A data answer has three parts: the figures, the comparison and the biological reason. The reason for changes in oxygen and carbon dioxide is usually cell respiration.
This lesson follows comparing gas exchange in selected organisms and belongs to the gas exchange section.
Worked example: a fictional jogging experiment
These are invented values for practice, not measured data. Air samples were collected before and after a student jogged.
| Sample | Oxygen (%) | Carbon dioxide (%) |
|---|---|---|
| Inhaled air | 20.9 | 0.04 |
| Exhaled air at rest | 16.8 | 3.6 |
| Exhaled air after jogging | 15.9 | 4.5 |
Calculate the change for each condition.
- Oxygen used at rest = 20.9 − 16.8 = 4.1 percentage points.
- Oxygen used after jogging = 20.9 − 15.9 = 5.0 percentage points.
- Carbon dioxide gained at rest = 3.6 − 0.04 = 3.56 percentage points.
- Carbon dioxide gained after jogging = 4.5 − 0.04 = 4.46 percentage points.
How do I write the explanation?
Link the figures to the biology in two sentences.
“After jogging, exhaled air has less oxygen (15.9%) and more carbon dioxide (4.5%) than exhaled air at rest (16.8% and 3.6%). The muscle cells respire faster during exercise, so they use more oxygen and produce more carbon dioxide, which the blood carries to the lungs.”
The first sentence carries the evidence. The second gives the cause.
The mistake that costs marks
The common slip is to describe only. A student writes that oxygen went down and carbon dioxide went up.
| Answer | Marks lost |
|---|---|
| “Oxygen decreased and carbon dioxide increased.” | No figures, no comparison and no reason |
| “Exhaled air has no oxygen left.” | Wrong: the table shows 16.8% and 15.9% |
| Correct: “Oxygen falls from 20.9% to 15.9% because muscle cells respire faster” | Figure, comparison and reason |
Also read the units. The values are percentages, so the differences are in percentage points.
Check yourself
Using the table, state how much more carbon dioxide was in the exhaled air after jogging than at rest, and give one reason.
Answer
4.5 − 3.6 = 0.9 percentage points more.
During jogging, the muscle cells respire at a faster rate, so they produce more carbon dioxide, and the blood carries it to the lungs to be breathed out.
What to study next
Test all four skills in the gas exchange practice set. To see why cells make carbon dioxide at all, read about cellular respiration. The graph evidence and fair-comparison lab offers practice with data questions.
For a teacher to go through your data answers, see online one-to-one Biology tuition.