Photosynthesis · Grade 7
How is a leaf built and what do the stomata do?
A leaf is built like a flat, wide factory: a protective layer on top, packing tissue rich in chloroplasts in the middle, veins bringing water in and carrying sugar out, and stomata on the underside. A stoma is a pore through which carbon dioxide enters — and through which, unavoidably, water leaves.
Learning objectives
- Label the epidermis, the mesophyll, the veins and the stomata on a cross section of a leaf
- Explain how a pair of guard cells opens and closes a stoma
- Connect the water lost through the stomata to the movement of water up from the roots
- Explain why a desert plant keeps its stomata shut during the day
What a cross-section shows
On top is the epidermis, a transparent protective layer coated in a wax that prevents evaporation. Light passes through it without being absorbed.
Beneath it lies the packing tissue — where most of the chloroplasts are and where most photosynthesis happens. Lower down the cells are more spaced out, with air spaces between them through which gas diffuses.
Veins run through the leaf: one type brings water and minerals up from the root, the other carries the sugar made here away to the rest of the plant. The veins you can see with the naked eye are exactly those.
The stoma and its guard cells
A stoma is a tiny pore, usually on the underside of the leaf, with a guard cell on each side. When they fill with water they swell and curve apart, and the pore opens; when they lose water they go limp and it closes.
Carbon dioxide enters through that pore and has no other route in. But water vapour leaves through the very same pore, and that is the trade the whole structure of the leaf is built around.
The water leaving pulls water up
Losing water through the stomata is not only a cost. It creates a pull in the veins that draws water up from the root along the entire stem — even in a tree tens of metres tall.
In a desert plant the trade is settled the other way: it closes its stomata during the hot hours and gives up photosynthesis to do so, because losing water in that heat is the greater danger.
Worked examples
Why are most stomata on the underside of a leaf?
- The upper side faces direct sun and heat
- A pore on a hot surface loses water faster
- On the shaded underside the loss is smaller
Answer: To take in gas while losing as little water as possible
A plant is wilting from lack of water. What happens to the stomata and to photosynthesis?
- The guard cells lose water and go limp
- The stomata close
- Carbon dioxide stops entering
Answer: Photosynthesis nearly stops, however much light there is
Vaseline is smeared on the underside of one leaf. How does it compare with an untreated leaf?
- The vaseline blocks the stomata
- No carbon dioxide gets in and no water gets out
- With no raw material there is no sugar made
Answer: The blocked leaf makes no starch and stays pale in an iodine test
Common mistakes
- Thinking the leaf absorbs carbon dioxide over its whole surface
- The epidermis is coated in an impermeable wax. Gas enters through the stomata alone, which is why blocking them stops the process.
- Treating water loss as a fault
- It is the unavoidable price of taking in gas, and it is also what pulls water up from the root. A plant that lost no water at all would struggle to transport any.
- Assuming a desert plant closes its stomata for lack of light
- It closes them precisely when there is light, because that is when the heat endangers it. It gives up photosynthesis to save water.
What to remember
- Gas enters through the stomata only.
- Guard cells open and close them by water content.
- Any opening for gas is an opening for water loss.
- Evaporation pulls water up from the root.