Gas Exchange in Leaves: Mesophyll and Stomata

0:23The leaf cross-section

A dicotyledon is a flowering plant such as a bean, a rose or an oak. Its leaf is a thin sheet with layers. On top is the waxy cuticle, which is waterproof, and gases cannot pass through it. Under that, the palisade cells, tightly packed. Then the spongy mesophyll, with lots of gaps between the cells.

0:45Stomata and guard cells

Mostly along the lower surface are tiny pores called stomata. One pore is a stoma. Each stoma is surrounded by two guard cells, which open and close it. When the stomata are open, carbon dioxide diffuses in from the air, and oxygen from photosynthesis diffuses out. When the guard cells close the stomata, for example to save water, the gas exchange slows down.

So the stomata are like a doorway, and the plant controls it.

1:15Air spaces in the mesophyll

Once inside, carbon dioxide meets the air spaces in the spongy mesophyll. Gases diffuse much faster through air than through water, so the carbon dioxide can spread right through the leaf to the cells that need it. Oxygen travels back the same way, to the stoma and out. Think of the air spaces as the corridors inside the building. They mean no cell is far from the doorway. Just remember, in the exam, write stomata and air spaces, not doorway or corridors.

1:48Mesophyll cells and the surface

Now zoom in. Mesophyll cells have much of their surface exposed to those air spaces. Together, the cells give a large surface area for gas exchange. The gases dissolve in the thin film of water on the cell walls, and then diffuse into the cell.

2:05The diffusion gradient

Why does the carbon dioxide keep coming in? Because photosynthesis uses it up. So the concentration inside the leaf stays low, and the concentration in the air stays higher. That keeps a steep concentration gradient, and diffusion carries on. A steep gradient means faster diffusion. So the leaf is never waiting for carbon dioxide.

2:28Gas exchange at night

One more idea, because it appears in the traps. Plants respire all the time, not only in the light. In the dark there is no photosynthesis. The leaf still respires, so oxygen diffuses in and carbon dioxide diffuses out.

3:13Examiner trap and recap

The trap. Plants do not only photosynthesise. They respire too. And gases do not cross the cuticle, they go through the stomata.

Recap. Gases enter through stomata and spread through air spaces. They reach mesophyll cells over a large surface, with photosynthesis keeping the gradient steep.

Next time, the catch. Open stomata let water escape, so how do insects and desert plants cope?