotted where the reaction time is shown on the X-axis and the quantity of product or substrate is shown on the Y-axis

Volume of a product produced against time graph

Graph produced when plotting the volume of a product produced against time

• The gradient is calculated from a point on the graph and used as a measure of the rate of reaction at that point in time
• A tangent must be drawn to calculate the change in x and y so the rate of reaction can be calculated
  • E.g. if calculating the initial rate of reaction 
    • Place a ruler on the point of origin and draw a line that corresponds to the curve during the early part of the reaction
    • Extend the line as far as is convenient to perform the calculations e.g. to 60 seconds

Drawing a tangent to calculate initial rate of reaction diagram

Drawing a tangent against the line through the origin to calculate the initial rate of reaction

Calculating the rate of reaction

• Once the tangent is drawn you can calculate the gradient of the line which is equal to the rate of the reaction
  • Initial rate = a ÷ b
  • Where
    • a = change in volume and
    • b = change in time
  • The units will be cm³ sec⁻¹ (this means volume per sec)

Calculating the rate of reaction from the tangent diagram
Rate of reaction is calculated by finding the gradient of the tangent from the origin