Demonstrate an understanding of rate calculations for enzyme activity

The idea

Rate of enzyme activity measures how quickly a substrate is used up or a product is formed per unit time. A common method: rate = amount of product formed ÷ time taken, or rate = amount of substrate used ÷ time. For example, if 30 cm³ of gas is collected in 20 s, the rate is 30 ÷ 20 = 1.5 cm³ s⁻¹. You may also calculate rate from the gradient of a tangent on a graph of product against time, or from the reciprocal of the time to complete a reaction. Units matter: keep the amount unit and the time unit consistent, and state the rate with combined units such as cm³ s⁻¹ or g min⁻¹. Comparing rates at different temperatures or pH values shows how conditions affect enzyme activity.

What earns the marks

• Rate = amount of product formed (or substrate used) ÷ time taken.
• Correctly substitute values and calculate, e.g. 30 cm³ ÷ 20 s = 1.5 cm³ s⁻¹.
• Use consistent units and express the rate with combined units such as cm³ s⁻¹ or g min⁻¹.
• Rate can be found from the gradient of a tangent to a product–time graph.
• Compare rates under different conditions (e.g. temperature, pH) to interpret enzyme activity.

Common mistakes and how to put them right

• Dividing time by amount instead of amount by time.

  Put it right by putting the amount of product or substrate on top.

• Mixing units, e.g. seconds and minutes, without converting.

  Put it right: Convert so both quantities use the same time unit.

• Reading the gradient of a curve as a single value.

  Put it right: Use a tangent at the chosen point for the rate at that moment.

Exam tips

• Write the formula before substituting numbers to avoid inverting the calculation.
• Always include units in the final answer, combining amount and time units.
• When using a graph, draw a tangent at the required point and calculate its gradient.