By the end of this lesson
- Relate gas exchange to photosynthesis.
- Relate gas exchange to respiration.
- Distinguish gross process rates from net exchange.
BIG QUESTION
When oxygen leaves a leaf, does that mean the leaf has stopped respiring?
Both processes continue in light; observed exchange is the net difference between their opposing gas effects.
Two processes affect the same gases
photosynthesisThe endothermic process in which green plants use light energy, absorbed by chlorophyll, to make glucose from carbon dioxide and water, releasing oxygen. lowers carbon-dioxide concentration and raises oxygen concentration in photosynthetic tissue. aerobic respirationRespiration that uses oxygen and fully oxidises glucose to carbon dioxide and water, releasing much more energy than anaerobic respiration. has the opposite overall effect.
In light, both processes occur. In darkness, photosynthesis stops but respiration continues.
Net exchange is what instruments observe
If photosynthesis is faster than respiration, a leaf shows net carbon-dioxide uptake and oxygen release. If respiration is faster, the net directions reverse.
Zero net exchange does not mean both processes have stopped; their rates may be equal at the compensation pointThe light intensity at which the rate of photosynthesis equals the rate of respiration, so there is no net exchange of carbon dioxide or oxygen..
Different tissues contribute differently
Mesophyll cells photosynthesise strongly in suitable light. Roots, many internal tissues and non-green organs respire but do not photosynthesise.
Whole-plant exchange combines all tissues, so a leaf-chamber result cannot automatically represent the entire plant.
Process and net gas directions
State whether you mean an individual process or the overall observation.
| Situation | CO₂ effect | O₂ effect |
|---|---|---|
| Photosynthesis alone | Consumed. | Produced. |
| Aerobic respiration alone | Produced. | Consumed. |
| Bright light; photosynthesis faster | Net uptake. | Net release. |
| Darkness | Net release. | Net uptake. |
| Compensation point | No net exchange. | No net exchange. |
Language that earns precision
- net exchange
- Overall movement after all simultaneous processes are considered.
- gross photosynthesis
- Total photosynthetic rate before respiratory gas exchange is subtracted.
- compensation point
- Light intensity where photosynthetic and respiratory gas effects balance.
- photosynthetic tissue
- Tissue containing functional chloroplasts and receiving suitable light.
Common misconceptions
Claim: Plants take in carbon dioxide and never oxygen.
Correction: Plant cells use oxygen in aerobic respiration.
Claim: Respiration stops when photosynthesis begins.
Correction: Respiration continues in light and dark.
Claim: No net gas exchange means the leaf is inactive.
Correction: Photosynthesis and respiration may be active at equal rates.
Match the answer to the command word
Use “net uptake” or “net release” when both processes occur.
Compare the two process rates before stating gas direction.
Zero crossing on a graph indicates equal gas effects, not zero metabolism.
Recall, apply, then justify
Explain why a leaf may release oxygen while still respiring.
Hint
Compare rates rather than switching one process off.
Suggested marking points
- Respiration consumes oxygen continuously.
- Photosynthesis in light produces oxygen.
- If photosynthesis is faster, production exceeds consumption and oxygen is released overall.
Explain gas exchange by a leaf in darkness.
Hint
Which process continues?
Suggested marking points
- Photosynthesis stops without light.
- Aerobic respiration continues, consuming oxygen.
- Carbon dioxide is produced and released overall.
What does zero net carbon-dioxide exchange indicate?
Hint
It need not mean zero reactions.
Suggested marking points
- Carbon dioxide uptake by photosynthesis equals release by respiration.
- The leaf may be at its compensation point.
Explain why net oxygen release cannot by itself reveal gross photosynthetic rate.
Planning hint
The measured result contains two rates.
What a strong design includes
- Photosynthesis produces oxygen.
- Respiration consumes oxygen simultaneously.
- Net release is production minus consumption.
- Different respiratory rates can give different net values for the same gross photosynthesis.
- Respiration must be estimated or measured separately to infer gross rate.