Plants make food through photosynthesis. Using their leaves, plants combine sunlight, carbon dioxide and water to make glucose and oxygen. A leaf is like a plant's food factory, collecting all of the components into one place so that photosynthesis can happen. Let's start with sunlight. The top of a leaf is exposed to the most sunlight, and so the cells specialised for trapping light are on top of the leaf. These specialised cells are called palisade mesophyll cells. They are packed full of chlorophyll - the green chemical that plants used to absorb light. Most leaves have a large surface area so that they can trap as much sunlight as possible.
Moving onto carbon dioxide. This is where the bottom of the leaf comes in. There are little pores on the bottom of the leaf called stomata. The stomata open up so that carbon dioxide can diffuse into the leaf. The stomata are controlled by 'sausage shaped' guard cells, which open up to let carbon dioxide in. The guard cells can also close the stomata, to stop other things inside the leaf, like water, from escaping.
The carbon dioxide comes in from the stomata, and then makes its way up through the leaf, through the gaps in the spongy mesophyll layer in the bottom part of the leaf and heads up to the palisade cells where photosynthesis occurs. Leaves are thin so that the carbon dioxide doesn't have too far to travel.
The final reactant needed for photosynthesis is water. Water comes into the plant through the roots, moves up the stem and enters the leaf through the vascular bundle. The vascular bundle contains a hollow tube specifically for water movement called the xylem. The veins on a leaf are actually the vascular bundle, allowing water to be spread out through the leaf.
The leaves palisade cells now have sunlight, carbon dioxide and water. They are ready to photosynthesis to make glucose and oxygen.
How do leaves manage to let in the wanted things (like water and carbon dioxide) but prevent unwanted things like bacteria getting in and also prevent the reactants from escaping before being used? At the top and bottom of the leaf are epidermis cells. These produce a protective waxy cuticle layer. The waxy cuticle seals up the leaf so that the only way in and out are through the stomata, which are regulated by the guard cells.
So from top to bottom, a leaf's structure:
- Waxy cuticle and epidermis cells
- Palisade cells (where photosynthesis occurs)
- Spongy mesophyll (with vascular bundle running through for water transport)
- Epidermis and cuticle, with stomata and guard cells spread throughout (allowing carbon dioxide in).
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