Lesson 6 of 12 / Nutrition and transport in flowering plants
Reading a limiting-factor graph
Why can extra light stop increasing the rate?
I can explain how light, carbon dioxide and temperature can limit photosynthesis.
I can investigate and describe light, CO2 and temperature effects on photosynthesis.Syllabus
Syllabus K224 / K225, Topic 8(h). Investigate and describe light, CO2 and temperature effects on photosynthesis.
About 6 min
Make a guess. You are not marked.
A greenhouse already has bright light. The grower adds more lamps but the plants grow no faster. Why?
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Something else, such as carbon dioxide, is now limiting.
Extra light helps only while light is in shortest supply. Then another factor holds the rate back.
The key idea
A limiting factor restricts the rate. Increasing it raises the rate only until another factor becomes limiting.
Relative photosynthetic rate
At low light, energy supply limits the rate. Extra CO2 has little effect while light remains limiting.
Brown: original CO2 availability. Green: higher CO2. Shading: selected light region. These are qualitative illustrative curves with no numerical scale, not measured data.
Explanation
A limiting factor is the factor in shortest supply, so it holds back the rate. Increasing the limiting factor increases the rate; increasing anything else does not. In dim light, light is usually the limiting factor. Giving more light then makes photosynthesis faster.
In bright light, the graph may level off. More light now makes little or no difference, because something else has become the limiting factor, such as carbon dioxide or temperature.
If adding more carbon dioxide in bright light makes the graph level off higher, then carbon dioxide was the limiting factor there. In dim light, extra carbon dioxide may make little difference, because light is still the limiting factor.
Exam graphs often show two curves, for example at two carbon dioxide concentrations. Where the curves lie on top of each other, the factor on the x-axis is the limiting factor. Where the curves separate, the factor that differs between them has become the limiting factor. Quote the levelled-off values with units, for example 5 cm3 per minute and 8 cm3 per minute.
Temperature affects the enzymes of photosynthesis. Warming can speed it up, up to an optimum. Above that, enzymes may be denatured and the rate falls.
Step by step
- 1
Locate the region
State the factor on the horizontal axis and where the observation lies.
- 2
Describe the pattern
Use rising, levelling off or falling, supported by the comparison.
- 3
Test the explanation
Use a second controlled treatment to identify a specific alternative limiting factor.
Worked example
Two light-response curves
At high light intensity, increasing CO2 raises oxygen output. At low light, it makes little difference. Explain both results.
One way to explain it
At high light, CO2 was limiting, so more CO2 allows a higher rate. At low light, light remains limiting, so extra CO2 alone cannot substantially increase the rate.
Why this answer works
- Use the effect of the changed CO2 treatment as evidence.
- Give separate explanations for the two light regions.
- Avoid saying that all factors are always equally limiting.
Watch out for this
A student says: "A flat light-response graph proves the plant has stopped photosynthesising." What is wrong with this?
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A plateau shows a steady rate, which can still be high. It means additional light is no longer increasing that rate under those conditions.
Check your understanding
A light-response graph reaches a positive plateau. What can be concluded from that graph alone?
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More light is not increasing the rate there; another factor may be limiting.
More evidence is needed to identify which other factor limits the rate.
Exam-style questions
Check your understanding
Invented data: at 0.01% carbon dioxide, the rate of photosynthesis stops rising above light intensity 3 units. At 0.04%, it rises until 6 units. What limits the rate at 0.01% above 3 units?
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Carbon dioxide concentration
Raising carbon dioxide raised the plateau, so carbon dioxide was limiting at 0.01%.
Quick recall
Why can extra light stop increasing the rate?