Required practical 1: Investigation into the effect of a named variable on…

The idea

You choose one named independent variable, temperature, pH, substrate concentration, enzyme concentration or inhibitor concentration, and measure a rate. The common systems are catalase from potato, celery or yeast acting on hydrogen peroxide, with the volume of oxygen collected in a gas syringe per unit time; amylase acting on starch, taking samples into iodine in potassium iodide solution at fixed intervals and timing how long the blue-black colour takes to disappear; and trypsin clearing a milk suspension, timed or followed with a colorimeter. Enzyme and substrate must be brought separately to the test temperature in a water bath before mixing, the pH held with a buffer, and every other volume and concentration kept the same, so that only the chosen variable changes. Timing starts at mixing.

What earns the marks

• Naming the independent variable and stating a suitable range and interval of values.
• A valid measure of rate, such as volume of oxygen collected per minute or one divided by the time for the colour to disappear.
• Two relevant controlled variables, such as volume and concentration of substrate, enzyme concentration, pH held with a buffer, or temperature held in a water bath.
• Equilibrating enzyme and substrate separately to the set temperature before mixing, and starting timing at the moment of mixing.
• Repeating at each value, discarding anomalies and calculating a mean.

Common mistakes and how to put them right

• Quoting the total volume of gas collected as the rate, instead of volume per unit time.

• Reading rates from the flattening part of a product against time curve rather than using initial rate from a tangent at the origin.

• Mixing enzyme and substrate before either has reached the water bath temperature, so the first readings are at the wrong temperature.

• Writing 'the same amount of enzyme' rather than the same volume and the same concentration.

• Saying repeats make the results accurate, when repeats improve repeatability and do nothing about a systematic error.

Exam tips

• Give rate with correct units: a volume of oxygen collected per second has units cm³ s⁻¹, while s⁻¹ applies only to a rate expressed as 1/time.
• Name the buffer and the water bath explicitly; 'kept pH the same' without saying how does not earn the control mark.
• If asked to improve the method, change one thing that reduces uncertainty, such as smaller intervals near the optimum or a data logger instead of a stopwatch.