The big idea: A leaf is a flat organ built for gas exchange and photosynthesis.
Photosynthesis needs carbon dioxide (CO₂) from the air, and it releases oxygen (O₂). So a leaf must let gases move easily between the air and its inner cells.
The leaf does this with small holes called stomata on its underside, and a loose, air-filled layer of cells inside called the spongy mesophyll.
A leaf in cross-section. From the top: waxy cuticle, upper epidermis, palisade mesophyll (tall, chloroplast-packed cells), spongy mesophyll (loose cells with air spaces) and lower epidermis with a stoma between two guard cells. CO₂ diffuses in and O₂ plus water vapour diffuse out through the open stoma.
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- Gas exchange
- The movement of gases (here CO₂ in and O₂ out) between an organism and its environment by diffusion.
- Stoma (plural: stomata)
- A small pore (hole) in the leaf surface, usually on the underside, through which gases enter and leave the leaf.
- Guard cells
- The two cells either side of a stoma that change shape to open or close the pore, controlling gas exchange and water loss.
- Palisade mesophyll
- Tall, chloroplast-packed cells just below the upper surface that carry out most of the leaf's photosynthesis.
- Spongy mesophyll
- Loosely-packed cells with large air spaces between them, through which gases diffuse to and from the photosynthesising cells.
- Epidermis
- The single layer of cells covering the upper and lower surfaces of the leaf; it protects the leaf and (on the underside) holds the stomata.
Why a leaf is thin and flat: A leaf is thin so that gases have only a short distance to diffuse between the air and the inner cells.
It is flat and wide to give a large surface area for catching light and for gases to enter — both make gas exchange and photosynthesis faster.
Each layer of the leaf has a job, and its structure is adapted to that job.
Reading from the top surface down to the bottom, the layers work together so that light reaches the chloroplasts and gases reach (and leave) every cell.
| Leaf layer (top to bottom) | What it is | How it helps gas exchange / photosynthesis |
|---|---|---|
| Waxy cuticle | A clear, waterproof wax coating on the surface | Reduces water loss; being transparent, it still lets light through |
| Upper epidermis | A single layer of clear, tightly-packed surface cells (no chloroplasts) | Transparent, so light passes straight through to the cells below |
| Palisade mesophyll | Tall, column-shaped cells packed with chloroplasts, just under the upper surface | Does most of the photosynthesis — its chloroplasts catch the most light |
| Spongy mesophyll | Loosely-packed rounded cells with large air spaces between them | The air spaces let CO₂ diffuse to every cell and O₂ diffuse away |
| Lower epidermis | A single layer of surface cells on the underside, containing the stomata | Holds the stomata, where gases enter and leave the leaf |
| Stoma + guard cells | A pore (stoma) flanked by two guard cells, mostly on the lower surface | The guard cells open and close the pore, controlling gas exchange and water loss |
The two mesophyll layers do different jobs: The palisade mesophyll sits near the top, where the light is brightest. Its cells are tall and packed with chloroplasts, so it does most of the photosynthesis.
The spongy mesophyll sits below it. Its cells are loosely packed with big air spaces between them. These air spaces let CO₂ diffuse in to every cell and O₂ diffuse out — they are the leaf's internal 'corridors' for gases.
Follow the carbon dioxide: A classic exam task is to trace the path of CO₂ from the air into a chloroplast. Follow it step by step:
1. CO₂ in the air diffuses in through an open stoma.
2. It spreads through the air spaces of the spongy mesophyll.
3. It diffuses across the cell wall and cell membrane of a mesophyll cell.
4. It crosses the cytoplasm and enters a chloroplast, where photosynthesis uses it.
Follow the blue arrow: CO₂ enters through the open stoma, fills the air spaces between the spongy mesophyll cells, then diffuses across the cell wall and membrane into the cytoplasm and finally into a chloroplast — where photosynthesis uses it.
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| Step | Where the CO₂ is | What it does next |
|---|---|---|
| 1 | In the air outside the leaf | Diffuses in through an open stoma |
| 2 | In the air spaces of the spongy mesophyll | Spreads through the air spaces to reach the cells |
| 3 | At the surface of a mesophyll cell | Diffuses across the cell wall and the cell membrane |
| 4 | In the cytoplasm of the cell | Diffuses on into a chloroplast |
| 5 | Inside a chloroplast | Is used in photosynthesis |
CO₂ entering the leaf
- Moves in through an open stoma
- Spreads through spongy mesophyll air spaces
- Crosses the cell wall and membrane
- Ends up in a chloroplast to be used in photosynthesis
O₂ + water vapour leaving
- O₂ made in photosynthesis diffuses out
- Travels back through the air spaces
- Leaves through an open stoma
- Water vapour is also lost this way (transpiration)
A memory hook for the pathway: Stoma → Spaces → Surface → Cytoplasm → Chloroplast.
Gases always move by diffusion, from where there is more of them to where there is less — no energy needed.
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How this is tested: On Paper 1B a short data-style question often shows an image of a leaf — a cross-section or the lower surface — and asks you to outline the path that carbon dioxide takes from the air into the chloroplasts. The marks are for the ordered steps: stoma → air spaces → across the cell wall/membrane → chloroplast.
On Paper 1A you may be shown a labelled leaf cross-section and asked to identify the structures marked X and Y (for example palisade mesophyll, spongy mesophyll, a stoma or a guard cell).
You may also be asked to mark or label the position of a guard cell or a stoma on an image of the leaf's lower surface.
IB-style question — trace the path of carbon dioxide
A dicot leaf is photosynthesising in bright light. Outline the path that a carbon dioxide molecule takes from the air outside the leaf to a chloroplast inside a mesophyll cell. [2]
How to score both marks
- Get into the leaf. The CO₂ diffuses in through an open stoma on the lower surface of the leaf.
- Move through the air spaces. It then spreads through the air spaces between the spongy mesophyll cells.
- Cross into the cell and reach the chloroplast. It diffuses across the cell wall and cell membrane, through the cytoplasm, and into a chloroplast. (Award 1 mark for entry through the stoma into the air spaces; 1 mark for crossing the cell wall/membrane into the chloroplast. Steps must be in order.)
Final answer
CO₂ diffuses in through an open stoma, through the air spaces of the spongy mesophyll, then across the cell wall and membrane into the cytoplasm and on into a chloroplast.
✓ Why this scores full marks: The answer is a clear, ordered sequence: stoma → air spaces → cell wall/membrane → chloroplast.
For a 2-mark 'outline the path', the examiner wants the steps in the right order, not just the word 'diffusion'. Naming the stoma and the spongy mesophyll air spaces is what scores.
Follow the blue arrow: CO₂ enters through the open stoma, fills the air spaces between the spongy mesophyll cells, then diffuses across the cell wall and membrane into the cytoplasm and finally into a chloroplast — where photosynthesis uses it.
Interactive diagram
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