Anatomy of a Leaf

So we know that plants make their own food through photosynthesis. But how does a plant get together the ingredients it needs for this: sunlight, carbon dioxide and water, and then combine them to create glucose and oxygen? Well, that's what leaves are for! The leaf is a plant's food factory — and its parts work together to get the reactants into one place so that photosynthesis can happen. Let's start with sunlight: Have a look at a leaf. The top of it is exposed to the most light—so the cells specialized for trapping light are on top. These cells are called palisade mesophyll and they're packed full of chlorophyll, the green chemical that plants use to absorb light. That's why the top side of the leaf is darker than the bottom side. Note that most leaves have a large surface area to trap as much sunlight as possible.

What else do we need for photosynthesis? Carbon dioxide. That's where the bottom of the leaf comes in. It's got these little pores called stomata, which open up so carbon dioxide can diffuse in to the leaf. They're controlled by sausage shaped guard cells, which open up to let carbon dioxide in. But they can also close up the stomata and prevent other things like water from escaping. So the carbon dioxide comes in through the stomata, makes it's way through gaps in the airy layer of cells (called spongy mesophyll) at the bottom part of the leaf, heads up to the palisade layer for photosynthesis. Leaves are thin, so the carbon dioxide doesn't have very far to travel.

So we've got sunlight and carbon dioxide now. What's left? Water. Water comes up through the roots and stem and enters the leaf through the vascular bundle, which contains a hollow tube for water transport called the Xylem. Look at how the vascular bundle spreads out in the leaf to form veins so it can spread water throughout the leaf. So now the leaf's palisade cells have water, carbon dioxide and sunlight, all they need to photosynthesise and make glucose (their food) and oxygen. But how does the leaf prevent unwanted intruders, like bacteria from getting in... and stop important reactants like water from escaping?

Above the palisade mesophyll and below the spongy mesophyll are epidermis cells, which produce a waxy coating called the cuticle. The cuticle seals up the leaf, so the only way in and out is through the stomata, which are regulated by the guard cells.

So let's go back through the parts of the leaf now, from top to bottom. We've got the thin, waxy cuticle... then the epidermis cells. These basically make up the leaf's skin. Then come the chlorophyll-rich palisade mesophyll, where the bulk of photosynthesis occurs. Below that is the spongy mesophyll, which have plenty space between them for reactants to move through. Through that space stretches the vascular bundle, with xylem to transport water and phloem to transport sugars. And below that, we've got another layer of epidermis and cuticle. Space around the lower epidermis and cuticle are the stomata, with guard cells on either side.

Leaves come in all different shapes and sizes—look at a cabbage leaf... and an oak tree leaf. But most of them have these same parts in common which allows them to get their reactants in and perform photosynthesis.