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 K327 / K328, 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 that is in shortest supply, so it holds back the rate of a process. Increasing a limiting factor increases the rate; increasing any other factor does not. At low light intensity, absorbed light energy may limit photosynthesis. Increasing light then increases the rate, provided enough carbon dioxide is available and temperature is suitable. A rising part of a rate-versus-light graph is consistent with light limiting the rate under those conditions.
At higher light intensity, a graph may level off. More light now produces little or no increase because another factor limits the rate. Carbon dioxide supply or temperature may be limiting. A plateau by itself does not identify which one; compare another controlled treatment to decide.
If increasing carbon dioxide at the same high light intensity raises the plateau, the original carbon dioxide availability was limiting there. At low light, extra carbon dioxide may have little effect because light still limits the process. Identify the region of the graph rather than assigning one limiting factor to the whole curve.
Exam graphs often show several curves, for example rate against light intensity at two carbon dioxide concentrations or two temperatures. Read them in three moves. First, where the curves overlap, the factor on the x-axis is the limiting factor, because changing the other condition makes no difference there. Second, where the curves separate, the condition that differs between them has become the limiting factor. Third, quote the plateau values with units, for example 5 cm3 per minute at the lower and 8 cm3 per minute at the higher carbon dioxide concentration.
Temperature affects enzyme-controlled reactions. Starting from a low temperature, warming can increase the rate up to an optimum under the stated conditions. Above a suitable range, enzyme activity can be impaired and enzymes may denature, reducing the rate. The optimum is not one fixed value for every species and environment.
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?