Choose a useful greenhouse change from limiting-factor evidence
Prioritise the condition that demonstrably restricts the stated plant response.
Section B6.1 · Photosynthesis
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall chloroplasts, enzymes and the difference between photosynthesis and respiration.
- Use the stated observations, quantities and controls; diagrams are schematic.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Can a currently non-limiting factor become limiting after another is improved?
What you will learn
- Prioritise the condition that demonstrably restricts the stated plant response.
- Use the stated evidence to explain the method and its limits.
Match the action to the conditions
A greenhouse provides ways to change light, carbon dioxide availability and temperature, but an effective intervention depends on what currently limits the photosynthetic rate. Increasing a plentiful factor may produce little benefit if a different requirement is restrictive.
At a low temperature, enzyme-controlled reactions can be slow even when light and carbon dioxide are available. Raising temperature within a suitable range may increase rate; excessive heat can damage tissues and reduce it. Do not describe warming as beneficial without a range and controlled evidence.
Consider a changing limit and evidence quality
In the supplied comparison, adding carbon dioxide at 10 °C leaves the rate at 0.2 cm³/min, while warming to 20 °C with the same light and carbon dioxide raises it to 0.4. At the warmer setting, a further carbon dioxide change may then become useful if a controlled follow-up supports it.
Rate evidence guides a biological choice but does not by itself establish financial value, crop yield, safe dosing or a complete greenhouse-control plan. A single short gas-release measurement is not guaranteed long-term growth. Repeat, check plant condition and compare the relevant outcomes.
Sources for this lesson
Pause and explain
What is wrong with always heating more to improve photosynthesis?
Worked example
At adequate matched light, carbon dioxide enrichment at 10 °C gives no change, but controlled warming to 20 °C doubles 0.2 to 0.4 cm³/min. What next step is biologically supported?
Show the worked solution
- Use the no-response carbon dioxide comparison.
- Use the positive warming response.
- Prioritise suitable temperature in this tested range and reassess other factors afterward.
Answer Suitable warming is supported over this tested range; reassess the limiting factor after conditions change.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
The supplied rate rises from 0.2 to 0.4 cm³/min after matched warming. Calculate the multiplier.
Give me a hint
Divide the new rate by the old.
Compare my reasoning
- 0.4 ÷ 0.2 = 2.
- This is a supplied measured ratio.
A twofold increase in the stated comparison.
Look for these in your work
- Correct ratio.
- Do not make it a universal temperature rule.
Extra light gives no improvement but a controlled carbon dioxide increase does. Which of the two interventions is currently better supported for increasing rate?
Give me a hint
Choose from the actual comparison.
Compare my reasoning
- Light alone had no measured effect.
- Carbon dioxide addition increased rate.
- Prioritise the tested carbon dioxide change, subject to safe practical conditions.
The controlled carbon dioxide intervention is currently better supported for increasing the measured rate.
Look for these in your work
- Use the intervention evidence.
- Avoid guaranteeing a long-term yield.
A grower sees a short gas-release increase and predicts a guaranteed doubled harvest. Explain the evidential gap.
Give me a hint
Photosynthetic rate and harvested yield are not identical outcomes.
Compare my reasoning
- The measurement is short-term gas release.
- Growth, allocation, stress and duration affect yield.
- Longer controlled yield evidence is needed.
A short-term rate increase does not establish a doubled harvest; verify growth and yield over appropriate controlled periods.
Look for these in your work
- Separate measured and claimed outcomes.
- Specify stronger evidence.
Common mistakes
- The best intervention can change after an improvement.
- A short-term rate is not a guaranteed harvest or financial return.
What can you explain now?
What is wrong with always heating more to improve photosynthesis?
Compare with the explanation
Excessive heat can reduce function and damage plants.
A suitable range and evidence are needed.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Choose an intervention from evidence and explain why the next limit may change.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.