Understanding how fast enzymes work explains why biological processes, like digestion, happen efficiently rather than taking days. Enzyme activity is a measure of the quantity of substrate converted to product per unit of time. We measure this using the rate of reaction, which tells us how quickly a substrate is used up or a product is formed.
The rate is not constant throughout a reaction. The initial rate of reaction (at ) is always the fastest because the substrate concentration is at its highest, leading to the maximum frequency of successful collisions. As the reaction progresses, the substrate is used up and the rate decreases.
The standard formula used to calculate this is:
Note: Edexcel interchangeably uses "amount of substrate used" or "change in substance" depending on the experiment.
In an experiment, catalase breaks down hydrogen peroxide. The oxygen gas produced is collected using a gas syringe. The reaction produces of oxygen gas in . Calculate the rate of reaction.
Step 1: Convert time into standard units (seconds).
Step 2: Substitute the experimental values into the formula.
Step 3: Calculate the final answer with units.
(Note: For Higher Tier papers, be prepared to write units using index notation, such as ).
You can easily measure the volume of a gas produced in a syringe, but try measuring the exact volume of starch disappearing on a spotting tile. For experiments with a fixed endpoint rather than a measurable volume, we calculate a "relative rate".
This is central to investigating the effect of pH on amylase. You record the time taken for starch to be completely broken down, indicated by the point where added iodine does NOT turn blue-black and stays orange/brown. The formula used is:
Amylase breaks down starch at pH 6. It takes for the iodine to stay orange/brown.
Step 1: Write down the formula.
Step 2: Substitute the experimental value.
Step 3: Calculate the final answer with standard units.
Because calculating often results in very small decimals that are difficult to plot on a graph, examiners frequently suggest using a multiplier. You can use to produce easy-to-graph whole numbers (e.g., arbitrary units).
Why do graphs of enzyme reactions curve instead of going up in a straight line? As the substrate is used up, the rate constantly drops. Eventually, the line forms a flat plateau, meaning the reaction has finished and the rate is zero.
To find the instantaneous rate at a specific point on the curve, you must draw a tangent. This is a straight line that touches the curve at a single point, but does NOT cross through it. You then calculate the gradient (steepness) of this line.
Calculate the rate of reaction at from a curved volume-time graph.
Step 1: Draw the tangent.
Step 2: Pick two easy-to-read points on your tangent line.
Step 3: Substitute into the gradient formula (Change in y Change in x).
Step 4: Calculate the final answer with units.
Students often calculate the mean rate (total volume divided by total time) when asked for the rate at a specific time. You must draw a tangent for specific time points, or you will score 0 marks.
When calculating an answer, examiners typically expect you to round your final value to 3 significant figures unless the question specifies otherwise.
If you are asked to draw a tangent to find the initial rate of reaction, ensure your straight line passes exactly through the origin (0,0).
When plotting the results of an investigation (e.g., how pH affects enzymes), always put the Rate on the y-axis and the independent variable (e.g., pH) on the x-axis.
Enzyme activity
A measure of the quantity of substrate converted to product per unit of time by an enzyme.
Rate of reaction
A measure of how quickly a substrate is used up or a product is formed over a specific period of time.
Initial rate of reaction
The speed of the reaction at the very start (t=0), representing the maximum rate before substrate concentration drops.
Tangent
A straight line drawn to touch a curve at a single point without crossing it, representing the gradient at that exact moment.
Gradient
The steepness of a line on a graph, calculated as the change in the vertical axis divided by the change in the horizontal axis.
Put your knowledge into practice — try past paper questions for Biology
Enzyme activity
A measure of the quantity of substrate converted to product per unit of time by an enzyme.
Rate of reaction
A measure of how quickly a substrate is used up or a product is formed over a specific period of time.
Initial rate of reaction
The speed of the reaction at the very start (t=0), representing the maximum rate before substrate concentration drops.
Tangent
A straight line drawn to touch a curve at a single point without crossing it, representing the gradient at that exact moment.
Gradient
The steepness of a line on a graph, calculated as the change in the vertical axis divided by the change in the horizontal axis.