A stoma is a pore in the leaf surface, bordered by two guard cells. The pore opens to let carbon dioxide in and closes to keep water from escaping.
This lesson belongs to leaves and photosynthesis. It builds on relating leaf structure to photosynthesis.
How do guard cells open the stoma?
The opening depends on water moving into and out of the guard cells. The steps come in a fixed order, so learn them as a chain.
- Water enters the guard cells by osmosis.
- The guard cells become turgid, because the vacuole swells and pushes on the wall.
- The wall on the inner side of each guard cell is thicker and stretches less.
- The outer wall stretches more, so the guard cells bend apart.
- The pore between them opens.
Closing is the reverse. Water leaves the guard cells, they become flaccid, the cells straighten and the pore shuts.
Why does the plant need to control the pore?
A plant faces a trade-off. Open stomata let carbon dioxide in for photosynthesis, but they also let water vapour out through transpiration.
The guard cells let the plant favour gas exchange when water is plentiful and water saving when it is scarce. Most application questions on this topic test this trade-off.
Worked example: a plant on a hot afternoon
Question: A potted plant sits in full sun at 2 pm on a hot, dry day. After a few hours, its stomata are mostly closed. Explain the effect on the plant.
Start with the cause. High temperature and dry air raise the rate of water loss from the leaf. The guard cells lose water, become flaccid and close the stomata.
Then give the effect. Less water is lost, which protects the plant from wilting. But less carbon dioxide enters the leaf, so the rate of photosynthesis falls even though light is strong.
A good answer finishes with the balance: “The plant gains water conservation but loses carbon dioxide intake.” That sentence shows you understand the trade-off, which is what the question is testing.
Which mistakes lose marks?
The most common is to say that guard cells “open by themselves.” The pore changes because water moves by osmosis, so your answer needs the word osmosis or turgid.
| Student wrote | Problem | Better version |
|---|---|---|
| “Guard cells open when it is sunny” | Gives a trigger with no mechanism | “In light, water enters the guard cells and they become turgid, so the pore opens” |
| “The stomata suck in carbon dioxide” | Gases move by diffusion | “Carbon dioxide diffuses in down its concentration gradient” |
| “Closed stomata stop photosynthesis” | Too absolute | “Closed stomata reduce carbon dioxide entry, so photosynthesis slows” |
Stomata can close because of water shortage or because of darkness. Both triggers are valid, so read what the question gives you.
Check yourself
At midnight, a leaf has most of its stomata closed. Explain why this does not harm the plant, and state what happens to the guard cells.
Answer
There is no light, so the plant is not photosynthesising and does not need carbon dioxide. Closed stomata prevent water vapour from leaving, so the plant loses less water.
The guard cells have lost water, so they are flaccid and the pore between them is closed.
What to study next
Use this chain on data in interpreting limiting-factor graphs, where a flat curve can mean carbon dioxide is short. The osmosis idea comes from movement across membranes, so revise it if the turgid step feels weak.
If you want a teacher to check your mechanism answers on a new question, see online one-to-one Biology tuition or the one-hour trial class (from RM50).