Plants need oxygen for the same reason you and
Ido -- without oxygen we can't convert the
carbohydrates, fats, and proteins we eat into
energy. We call this process respiration,
and the formula for this sort of reaction is like
sugar + oxygen --> carbon dioxide +
water + energy
So we breathe in oxygen and eat food, and we
exhale carbon dioxide and excrete water.
This exact same reaction goes on in every
living cell, including all plant cells. But
of course plants don't have to eat food,
because they make their own food using
The formula for photosynthesis is basically
carbon dioxide + water + sunlight -->
sugar + oxygen
You can see that this is basically the reverse
of respiration, but plants convert the energy in
sunlight into the chemical bonds of the sugar.
When cells respire, they break those bonds and get
the energy out of them. Anyway, you can see that
photosynthesis produces oxygen as a waste
product, so for the most part plants don't
have to breathe in extra oxygen -- they can just
use the oxygen that they produce during
photosynthesis. However, plants only perform
photosynthesis in the green parts,
like leaves and stems, but all plant cells need
oxygen to respire. Cells in the leaves get
plenty of oxygen from photosynthesis, but cells in
the roots often need to get oxygen from the
environment to stay alive. Even though roots
are buried, they can absorb oxygen from the small
air spaces in soil. This is why it's possible
to 'drown' plants by watering them too much.
If the soil is way too wet, the roots are
smothered, the roots can't get any oxygen from the
air, and the cells in the roots die. Without
those root cells, the rest of the plant dies.
Some plants have evolved adaptations to deal with
extremely wet soil.
Mangroves are trees that live in swampy
environments along the coast in the tropics. The
roots of mangroves are often entirely under
saltwater, so they have special structures called
pneumatophores (Greek for "air carrier")
that act like snorkels, sticking up out of the
water to get a oxygen for the roots.
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