1Adapted for photosynthesis
A leaf is broad, with a large surface to absorb light, and thin, so gases only have a short way to diffuse. Veins bring water and take away sugars.
adaptation
Lesson 19.2 notes
Leaves
How a leaf is adapted: inside a leaf, palisade cells, air spaces, stomata and guard cells, veins, and transpiration.
2Inside a leaf
A thin slice of leaf under a microscope shows layers: a waxy cuticle, the upper epidermis, the palisade layer, the spongy layer, and the lower epidermis.
cuticleepidermispalisadespongy
3Palisade cells
Most photosynthesis happens in the palisade cells, near the top of the leaf where the light is brightest. They are tall, packed close together and full of chloroplasts. The epidermis above them is clear.
palisade cells
4Air spaces
The spongy layer has air spaces between its cells. Carbon dioxide diffuses through them to every cell, and oxygen diffuses out.
air spacesdiffusion
5Stomata and guard cells
Gases pass in and out through stomata, tiny holes mostly on the underside of a leaf. Two guard cells open each stoma in the light and close it at night.
stomataguard cells
6Veins
Veins contain xylem, which carries water and minerals up from the roots, and phloem, which carries sugars made in the leaf to the rest of the plant.
xylemphloem
7Transpiration
Water evaporates inside the leaf and escapes as water vapour through the stomata. This is transpiration. It pulls more water up the xylem from the roots.
transpiration
Explore: Leaf explorer
Pick a part of a leaf to see where it is and how it helps photosynthesis, then switch between day and night.
Common mistake: Stomata let water vapour out too
Stomata let carbon dioxide in and oxygen out, but water vapour escapes through them as well. So guard cells close the stomata at night, and when the plant is short of water.
Remember
- A leaf is broad to absorb light, thin so gases diffuse a short way, and has veins to bring water and take away sugars.
- Palisade cells near the top are packed with chloroplasts. Air spaces in the spongy layer let gases reach every cell.
- Carbon dioxide diffuses in and oxygen out through stomata. Guard cells open them in the light and close them at night.
Going further at GCSE