Graph questions ask you to describe how rate changes with temperature or pH, to explain the change, and to use values from the graph. Keep these three jobs separate and you will collect most of the marks.
This lesson belongs to biological molecules and enzymes and follows on from enzyme specificity.
What does each part of the curve mean?
A temperature curve rises, peaks at the optimum, then falls steeply. A pH curve rises to a peak and falls on both sides.
Rising part: the rate increases as conditions approach the optimum.
Peak: the optimum, where the rate is highest.
Falling part: the rate decreases beyond the optimum, because the active site is changing shape.
A routine for graph answers
- Describe using direction words: increases, reaches a maximum, decreases.
- Quote data: a value and its unit from the axis.
- Explain with enzyme ideas.
- Limit your claim to the readings you actually have.
Worked example
The data below are invented for practice. They show the rate of an enzyme-catalysed reaction at different temperatures.
| Temperature (°C) | 10 | 20 | 30 | 40 | 50 | 60 |
|---|---|---|---|---|---|---|
| Rate (units per minute) | 2 | 5 | 10 | 18 | 6 | 0 |
Describe: the rate rises from 2 units per minute at 10 °C to 18 units per minute at 40 °C. It then falls to 6 at 50 °C and 0 at 60 °C.
Compare: between 20 °C and 30 °C the rate doubles, since 5 × 2 = 10.
Optimum: the highest reading is at 40 °C, so the optimum is about 40 °C. It could lie anywhere between 30 and 50 °C, since those are the neighbouring readings, so testing at 35 °C and 45 °C would give a better estimate.
Explain the fall: above the optimum the enzyme’s active site changes shape, so the substrate no longer fits.
A rate from time data. In a second experiment, starch disappeared in 40 seconds at pH 7 and in 100 seconds at pH 5. Using rate = 1000 ÷ time, pH 7 gives 1000 ÷ 40 = 25 units and pH 5 gives 1000 ÷ 100 = 10 units. The reaction is 25 ÷ 10 = 2.5 times faster at pH 7.
The mistake to watch for
A student writes: “The optimum is exactly 40 °C because that is the peak.” The data only have readings every 10 °C, so the peak cannot be pinpointed.
Another slip is treating a longer time as a faster rate. Time and rate move in opposite directions.
Check yourself
1. Using the table above, state the rate at 50 °C and describe the change from 40 °C.
Show answer
The rate at 50 °C is 6 units per minute. It decreases from 18 at 40 °C, a fall of 12 units per minute.
2. A reaction takes 20 s at 35 °C and 80 s at 15 °C. How many times faster is it at 35 °C?
Show answer
The time is 80 ÷ 20 = 4 times shorter, so the rate is 4 times faster at 35 °C.
3. Pepsin works well at about pH 2. State what you would expect at pH 8 and why.
Show answer
The rate would be very low or zero. At pH 8 the pepsin’s active site changes shape, so the substrate cannot fit.
Where this leads next
The steep fall above the optimum is a different process from the slow rate in the cold, which the next lesson on denaturation separates. The practice set gives mixed graph questions.
Graph answers lose marks when the explanation is missing, and that is hard to notice alone. A teacher in online one-to-one Biology tuition can ask “why?” after each sentence until your answer is complete.