AQA Biology Required Practical 1: Effect of a named variable on the rate of an enzyme-controlled reaction

To accurately measure how a specific concentration affects the rate, that concentration must actually be present.

Answer: Because as the reaction proceeds, substrate concentration decreases, becoming a limiting factor and slowing the rate.

Required practical 1. The initial rate is measured (usually by drawing a tangent at $t=0$) because at the very start of the reaction, the substrate concentration is known and is not yet acting as a limiting factor.

If investigating temperature (the independent variable), all other factors that affect enzyme activity must be controlled.

Answer: To maintain a constant pH, ensuring that any change in the rate of reaction is solely due to the temperature change.

Required practical 1. A buffer maintains a constant pH. pH is a control variable when investigating temperature; if the pH fluctuates, it could denature the enzyme, invalidating the results.

Boiling denatures the enzyme. Compare this tube with the treatment containing active enzyme.

Answer: To provide a comparison showing what happens when the enzyme has been denatured, helping test whether substrate breakdown depends on active enzyme.

A boiled-enzyme control provides a comparison in which the enzyme has lost its functional active-site shape. If substrate breakdown occurs with active enzyme but not with boiled enzyme, that supports the conclusion that the observed reaction depends on active enzyme.

If you mix a cold enzyme with a cold substrate and put them in a hot water bath, the initial reaction happens while the liquids are still cold.

Answer: To allow both solutions to equilibrate to the target temperature so the reaction occurs at the correct temperature from the very first second.

Required practical 1. Equilibration is crucial for validity. If the solutions are mixed before reaching the target temperature, the initial rate of reaction will be measured at a lower, incorrect temperature.

Rate is a measure of how fast something happens. If it takes a long time, the rate is low. If it takes a short time, the rate is high.

Answer: Rate = $1 / time$

Required practical 1. When the end-point of a reaction is measured by time taken (e.g., time for an 'X' to disappear in a milk powder/trypsin experiment), the rate is inversely proportional to time ($1/t$).

Buffers resist changes in hydrogen ion ($H^+$) concentration.

Answer: To keep the pH constant, ensuring the ionic bonds holding the enzyme's tertiary structure and active site together are not disrupted.

Required practical 1. pH affects the charges on the amino acids making up the active site. A buffer acts as a control variable, preventing pH fluctuations from acting as a confounding variable.

The negative control shows what happens when the active enzyme is omitted but the other conditions are kept the same.

Answer: The substrate solution, the buffer, and a volume of distilled water equal to the volume of the enzyme solution.

Replacing the enzyme solution with the same volume of water provides a negative control. If substrate breakdown occurs in the enzyme treatment but not in the control, that supports attributing the difference to the presence of active enzyme.

Enzymes are not used up, but the thing they are breaking down is.

Answer: The substrate concentration decreases over time and eventually runs out, meaning no more enzyme-substrate complexes can be formed.

Required practical 1. As the reaction proceeds, substrate is converted into product. The substrate concentration falls until it is fully depleted, at which point the reaction stops and the graph plateaus.