A limiting-factor graph shows which condition is holding the rate of photosynthesis back. On a rising section, the factor on the x-axis is limiting. On a flat section, something else is. Your job is to say which factor, and to use the evidence on the graph.
This lesson follows inputs and products of photosynthesis and belongs to plant nutrition.
How do I read the graph step by step?
- Read the axes. Note what is on the x-axis (the factor being changed), what is on the y-axis (the rate) and the units.
- Find the rising section. Here the factor on the x-axis is limiting, because raising it raises the rate.
- Find where the curve flattens. Here the x-axis factor is no longer limiting.
- Name what limits the flat section. Use the conditions stated in the question, such as the carbon dioxide level or temperature.
- Compare curves if there are two. If a second curve goes higher at the same x value, the factor that was changed between the curves is limiting the lower curve.
Worked example
The data below are invented for practice. A student measured bubbles of oxygen per minute from pondweed at two carbon dioxide levels, with temperature kept constant.
| Light intensity (arbitrary units) | 0 | 2 | 4 | 6 | 8 | 10 |
|---|---|---|---|---|---|---|
| Rate, low CO₂ (bubbles/min) | 0 | 8 | 16 | 20 | 20 | 20 |
| Rate, high CO₂ (bubbles/min) | 0 | 8 | 16 | 24 | 30 | 30 |
Question: What limits the rate at light intensity 8 on the low CO₂ curve? Give evidence.
Step 1, locate the point. At 8 units the low CO₂ curve is flat at 20 bubbles per minute. It has been 20 since intensity 6.
Step 2, rule out light. Between 6 and 8 the light increased, yet the rate did not change. So light is not limiting here.
Step 3, compare the curves. At the same intensity of 8, the high CO₂ curve gives 30. The only difference is the carbon dioxide level.
Step 4, calculate the gain. 30 − 20 = 10 bubbles per minute more. As a percentage of 20, that is 10 ÷ 20 × 100 = 50%.
Answer: Carbon dioxide concentration is limiting at intensity 8, because raising it from low to high increased the rate from 20 to 30 bubbles per minute while light stayed the same.
For the rising part, the rate climbs 8 bubbles for every 2 units of light, which is 4 bubbles per unit. That steady rise shows light is limiting from 0 to about 4 on the low CO₂ curve, and to about 6 on the high one.
The mistake to watch for
A common slip is to say light is limiting on the flat section because “the light is at its maximum”.
Mistaken answer: “At the flat part light is the limiting factor because it cannot increase any more.”
The graph shows light still increasing along the x-axis, so the light has not run out.
The correction is that a flat line means the x-axis factor has stopped limiting. Name a different factor, and support it with the second curve or the conditions given.
Check yourself
Use the table above.
1. What limits the high CO₂ curve at light intensity 4?
Show answer
The curve is still rising at this point (16 at 4, 24 at 6), so light intensity is limiting.
2. What is the rate on the high CO₂ curve at intensity 10, and why does it stop rising after 8?
Show answer
The rate is 30 bubbles per minute. After intensity 8 light is no longer limiting, so another factor, such as temperature, now limits the rate.
3. On the high CO₂ curve, light doubles from 4 to 8. Does the rate double? Explain.
Show answer
The rate goes from 16 to 30. Double 16 is 32, so it does not quite double. By intensity 8 another factor has begun to limit the rate.
Where this leads next
Next, see how leaf structure supports the supply of light and carbon dioxide in leaf adaptations for exchange. The mistake log tool is a handy place to record which flat-line explanations you got wrong.
Flat-line explanations often lose marks on wording, not understanding. A teacher in online one-to-one Biology tuition can go through your written answers and show you what to add.