Gas exchange in leavesSpec B3.1.7, B3.1.8
In short
A leaf is adapted for gas exchange by stomata, opened and closed by guard cells, which let carbon dioxide in and oxygen out, and by air spaces in the spongy mesophyll that give a large moist surface. The waxy cuticle and epidermis reduce water loss, and veins bring water and remove sugars.
In the light a leaf takes in carbon dioxide for photosynthesis and releases oxygen. The challenge is to let gases in and out without losing too much water.
(opens full size in a new tab)| Structure | Adaptation |
|---|---|
| Waxy cuticle | Waterproof layer on the epidermis that reduces water loss by evaporation. |
| Epidermis | A single, tightly packed layer of cells that secretes the cuticle and protects the leaf; it is transparent, so light reaches the mesophyll. |
| Stomatal guard cells | Pairs of guard cells open the stoma between them to let CO₂ in and O₂ out, and close it to reduce water loss. |
| Air spaces | Connect the stomata with the mesophyll so gases diffuse quickly through the leaf. |
| Spongy mesophyll | Loosely packed cells with moist walls give a large surface area for gas exchange. |
| Veins | Xylem brings water to replace the water lost; phloem carries away the sugars made in photosynthesis. |
Plan diagram of a leaf
A plan diagram shows the distribution of tissues as areas outlined by single lines, with no individual cells drawn. Draw it large, use a sharp pencil, no shading, and label with ruled lines that do not cross.
(opens full size in a new tab)In the vein, xylem is towards the upper surface and phloem towards the lower surface. Examiners check this in leaf plan diagrams.
Written and checked against the IB Biology HL specification · Updated October 2026