Where plants exchange gases
Like animals, plants must take in and release gases, but they have no lungs. Gases move in and out by diffusion through tiny pores. In leaves these pores are called stomata (singular: stoma), found mostly on the lower epidermis. Each stoma is guarded by two guard cells that open and close it. In woody stems, gases pass through small openings called lenticels.
Key idea
Stomata are controlled by guard cells: when the guard cells become turgid (full of water) the stoma opens; when they lose water the stoma closes.
Photosynthesis and respiration
Plants carry out two gas-using processes. Respiration happens all the time and uses oxygen while releasing carbon dioxide. Photosynthesis happens only in light and does the opposite: it uses carbon dioxide and releases oxygen.
| Time | Net gas taken in | Net gas released |
|---|---|---|
| Daytime (bright light) | Carbon dioxide | Oxygen |
| Night (no light) | Oxygen | Carbon dioxide |
Inside the leaf, loosely packed cells of the spongy mesophyll leave many air spaces. These air spaces let gases diffuse easily between the stomata and the cells that need them.
Example
On a sunny afternoon a leaf takes in more carbon dioxide than it makes, so the net movement is carbon dioxide in and oxygen out.