These questions ask you to separate respiration, photosynthesis, and gas exchange, then compare their rates in light or darkness. Track each process first and calculate the overall movement of gases only after that.
Keep process and balance separate
Respiration is a chemical reaction inside every living plant cell. Its word equation is glucose + oxygen → carbon dioxide + water. The reaction releases stored energy for growth and other life processes. It does not make food or create energy. Root cells, leaf cells, and other living cells all respire during the day and at night.
Photosynthesis has a different schedule because it needs light. In daylight a green leaf can photosynthesise and respire at the same time. In darkness photosynthesis stops, but respiration continues. The processes are not a day-and-night pair that take turns.
| Condition | Processes and overall gas movement |
|---|---|
| Bright light | Both processes occur; faster photosynthesis causes overall carbon dioxide uptake and oxygen release |
| Darkness | Only respiration occurs; oxygen enters overall and carbon dioxide leaves overall |
| Equal rates near dawn or dusk | Both processes occur, but their gas movements cancel, giving no net exchange |
The word net means the balance after both processes are counted. In bright light, respiration still uses some oxygen and releases some carbon dioxide, but faster photosynthesis masks those movements. Some oxygen made by photosynthesis is used immediately in respiration, and only the surplus leaves the plant. At equal rates, no overall gas movement does not mean that the cells are inactive.
Gas exchange is the movement of gases, not respiration itself. Carbon dioxide and oxygen diffuse through stomata because of concentration differences. Guard cells alter the pore width, but they do not pump gases. Respiration is the reaction using oxygen and glucose inside cells; stomata are only part of the route connecting leaf tissues with the air.
Common process mix-ups
| Tempting route | How to reject it |
|---|---|
| Say respiration begins at night or stops in bright light | Living cells need a continuous energy supply, so respiration continues in light and darkness. |
| Explain night-time gas movement by respiration switching on | The visible change happens because photosynthesis switches off; respiration was already running. |
| Say respiration makes food or makes new energy | Respiration breaks down glucose and releases energy that was already stored. |
| Send gases through roots or pump them through stomata | Leaf gases move passively through stomata by diffusion. |
| Restrict respiration to green leaf cells | Every living cell respires, including cells in non-green roots. |
| Read no net exchange as no activity, or store all oxygen until dark | Opposing movements can cancel while both processes continue, and respiration uses some oxygen immediately. |
Try it
Which word equation lists the substances used and made in aerobic respiration in a plant?
- carbon dioxide + water → glucose + oxygen
- glucose + carbon dioxide → oxygen + water
- glucose + oxygen → carbon dioxide + water
- glucose + water → carbon dioxide + oxygen
A wall poster on gas exchange in plants tells its readers that plants breathe through their stomata. Why is calling that respiration wrong?
- Respiration is a chemical reaction inside cells, not the movement of gases
- Plants breathe through their roots rather than through their leaves
- Respiration is simply another name for the movement of gases through the pores
- Plants do not exchange gases with the air at all