By the end of this lesson
- Explain day and night gas exchange.
- Interpret net-exchange graphs.
- Identify and explain the compensation point.
BIG QUESTION
At what light intensity does a leaf switch from being a net carbon source to a net carbon sink?
The switch occurs at the compensation point, where photosynthetic carbon-dioxide uptake equals respiratory release.
Respiration continues throughout the cycle
Living plant cells need ATP continuously, so respirationA series of enzyme-controlled chemical reactions in cells that release energy from nutrient molecules, such as glucose; it is not the same as breathing. occurs in light and darkness. Darkness does not switch plant metabolism off.
In darkness, net oxygen enters and net carbon dioxide leaves because photosynthesis has stopped.
Increasing light changes the balance
As light intensity increases, photosynthesis rate rises while respiration may remain approximately steady over a short controlled experiment.
At low light respiration exceeds photosynthesis; above a threshold, photosynthesis exceeds respiration.
The compensation point has zero net exchange
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., carbon-dioxide uptake equals carbon-dioxide release and oxygen production equals oxygen consumption.
Zero net gas exchange means equal process rates, not no photosynthesis or respiration.
High light eventually meets another limit
At greater light intensity, net photosynthesis may plateau because carbon dioxide, temperature or biochemical capacity becomes limiting.
A graph of net exchange must be read with its sign convention: some axes plot uptake as positive, while others plot release as positive.
Graph skills: SLAPUK and choosing a scaleNet gas exchange: axes that cross zero.
Light intensity and net exchange
Compare photosynthesis and respiration at each light level.
| Condition | Relative rates | Net exchange |
|---|---|---|
| Darkness | Respiration only. | O₂ in; CO₂ out. |
| Below compensation point | Respiration > photosynthesis. | O₂ in; CO₂ out. |
| At compensation point | Respiration = photosynthesis. | No net exchange. |
| Above compensation point | Photosynthesis > respiration. | CO₂ in; O₂ out. |
Language that earns precision
- net exchange
- Observed gas movement after opposing process rates are combined.
- compensation point
- Light intensity where photosynthesis and respiration have equal gas effects.
- carbon sink
- A system taking up more carbon than it releases over a stated interval.
- carbon source
- A system releasing more carbon than it takes up over a stated interval.
Common misconceptions
Claim: Plants stop respiring during the day.
Correction: Respiration continues while photosynthesis also occurs.
Claim: No net exchange means the leaf is inactive.
Correction: Photosynthesis and respiration may be occurring at equal rates.
Claim: Any light causes net carbon-dioxide uptake.
Correction: Below compensation intensity, respiration can still exceed photosynthesis.
Match the answer to the command word
Check axis direction and locate darkness, zero crossing and plateau.
Compare process rates instead of saying the leaf changes from respiration to photosynthesis.
Read the compensation point where the net-exchange curve crosses zero.
Recall, apply, then justify
Explain net gas exchange by a plant at night.
Hint
One process stops; one continues.
Suggested marking points
- No light means photosynthesis stops.
- Respiration continues and consumes oxygen.
- Carbon dioxide is released overall.
Explain what occurs at the compensation point.
Hint
Equal rates and zero net movement.
Suggested marking points
- Photosynthesis and respiration occur at equal rates.
- Photosynthetic CO₂ uptake equals respiratory CO₂ release.
- There is no net gas exchange although both processes continue.
Suggest why a net-exchange curve plateaus at high light.
Hint
Light is no longer limiting.
Suggested marking points
- Light intensity is no longer the limiting factor.
- Carbon dioxide, temperature or another biochemical capacity limits photosynthesis.
Predict how raising temperature could shift the light compensation point and explain why the prediction is conditional.
Planning hint
Temperature affects both processes.
What a strong design includes
- A temperature rise may increase respiration rate.
- More photosynthesis/light may then be needed to balance respiratory carbon dioxide.
- The compensation point could shift to higher light.
- Photosynthesis is also enzyme-controlled and may increase below its optimum.
- Above an optimum, photosynthesis may fall.
- The direction therefore depends on the temperature responses of both processes.