Identify light and carbon dioxide limiting factors from data
Use controlled changes to explain which supply currently restricts rate.
Section 6.1 · Photosynthesis
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
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
What makes a factor limiting in the current comparison?
What you will learn
- Use controlled changes to explain which supply currently restricts rate.
- Use the stated evidence to explain the method and its limits.
Use a tested intervention
A limiting factor restricts the rate under the current conditions because its available supply or level is insufficient for a higher rate. The limiting factor can change: brighter light may matter at one setting while carbon dioxide availability matters after that change.
Use controlled responses rather than choosing whichever factor sounds smallest. If increasing light while keeping other conditions matched raises the measured rate, the evidence supports light limitation over that tested interval. If extra light has no effect but added carbon dioxide raises the rate, the supplied evidence supports carbon dioxide limitation at that setting.
Read plateaux carefully
In the supplied model, increasing measured light from 20 to 40 units raises gas release from 0.2 to 0.4 cm³/min, while 40 to 80 leaves it at 0.4. At the brighter setting, added carbon dioxide raises release to 0.7. This intervention supports a change in the currently restrictive supply.
A plateau alone does not identify a unique limiting factor. Plant damage, apparatus capacity, unmeasured temperature effects or more than one constraint can also prevent an increase. Match conditions and test the proposed factor before assigning a specific cause. These data are an original teaching model, not a universal plant response.
Pause and explain
Why is a flat light-response graph alone insufficient to name carbon dioxide as the unique limit?
Worked example
At a fixed temperature, extra light gives no increase from 0.4 cm³/min, but increased carbon dioxide at that same light raises the rate to 0.7. Interpret.
Show the worked solution
- Light alone did not increase the measured rate in this interval.
- The controlled carbon dioxide increase did.
- Identify carbon dioxide limitation at the tested brighter setting.
Answer The intervention supports carbon dioxide being limiting under those tested conditions.
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.
A controlled light increase raises rate from 0.2 to 0.4 cm³/min. What limiting-factor inference is supported over that interval?
Give me a hint
Use the response to the changed factor.
Compare my reasoning
- Only light was increased under matched conditions.
- Rate increased.
- Light availability restricted the lower-setting rate.
The evidence supports light limitation over the tested interval.
Look for these in your work
- Tie the inference to the intervention.
- Limit it to the tested conditions.
At high light, carbon dioxide addition raises 0.4 to 0.7 cm³/min. Calculate the absolute and percentage increase.
Give me a hint
Use the initial value for percentage change.
Compare my reasoning
- Difference: 0.7 − 0.4 = 0.3 cm³/min.
- 0.3 ÷ 0.4 × 100 = 75%.
An increase of 0.3 cm³/min, or 75% of the original rate.
Look for these in your work
- Correct denominator.
- Distinguish rate units and percent.
A gas collector reaches its maximum capacity and subsequent readings are flat. Why must that be checked before claiming a biological limiting factor?
Give me a hint
The apparatus may be imposing the apparent plateau.
Compare my reasoning
- A full collector cannot show further collection normally.
- The graph could reflect measurement capacity.
- Use suitable capacity and verify continued measurements.
An apparatus limit can mimic a biological plateau; correct the collection method before interpreting plant limitation.
Look for these in your work
- Name the measurement alternative.
- Suggest an appropriate check.
Common mistakes
- A plateau alone does not name the cause.
- Limiting factors can change with conditions.
What can you explain now?
Why is a flat light-response graph alone insufficient to name carbon dioxide as the unique limit?
Compare with the explanation
Several causes can produce a plateau.
A further controlled intervention is needed.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain how a controlled follow-up identifies a limit more strongly than a plateau alone.
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.