Observation and explanation are different jobs
When a question says describe, report what the data show: direction, shape, maximum or minimum, plateau, anomaly and relevant values. Do not explain why the pattern occurs unless asked. When it says explain, connect the pattern to a biological mechanism.
A useful description often follows overview, detail, evidence: state the overall relationship, identify a change in pattern, then quote paired values with units. This is stronger than listing every point or writing only 'it increases.'
| Command | What to do |
|---|---|
| State | Give a concise answer without working or explanation |
| Describe | Report patterns or features visible in data |
| Explain | Give linked biological reasons |
| Compare | Use paired similarities and differences |
| Suggest | Apply biology to an unfamiliar situation |
Read the axes before reading the story
Identify the independent variable on the x-axis, the dependent variable on the y-axis, units and scale intervals. Check whether the axis begins at zero and whether points represent discrete categories or continuous measurements.
For a line graph, plot small accurate points, use the required best-fit line or smooth curve, and avoid forcing a curve through an anomalous value. For a bar chart, keep bars separate when categories are discontinuous.
Variables, units, scale and legend.
Overall pattern, changes and anomaly.
Quote paired values with units.
Link the pattern to a mechanism.
Consider repeats, range and limitations.
Make calculations visible and unit-safe
Write the formula, substitute values, show working and give the final unit. Percentage change is (new − original) ÷ original × 100. Rate is change divided by time. Magnification is image size divided by actual size after converting both to the same units.
Check whether a negative percentage is meaningful and whether the question asks for decrease as a positive percentage. Keep enough working figures to avoid rounding error, then round only the final answer to a sensible precision.
Build explanations as linked steps
A high-quality explanation makes each consequence cause the next. For exercise and breathing rate: muscle contraction increases ATP demand; respiration rate rises; more carbon dioxide is produced; blood carbon dioxide and acidity change; receptors and the breathing centre increase ventilation.
Avoid jumping from stimulus directly to final response. Middle links are often where marks sit. Use biological nouns and verbs: enzyme–substrate collisions increase, stomata close, diffusion gradient becomes steeper, or water potential decreases.
Evaluate without writing generic criticism
Identify a specific limitation, explain how it could affect results, and propose a matching improvement. If temperature varied, enzyme activity may have changed; use a thermostatically controlled water bath. If colour was judged by eye, readings may be subjective; use a colorimeter.
A larger sample improves confidence only when sampling is representative and the method is consistent. More repeats cannot repair a systematic bias, so improvement must address the actual weakness.
The ideas to carry forward
- Describe data first; explain mechanisms only when requested.
- Use variables, units and paired values to support patterns.
- Show formulas and convert units before calculating.
- Match every evaluation point with a relevant improvement.
Answer first. Then reveal the marking logic.
01What is missing from the description 'the rate increased'?2 marks · show the biological link
Answer: It lacks the range, shape, any change or plateau, and numerical evidence with units.
02Why do repeated measurements not remove a systematic error?2 marks · show the biological link
Answer: A systematic error shifts measurements consistently in the same direction, so repeating the same biased method reproduces the bias.
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