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Enzyme Reaction Rates: Skills (HL IB Biology)

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Determining Enzyme Reaction Rates

  • 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:

Rate of reaction formula

  • 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: 

rates-of-reaction-2

    • 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 graphTangent initial reaction rate (1), downloadable AS & A Level Biology revision notes

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-line

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 diagramcalculating-the-rate-from-a-tangent
Rate of reaction is calculated by finding the gradient of the tangent from the origin

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Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.