Carbon dioxide is made all the time by
plants. Every cell in a plant needs energy in
the form of ATP. Plants get the energy to make
ATP by breaking down sugar (or similar
molecules) into carbon dioxide and water. This
requires oxygen. This process is called
cellular respiration. It’s the same thing that
we animals do. Fungi and many one-celled
organisms do it too.
We get our sugar and other foods by eating.
Plants get their sugar through
photosynthesis. The first stage of
photosynthesis has to happen in the light.
Plants use light energy to make ATP and another
energy-carrying molecule called NADPH. They
can “spend” the energy in these molecules to put
CO2 together (along with the hydrogen
from water) to make sugar. Oxygen is a waste
Now think about a plant. How much of the plant
can do photosynthesis? Not the roots; they
don’t get any light. Not the branches; they
don’t have the green pigment that is needed for
photosynthesis. Not the cells deep inside the
plant;they don’t get any light either. In most
plants, photosynthesis happens mostly in the
leaves. The sugars made in the leaves travel
through the plant to all of the other cells.
The sugar water (sap) travels in tubes called
So cells all over the plant are producing carbon
dioxide all the time. Some of this may be used
in photosynthesis, but that’s not happening in
many areas of the plant. The CO2
just diffuses out through the cell membrane and
holes in the plant cell walls.
There are actually some plants that only open
their stomata at night. Why do you think they
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