A limiting-factor graph plots the rate of photosynthesis against one factor, such as light intensity. You read the rising section, read the flat section, and say what limits the rate in each.
This lesson belongs to leaves and photosynthesis. The idea also appears in distinguishing limiting factors for enzymes.
What is the two-part reading routine?
Treat the curve as two sections and say something different about each.
- Rising section: the factor on the x-axis is limiting. Increasing it raises the rate.
- Flat section: the x-axis factor is no longer limiting. Another factor, held constant, now limits the rate.
In the rising section, the rate goes up in step with the factor. In the flat section, the plant is already using all the light it can with the carbon dioxide and temperature it has.
Worked example with two curves
A student measures the rate of photosynthesis in pondweed at increasing light intensity. She uses two water tanks: one with normal carbon dioxide and one with added carbon dioxide. All readings are in arbitrary units.
| Light intensity | Rate, normal CO₂ | Rate, added CO₂ |
|---|---|---|
| 0 | 0 | 0 |
| 2 | 10 | 10 |
| 4 | 20 | 20 |
| 6 | 30 | 30 |
| 8 | 30 | 40 |
| 10 | 30 | 50 |
Question 1: What limits the rate in both tanks between light intensity 0 and 6?
In both tanks the rate rises by 10 units for every 2 units of light. The two curves are identical, so light is the limiting factor in this range.
Question 2: What limits the rate in the normal tank from 6 to 10?
The rate stays at 30, so light is no longer limiting. The added-carbon-dioxide tank continues to rise at the same light intensities, so carbon dioxide is the limiting factor in the normal tank.
Question 3: What is the evidence that carbon dioxide, not temperature, is limiting?
The only difference between the two tanks is the carbon dioxide level. At light intensity 10 the rate is 30 with normal carbon dioxide and 50 with added carbon dioxide, so the added carbon dioxide raised the rate by 20 units.
A well-built answer quotes those numbers. “Carbon dioxide limits the rate because at light intensity 10 the added tank is 50 and the normal tank is 30.”
The mistake that costs marks
The common error is to say the flat section means “photosynthesis has stopped.” It has not stopped. The rate is steady, but at a maximum set by another factor.
Another slip is to say “light is limiting” on the flat section, because light is what the x-axis shows. On a flat section the plotted factor is exactly the one that is not limiting.
| Part of curve | Wrong | Right |
|---|---|---|
| Flat at 30 | “Light is limiting” | “Light is no longer limiting; carbon dioxide or temperature limits the rate” |
| Flat at 30 | “Photosynthesis stopped” | “The rate is constant at a maximum” |
| Rising | “The rate is high” | “The rate rises because light is limiting” |
Check yourself
In another experiment, leaves are kept at 20 °C and given enough light and carbon dioxide. The rate is 18 units. At 30 °C, with the same light and carbon dioxide, the rate is 27 units. Name the limiting factor at 20 °C and explain your choice.
Answer
Temperature is the limiting factor at 20 °C. Light and carbon dioxide are the same in both trials, yet the rate increases by 9 units (27 − 18) when only temperature rises.
Raising temperature increases enzyme activity in the light-independent stage, so more glucose is made.
What to study next
Use comparing light-dependent and light-independent processes to see why temperature and carbon dioxide act on one stage and light on the other. Try more tables in the graph evidence and fair-comparison lab.
If you want a teacher to go through a new graph with you section by section, see online one-to-one Biology tuition or the one-hour trial class (from RM50).