Lesson 5 of 5 / Photosynthesis and energy capture
Identify a limiting factor
What does a plateau reveal, and what further comparison tests it?
I can investigate effects of light, CO2 and temperature on photosynthesis.
I can investigate limiting factors in photosynthesis.Syllabus
Syllabus 9477, Core 3(e). Investigate limiting factors in photosynthesis.
About 7 min
Make a guess. You are not marked.
A light-response curve levels off at high light. Raising CO2 makes the plateau higher. What was limiting the plateau?
Show the answer
CO2 concentration.
If raising a factor raises the rate, that factor was limiting there.
The key idea
A limiting factor is identified by changing it and observing a response while other conditions are controlled.
Explore the idea
Does extra CO2 raise this part of the curve?
The green curve rises and then plateaus. This alone does not name the limiting factor. Add the controlled CO2 comparison to test one explanation.
Illustrative data, with lines joining three measured conditions rather than a fitted physiological curve. Hold temperature and sample amount constant and repeat readings. Net gas exchange includes respiration. An excessive temperature can lower rate even if light and CO2 are plentiful.
Explanation
At low light, increasing light can raise photosynthetic rate by supplying more energy for light-dependent reactions. At high light a plateau may occur because another factor, such as CO2 supply or enzyme capacity at the current temperature, limits the rate. A plateau alone does not identify which factor is responsible.
Compare curves at different CO2 concentrations while keeping light and temperature controlled. If higher CO2 raises the plateau, CO2 had limited that region of the lower-CO2 curve. At very low light, increasing CO2 may have little effect because energy supply remains limiting.
Temperature affects enzyme-catalysed carbon fixation and other processes. Increasing temperature within a suitable range can increase rate, while excessive heat disrupts proteins and can alter stomatal behaviour or water status in intact leaves. Control temperature when using a lamp so that light and heating effects are not confused.
Measure oxygen volume or CO2 exchange with appropriate controls and repeated samples; bubble counts are less precise because bubble size varies. Net oxygen production reflects photosynthesis minus respiration. Standardise sample leaf area or biomass and account for background gas exchange before drawing quantitative comparisons.
Step by step
- 1
Describe the observed curve
Give the rising and plateau regions.
- 2
Use a second controlled condition
Ask whether extra CO2 or changed temperature raises the rate.
- 3
Evaluate the measurement
Consider gas dissolution, respiration and sample size.
Worked example
Work through the evidence
At high light, rate is 8 units with low CO2 and 14 with high CO2. At low light both are 2. Interpret.
One way to explain it
CO2 limits the high-light rate in the lower-CO2 condition, while light limits the low-light region strongly enough that extra CO2 gives no measured increase.
Why this answer works
- Use the response to the changed factor.
- Do not extrapolate one limiting factor across the entire graph.
Watch out for this
A student says: "The factor on the x-axis must limit every point of the curve." What is wrong with this?
Show the answer
Once the curve plateaus, another condition may limit further response.
Check your understanding
Which design best isolates a light-intensity effect?
Show the answer
Vary light while controlling temperature, CO2 and sample amount.
This helps attribute rate differences to light.
Quick recall
What does a plateau reveal, and what further comparison tests it?