By the end of this lesson
- Identify the metabolic origins of carbon dioxide and oxygen.
- Explain gas loss through stomata by diffusion.
- Distinguish process production from net exchange.
BIG QUESTION
How can the same leaf release two different waste gases at different times?
Respiration and photosynthesis generate different gases; net loss depends on their relative rates and diffusion gradients.
Respiration generates carbon dioxide
Aerobic respiration releases energy from glucose and produces carbon dioxide and water. Carbon dioxide can diffuse from respiring cells into leaf air spaces and out through stomata.
Respiration continues by day and night, although its product may be reused in photosynthesis.
Photosynthesis generates oxygen
During photosynthesis, water is split and oxygen is produced. Oxygen not used in respiration can diffuse into leaf air spaces and leave through stomata.
Calling oxygen a waste product here means it is an excess product of this metabolic pathway, not that oxygen is always useless to the plant.
Net loss depends on both processes
In bright light, photosynthesis may exceed respiration, giving net oxygen loss and carbon-dioxide uptake. In darkness, respiration continues without photosynthesis, giving net carbon-dioxide loss and oxygen uptake.
Gases move down concentration gradients through moist mesophyll surfaces, air spaces and stomatal pores.
Metabolic gases in a leaf
Separate where a gas is produced from its overall movement.
| Process | Gas produced | Likely net effect |
|---|---|---|
| Aerobic respiration | Carbon dioxide. | CO₂ may leave in darkness. |
| Photosynthesis | Oxygen. | O₂ may leave in sufficient light. |
| Bright light overall | Both processes occur. | Net CO₂ uptake and O₂ loss are common. |
| Darkness overall | Respiration only. | Net O₂ uptake and CO₂ loss. |
Language that earns precision
- excretory product
- Waste product of metabolism that is removed from an organism.
- net exchange
- Overall movement after simultaneous processes are considered.
- stoma
- Adjustable epidermal pore through which gases diffuse.
Common misconceptions
Claim: Plants only excrete oxygen.
Correction: Respiration produces carbon dioxide, and net exchange changes with conditions.
Claim: Oxygen is always harmful waste to a plant.
Correction: It is a photosynthetic product and is also used in aerobic respiration.
Claim: Stomata actively pump gases out.
Correction: Gases diffuse through stomata down concentration gradients.
Match the answer to the command word
Name the metabolic process, its gas product and diffusion through stomata.
Use net uptake or net loss when both processes occur.
Recall, apply, then justify
Explain why a leaf may lose oxygen in bright light.
Hint
Compare production and use.
Suggested marking points
- Photosynthesis produces oxygen.
- Photosynthesis may produce oxygen faster than respiration uses it.
- The excess diffuses out through stomata.
Explain gas exchange by a leaf in darkness.
Hint
Which metabolic process continues?
Suggested marking points
- Photosynthesis stops without light.
- Respiration continues.
- Respiration consumes oxygen and produces carbon dioxide.
- Carbon dioxide diffuses out and oxygen diffuses in through stomata.
Explain why measuring oxygen leaving a leaf underestimates its gross photosynthetic oxygen production.
Planning hint
Some oxygen is used before it leaves.
What a strong design includes
- Photosynthesis produces oxygen.
- Respiration occurs at the same time.
- Some produced oxygen is consumed by respiration.
- Only the excess contributes to net release.
- Measured net release equals production minus consumption.
- Gross production is therefore greater than net release.