Rate Calculations for Enzyme Activity (Edexcel GCSE Combined Science): Revision Note
Exam code: 1SC0
Rate Calculations for Enzyme Activity
Rate calculations are important in determining how fast an enzyme is working (i.e. the rate of reaction)
To perform a rate calculation, use the following formula:
rate = change ÷ time
Change = the change in the substance being measured
E.g. the amount of substrate used up in the reaction or the amount of product formed
Time = the time taken for that change to occur
Worked Example
Amylase catalyses the breakdown of starch into maltose. 15 grams of starch were added to a solution containing amylase. It took 2 hours for all the starch to be broken down.
Calculate the rate of reaction.
Answer:
Step 1: write out the equation for calculating the rate of enzyme activity
rate = change ÷ time
In this case: rate = amount of substrate used ÷ time
Step 2: substitute in the known values and calculate the rate
rate = 15 g ÷ 2 hours
rate = 7.5 g / hr or 7.5 g hr⁻¹
Worked Example
The enzyme catalase catalyses the breakdown of hydrogen peroxide into water and oxygen. In one experiment, a student found that 45 cm³ of oxygen was released in 5 minutes.
Calculate the rate of reaction.
Answer:
Step 1: write out the equation for calculating the rate of enzyme activity
rate = change ÷ time
In this case: rate = amount of product formed ÷ time
Step 2: substitute in the known values and calculate the rate
rate = 45 cm³ ÷ 5 minutes
rate = 9 cm³ / min or 9 cm³ min⁻¹
In some situations you may not be told how much something has changed during a reaction; instead, you may only be told the time taken for the reaction to occur
In this case you can still calculate the rate of reaction by using the following (slightly different) formula:
rate = 1 ÷ time
Worked Example
A student adds a set volume of starch solution to a set volume of amylase solution at a range of different pH values. At each pH, the student times how long it takes for the amylase to break down all of the starch. At pH 6 the time taken for amylase to break down all of the starch was 50 seconds.
Calculate the rate of reaction at pH 6.
Answer:
Step 1: write out the equation for calculating the rate of enzyme activity
rate = 1 ÷ time
Step 2: substitute in the known values and calculate the rate
rate = 1 ÷ 50 seconds
rate = 0.02 s⁻¹
The units for the calculation above are in s⁻¹ because rate is given per unit time.
Examiner Tips and Tricks
In an exam you could be asked to plot the reaction rates (from an enzyme catalysed reaction) on a graph. However, using the equation 'rate = 1 ÷ time' often gives small numbers that are difficult to plot on a graph. In these cases, you can also use the equation:
rate = 1000 ÷ time
This equation give you bigger numbers that are easier to plot on a graph. So, for the calculation in the worked example above, you would get:
rate = 1000 ÷ 50 seconds
rate = 20 s⁻¹
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