Photosynthesis and respiration
Trace matter and energy without assuming plants only photosynthesise.
Before you begin
- Chemical reactions
- Cells and organelles
Distinguish the overall processes
Photosynthesis uses light energy to build carbohydrates from carbon dioxide and water, releasing oxygen. Aerobic respiration transfers energy from organic molecules using oxygen, producing carbon dioxide and water. These overall descriptions do not imply a single reversible reaction: the cell uses different pathways. Plants carry out respiration both in the light and in darkness.
Separate gross processes from net exchange
Observed gas exchange is the combined effect of simultaneous processes. In sufficient light, photosynthesis may consume carbon dioxide faster than respiration releases it, producing net uptake. In darkness, photosynthesis stops but respiration continues, producing net carbon dioxide release. Other limiting factors can restrict photosynthesis even when light is abundant.
Worked example
A leaf takes in carbon dioxide overall in bright light. Does that prove respiration has stopped?
Show the worked solution
- The observed measurement is net exchange, not a direct measurement of just one pathway.
- Photosynthesis can consume more carbon dioxide than respiration releases.
- Respiration may therefore continue while net uptake is recorded.
No. Photosynthesis can exceed respiration while both occur.
Try it yourself
Which statement about a living green plant is correct?
Common mistakes
- Net gas exchange does not identify the rate of one pathway on its own.
What can you explain now?
Why might more light stop increasing photosynthesis rate after a certain point?
Compare with the explanation
Another factor, such as carbon dioxide availability or temperature, may become limiting.
Increasing one input helps only while it limits the process. A plateau does not show that the plant no longer needs light.
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