To test a claim, decide what the data would look like if the claim were true, then compare that with the numbers you are given. Your conclusion should quote figures and say what the results support, not what they prove.
This lesson applies limiting-factor reading to a written data set. It belongs to plant nutrition.
What steps should I follow?
- Restate the claim in one sentence.
- Write the prediction. If the claim is true, what should happen to the results?
- Process the data. Find means from repeats, and work out ratios or differences.
- Compare prediction with results, quoting numbers.
- Write the conclusion, using “supports” or “does not support”.
- Comment on reliability: controls, repeats and anything that could have changed unintentionally.
Worked example
The data are invented for practice. A student counted oxygen bubbles per minute from pondweed at three light intensities, with three repeats each. Water temperature was not monitored.
Claim: “The rate of photosynthesis is directly proportional to light intensity.”
| Relative light intensity | Repeat 1 | Repeat 2 | Repeat 3 | Mean |
|---|---|---|---|---|
| 1 | 4 | 5 | 6 | ? |
| 2 | 8 | 9 | 10 | ? |
| 4 | 9 | 10 | 11 | ? |
Step 1, prediction. If the rate is proportional to light, doubling the intensity should double the rate.
Step 2, means. Intensity 1: (4 + 5 + 6) ÷ 3 = 15 ÷ 3 = 5. Intensity 2: (8 + 9 + 10) ÷ 3 = 27 ÷ 3 = 9. Intensity 4: (9 + 10 + 11) ÷ 3 = 30 ÷ 3 = 10.
Step 3, ratios. From intensity 1 to 2, the rate goes 5 to 9, a factor of 9 ÷ 5 = 1.8. From 2 to 4 it goes 9 to 10, a factor of 10 ÷ 9 ≈ 1.1.
Step 4, compare. Doubling from 1 to 2 gives nearly double (1.8). Doubling from 2 to 4 gives only about 1.1 times the rate.
Step 5, conclusion. The results support the claim at low intensity but do not support it across the whole range, because the rate levels off between 2 and 4. Another factor, such as carbon dioxide or temperature, is probably limiting at higher intensity.
Step 6, reliability. Temperature was not monitored, and a lamp moved closer can warm the water. That change could affect the rate. A better design would keep temperature constant, for example with a water bath, and add carbon dioxide so it is not limiting.
Answer: The claim is only partly supported: proportional at low light, not at 4 units.
The mistake to watch for
A common slip is to state that the data “prove” a claim, or to judge it from one reading.
Mistaken answer: “Reading 9 at intensity 2 is almost double 5, so the experiment proves the claim.”
This uses one comparison and ignores the flat section between 2 and 4.
The correction is to use all of the data, calculate the ratio at every step, and use careful wording: “supports”, “does not support”, “suggests”. Check also whether a variable that was not controlled could explain the pattern.
Check yourself
Use invented data again.
1. A covered part of a leaf stayed orange-brown after the iodine test while the uncovered part turned blue-black. What does this support?
Show answer
Blue-black shows starch. The results support the claim that light is needed to make starch, because the part kept from light had none. A control leaf kept entirely in the light would strengthen this.
2. Bubbles per minute at one intensity were 12, 14 and 13. Find the mean.
Show answer
(12 + 14 + 13) ÷ 3 = 39 ÷ 3 = 13 bubbles per minute.
3. Give one variable that must be controlled in the bubble experiment.
Show answer
Any one of: temperature, carbon dioxide concentration, the species or amount of pondweed, or the time over which bubbles are counted.
Where this leads next
Next, separate the plant’s carbon source from the minerals it takes from soil in mineral uptake versus carbon acquisition. Practise the whole route in the plant nutrition practice set.
Conclusions are easy to over-claim without noticing. A teacher in online one-to-one Biology tuition can check your wording against the data, one sentence at a time.