Linking a mechanism with evidence means using a figure from the results to show that your explanation is true. Questions say “explain the results” or “use the data to support your answer”. It appears in enzyme, osmosis, transpiration, photosynthesis and inheritance questions.
It builds on the chain from structure-function explanations, now with a number at one link.
How do you join a mechanism to a number?
Use three moves. Each is one short sentence.
- Pattern with a figure: say what the numbers do, quoting values and units.
- Mechanism: name the process that causes the pattern and how it works.
- Tie-back: say how the mechanism explains the specific figure you quoted.
A useful test: cover your answer except the numbers. If the marker cannot tell which figures you used, the evidence is not linked.
Worked example
All data here are invented for practice. A student added a catalase solution to hydrogen peroxide at different temperatures and collected the oxygen produced in 2 minutes.
| Temperature (°C) | Oxygen collected (cm³) |
|---|---|
| 20 | 4 |
| 30 | 9 |
| 40 | 16 |
| 50 | 7 |
| 60 | 1 |
Question: Explain the results.
Step 1, pattern with figures: oxygen rises from 4 cm³ at 20 °C to 16 cm³ at 40 °C, then falls to 7 cm³ at 50 °C and 1 cm³ at 60 °C.
Step 2, rising part: as temperature rises, enzyme and substrate molecules move faster, so they collide more often and with more energy. More substrate is broken down per minute. That explains the increase of 7 cm³ between 30 and 40 °C.
Step 3, falling part: above about 40 °C the enzyme begins to denature. The shape of the active site changes, so the substrate no longer fits. That explains why 50 °C gives 9 cm³ less than 40 °C.
Step 4, conclusion: the optimum in these results is close to 40 °C. Testing 35 °C and 45 °C would locate it more precisely.
The increase from 30 to 40 °C is 16 − 9 = 7 cm³, and the drop from 40 to 50 °C is 16 − 7 = 9 cm³, so both quoted differences are correct.
The mistake to watch for
A common slip is to write the theory and the data as two separate answers.
Mistaken answer: “Enzymes have an optimum temperature and denature when too hot. The results go up and then down.”
Both sentences are true, but no figure is used and nothing ties the two together.
The correction names a figure and then the mechanism that explains it: “At 50 °C only 7 cm³ was collected, because the enzyme has begun to denature and the substrate no longer fits the active site.”
Check yourself
Try each one, then open the answer. Figures are invented.
1. A pea seedling root loses mass in a strong sugar solution. Its mass was 2.0 g at the start and 1.7 g after an hour. Calculate the percentage change and explain it.
Show answer
Change = 1.7 − 2.0 = −0.3 g. Percentage change = −0.3 ÷ 2.0 × 100 = −15%. Water left the cells by osmosis, because the sugar solution had a lower water potential (more solute) than the cell contents.
2. In the table above, which figure would you quote to support the idea that the enzyme is almost fully denatured at 60 °C?
Show answer
1 cm³ at 60 °C, compared with 16 cm³ at 40 °C. Nearly all activity is lost, which fits denaturation of most enzyme molecules.
3. A model predicts Aa × Aa pea plants will give dominant and recessive offspring in a 3 : 1 ratio. A real sample of 8 offspring had 5 dominant and 3 recessive. Does this prove the model wrong?
Show answer
No. The model expects 6 dominant and 2 recessive from 8, and 5 and 3 is close. Fertilisation is random, so small samples vary by chance. A larger sample should be closer to the predicted ratio on average. You can explore such models on the inheritance model board or list outcomes with the counting board.
Where this leads next
A well-linked answer is easier to draw too, so continue to labelling an original diagram without overcrowding. If your evidence sentences drift into teleological wording, the later lesson on adaptation language covers that.
Some students can see the trend and explain the theory separately, yet lose marks at the joining sentence. That is the exact step our teachers practise in online one-to-one Biology tuition.