12.2.11—Energy yield

Temperature and substrate concentration can change the rate of respiration because respiratory reactions depend on enzyme activity and on the availability of substrate. The rate should be inferred from a time-based or volume-based readout under controlled conditions, not from an unsupported endpoint comparison.

• Temperature: changing temperature changes molecular kinetic energy and enzyme-catalysed collision frequency. Rate may rise as temperature increases, but very high temperature can denature respiratory enzymes and lower the rate; do not assign a universal optimum value.
• Substrate concentration: increasing the available respiratory substrate can increase rate while substrate availability limits the pathway. Comparisons must keep the substrate type and all other relevant conditions consistent.
• Redox-indicator readout: in a yeast suspension, DCPIP or methylene blue can accept hydrogen and change from blue to colourless when reduced. Faster discolouration indicates faster hydrogen release and therefore a faster respiration rate. For a fixed endpoint, rate is inversely related to the time taken.
• Oxygen-consumption readout: a respirometer can measure oxygen consumption by a suitable aerobic organism; a controlled temperature and a time-normalised gas-volume change provide a rate. This readout is not a measure of anaerobic respiration, which does not consume oxygen.

1. Choose one independent variable: a range of temperatures or a range of substrate concentrations.
2. Keep yeast-cell volume and concentration, substrate type, dye volume and concentration, temperature of added solutions, reaction time and other relevant conditions matched. Use a no-dye control when the yeast suspension has its own colour.
3. Measure the same defined readout for every condition: for example, time to the same indicator endpoint or oxygen-volume change per unit time.
4. Repeat each condition, calculate a representative rate, and compare the rates or plot rate against the independent variable.
5. Interpret the pattern cautiously: use the early, comparable rate rather than allowing a late endpoint or exhausted substrate to dominate the conclusion.

A fair test changes one independent variable at a time. Do not infer a precise optimum temperature, invent apparatus specifications or numerical values, or treat colour-change time as a rate without accounting for the inverse relationship. Distinguish the yeast indicator investigation of anaerobic respiration from an oxygen-consuming respirometer investigation of aerobic respiration.