So we know that plants make their own food
through photosynthesis. But how does a plant ;0 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.