How carbon dioxide gets in
The second raw material arrives by a completely different route, and it needs no plumbing at all — only a hole and the ordinary behaviour of a gas.
Diffusion
The movement of the particles of a substance from a place where they are more concentrated to a place where they are less concentrated, until they are evenly spread. No pump and no energy are needed; it happens on its own.
The route, step by step
- Carbon dioxide is taken from the atmosphere.
- It enters the leaf through the stomata, by diffusion.
- It moves through the intercellular spaces inside the leaf.
- It reaches the leaf cells.
You have seen both of those structures already
Stomata are the pores in a leaf, each surrounded by two guard cells — grade 10 named them as the main route for gas exchange. The intercellular spaces are the gaps between parenchyma cells that chapter 1 listed as a characteristic, and they are largest of all in the spongy parenchyma. That loose, airy layer is not loosely packed by accident.
Why diffusion is enough
A leaf that is photosynthesising is using carbon dioxide up, so the concentration inside it is always lower than the concentration in the air outside. Gas therefore moves inwards of its own accord, for as long as the process continues. The plant spends nothing on getting its carbon dioxide.
Stomata are the way out as well as the way in
The same pores admit carbon dioxide and release oxygen, and in respiration they do the reverse. A stoma is a hole, not a valve with a direction. What moves through it, and which way, depends only on which gas is more concentrated on which side.