Good question. Plants sort of breathe through
holes in their leaves called stomata. If they
have more carbon dioxide than they can use
immediately for photosynthesis, it just
exits through the stomata.
The stomata are a problem for plants in dry
climates because when they are open to allow
carbon dioxide and oxygen to move into or out of
the leaf, water also evaporates. When the plant
gets dry, the cells that “guard the door” of the
stomata shrink and they close. When there’s
enough water, these cells swell up and open the
Too much oxygen from photosynthesis can be a
problem because if it builds up, it blocks
the enzyme that should be grabbing onto carbon
dioxide to complete photosynthesis. Some plants
have special adaptations that allow them to avoid
this problem. Check out C4 plants for
more information. Some plants, like cacti,
only open their stomata at night, when the air is
cooler. This saves on water loss, but means they
have less time to do gas exchange.
So there’s a whole complex balance of water,
gas exchange, cellular respiration, and
photosynthesis. Here’s another wrinkle, water
evaporating through stomata is what pulls water up
from the roots. If plants can’t pull water
up, what else will they not be able to get?
Thanks for asking.