Distinguish guard cells from stomatal pores
Connect a controlled opening with carbon dioxide entry and water loss.
Section B6.2 · Leaf structure
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall chloroplasts, enzymes and the difference between photosynthesis and respiration.
- Use the stated observations, quantities and controls; diagrams are schematic.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which part is the stomatal pore?
What you will learn
- Connect a controlled opening with carbon dioxide entry and water loss.
- Use the stated evidence to explain the method and its limits.
Name the cells and the gap separately
A stoma is a pore in the epidermis; stomata is the plural. A pair of guard cells borders and controls the pore’s opening. In a surface view, identify the gap between the pair rather than labelling an entire guard cell as the pore.
Guard cells contain chloroplasts in the supplied dicotyledonous model. Ordinary epidermal cells generally lack chloroplasts. The presence of chloroplasts does not mean that the space between guard cells is another chloroplast.
Link exchange and regulation
When open, stomata provide access for carbon dioxide diffusion into the leaf and allow other gases and water vapour to leave or enter according to gradients. Guard-cell control changes the aperture and can balance gas access with conservation of water.
An open pore does not force every gas to move in only one direction at every time. Photosynthesis and respiration affect internal concentrations. The detailed mechanism of turgor change belongs to related water-relations teaching; here the structure-function link is control of the gas-exchange opening.
Sources for this lesson
Pause and explain
How do guard cells support the leaf’s exchange function?
Worked example
A surface image shows two curved cells with a dark gap between them. Label the three features.
Show the worked solution
- The two cells are guard cells.
- The intervening gap is the stomatal pore.
- Small documented organelle signals inside the cells can represent chloroplasts.
Answer Guard-cell pair around a stomatal pore, with chloroplasts inside the guard cells when resolved by the supplied image method.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Name the cells that border and control a stoma.
Give me a hint
Use the cell name, not the gap.
Compare my reasoning
- Identify the pair on either side.
- Name guard cells.
Guard cells.
Look for these in your work
- Correct name.
- Distinguish them from the pore.
Explain a photosynthetic benefit of an open stomatal pore.
Give me a hint
Connect the opening with a raw material.
Compare my reasoning
- Carbon dioxide can diffuse through it.
- Internal spaces distribute the gas to mesophyll.
The opening permits carbon dioxide entry by diffusion toward photosynthetic tissue when the gradient supports it.
Look for these in your work
- Name carbon dioxide and diffusion.
- Retain the gradient condition.
A leaf closes many stomata in dry conditions. Explain one possible trade-off for photosynthesis.
Give me a hint
Relate water conservation to gas access.
Compare my reasoning
- Reduced aperture can reduce water-vapour loss.
- It can also restrict carbon dioxide access.
- Photosynthetic rate may be affected under those conditions.
Closing stomata can conserve water while restricting carbon dioxide supply for photosynthesis.
Look for these in your work
- Include both effects.
- Avoid claiming every closure guarantees zero synthesis.
Common mistakes
- Do not label a guard cell itself as the pore.
- Gas direction depends on gradients and process balance.
What can you explain now?
How do guard cells support the leaf’s exchange function?
Compare with the explanation
They control the aperture of the pore.
This regulates access and water loss.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw the pair, pore and chloroplasts and link aperture to exchange.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.