Turn up the heat.
Cook an enzyme until it stops working, leave potato in sugar solution overnight, and move a lamp until light stops being the limiting factor — three GCSE Biology practicals.
An enzyme has one favourite temperature and one favourite pH
FIG. 01 · ENZYMES · 4.2.2.1Amylase breaks starch into maltose in the mouth and small intestine. It works best around pH 7.
Heat gives particles more kinetic energy, so enzyme and substrate collide more often — up to a point.
The dashed curve shows the same enzyme at its optimum pH, so you can see how much the pH is costing you.
In the exam: never write that an enzyme is "killed" — it is not alive. Say that the wrong temperature or pH changes the shape of the active site, so it is no longer complementary to the substrate, so fewer enzyme–substrate complexes form and the rate falls. Below the optimum the rate is low because particles have less kinetic energy and collide less often; above it the enzyme denatures, and that change is permanent. The required practical uses iodine to time how long amylase takes to digest starch at different pH values.
Leave potato in sugar water and weigh it again
FIG. 02 · OSMOSIS · 4.1.3.2Distilled water is 0.00. The cylinder is left for 30 minutes, blotted dry and reweighed.
Where the line crosses zero, the solution and the potato cells have the same concentration, so there is no net movement of water. That point estimates the cells' own internal concentration.
In the exam: osmosis is the movement of water from a dilute solution to a concentrated one through a partially permeable membrane. In dilute solutions the potato cells gain water, become turgid and the cylinder gains mass; in concentrated solutions they lose water, become flabby and eventually plasmolysed, and the cylinder loses mass. Percentage change in mass, not raw change, is what you plot — because the cylinders never start exactly the same size, and percentages make them comparable.
Move the lamp until light stops being the problem
FIG. 03 · PHOTOSYNTHESIS · 4.4.1.2Halve the distance and the intensity goes up four times — the inverse square law, and the reason distance is a poor x-axis on its own.
Above about 40 °C the enzymes controlling photosynthesis denature and the rate collapses, so growers stop well short.
In the exam: a limiting factor is the one in shortest supply — the factor that, if you increased it, would increase the rate. On the steep part of the curve, light is limiting; once the line flattens, adding light does nothing and something else (carbon dioxide or temperature) has taken over. Greenhouse questions are the same graph in disguise: growers pay for extra CO₂ and heat only because they have already stopped light being the limit. The pondweed practical measures rate as bubbles of oxygen per minute at different lamp distances.