Good question. The answer is that they do
recycle, to some extent. Let’s start with
CO2. Plants take it in, combine it
with water, and use energy (which they get from
light) to build molecules like sugar, starch,
and wood. That’s photosynthesis. The sugar may
be broken down quickly by cells needing energy,
but the other molecules become part of the
plant’s “body.” When you look at a flower or
tree, most of what you see is carbon, oxygen,
and hydrogen that are tied up in plant tissues.
To do more photosynthesis, the plant needs fresh
stores of CO2 and water.
Water is also evaporating all the time.
Obviously, this happens more in dry climates.
Every time the plant opens up the holes in the
leaves (stomata) to move gases like oxygen and
carbon dioxide, even more water evaporates.
Desert plants have evolved adaptations to limit
this, such as only opening the holes at night,
and having a waxy coating to reduce their water
Plants can use their waste oxygen when
they’re breaking down the sugar in cellular
respiration, so they are usually not limited by
a lack of oxygen. They can’t hold onto oxygen
long, though. The reason is a little
complicated. Enzymes are biological catalysts—
chemicals that speed up chemical reactions.
Many enzymes are needed to catalyze all the
reactions of photosynthesis and cellular
respiration. Putting the CO2
together to make sugar uses an enzyme called
rubisco. The problem is that oxygen “clogs” the
active site on the enzyme rubisco so that it
can’t do its job. This means that the plant has
to let out the oxygen. Some plants have evolved
ways to isolate the rubisco so that it’s not
getting clogged up with oxygen. These are
called C4 plants.
What parts of the world do you think have the
most C4 plants?
Thanks for asking.
This is a great question. Plants actually do
use the oxygen they produce from photosynthesis
for respiration and the carbon dioxide they
produce from respiration for photosynthesis.
However, (and fortunately for us!) plants use
much less oxygen for their own respiration than
they produce from photosynthesis, and they use
much more carbon dioxide for photosynthesis than
what they produce from respiration. So the net
result is that they expel the extra oxygen, and
take in some extra carbon dioxide. I hope this
Well, to begin with, plants have only been
around for about 400 million years.
The answer is that there is only so much
space in the interior of a leaf to store gasses
like CO2 and water vapor, so the
plant has to get rid of it - holding it in would
increase the pressure and blow the leaf apart.
Single-celled organisms have even less room to
store excess gasses.
When a plant is actively photosynthesizing as
well as respiring, they do use any gasses they
have available in their internal pore spaces -
which includes their waste products from
respiration (and photosynthesis, too - that
makes oxygen, which is a really nasty waste
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