CIE 9700 Biology Enzymes: Temperature, pH and Exam Graphs
Enzyme questions can look like graph-reading exercises, but the marks are in explaining the mechanism behind the line. For Cambridge AS Biology 9700, a curve rising, reaching a maximum and falling is not enough on its own: you must know what is happening to molecules and the active site.
Temperature: two effects compete
At low to moderate temperatures, an increase normally raises enzyme activity because molecules have more kinetic energy and successful collisions occur more frequently. The rate rises towards an optimum under the stated conditions.
Above the optimum, the enzyme's structure can be disrupted. Changes in bonding alter the active site's shape, making successful enzyme-substrate complex formation less likely. Do not describe every small increase in temperature as instantly 'killing' an enzyme.
pH: the active site depends on charge and structure
Changes in pH alter the ionisation of groups on amino acid side chains. That can disturb interactions that maintain the enzyme's three-dimensional structure or directly affect binding at the active site.
An enzyme has an optimum pH under particular conditions, not a universal optimum shared by all enzymes. For a graph question, describe the pattern actually shown instead of assuming the peak must occur at pH 7.
Substrate concentration and the plateau
At low substrate concentration, adding substrate increases the chance of collisions with unoccupied active sites. When enzyme concentration is fixed, the rate eventually levels off as the active sites become occupied most of the time.
A plateau is not evidence that the reaction has 'stopped'. It means adding more substrate under those conditions does not substantially increase the rate. Raising enzyme concentration may change the maximum rate.
Inhibitors: ask what changes
Competitive inhibition involves an inhibitor competing with substrate for the active site. Non-competitive inhibition reduces activity through binding away from the active site and changing the enzyme's behaviour. Questions may test how the rate changes across different substrate concentrations.
The key is to reason from binding and site availability, not memorise the shape of a graph without understanding what the axes measure.
How to tackle an enzyme MCQ
Before looking at answers, identify the variable changed, the variable measured and what is held constant. Describe the plotted relationship in one sentence, then propose the enzyme-level explanation.
Work through topic notes, test yourself on a few paper questions, and return to the original graph whenever an answer surprises you. That sequence helps you distinguish genuine biological reasoning from recognising a familiar picture.
Continue revising with EIE
Read the syllabus-mapped material for the exact topic, then test your understanding. Use the linked practice resources where available and keep a short record of what you need to revisit.