The short answer is that when plants are doing
cellular respiration, they produce carbon dioxide
and water. When they are doing
photosynthesis, they make oxygen and sugar.
So it’s not as strange as it seems. It’s just that
they can use 2 different processes
depending on the conditions.
Let’s look at the conditions. Like us, they
need oxygen and sugar to do cellular
respiration. Like us, they use the energy
from breaking down the sugar to make ATP, a
source of energy that all cells can use. Carbon
dioxide and water are waste products. We get
rid of them in different ways, of course. The
reason you don’t weigh as much as all the
groceries you eat is that you breathe out a lot
of carbon dioxide and get rid of the water. We
lose water in a lot of ways, including urination
and sweating. Plants let it evaporate of their
leaves. This also powers the movement of water
up from the roots, so it’s important.
When they do photosynthesis, they need carbon
dioxide, water, and light energy. They
produce oxygen and sugar. The sugar can
then be built up into things like starches,
cellulose, and wood. Notice that light is
a big deal. We talk a lot about plant cells
doing photosynthesis, but lots of plant
cells never do photosynthesis because they’re in
parts of the plant that are not exposed to the
light. They may be underground in roots or deep in
the wood of a tree. They all need ATP, though.
They get their sugar from leaf cells and do
cellular respiration. The sugar travels to them
via tubes that sort of act like our blood vessels.
If I look out my window now, I see huge trees with
no leaves. Their cells have to make it through the
winter burning the sugar they’ve been making since
last spring. So all the cells left in the tree
are doing cellular respiration.
So here’s a puzzle for you. The sun still shines
here (sometimes) in the winter, so why don’t
trees like oaks, maples, and aspens just keep
their leaves? Hint: I also see snow out
Thanks for asking,
Respiration in plants can occur in the dark or
under sunlight. Respiration in the dark is
commonly just called respiration; this
process is very important to plants at night for
energy production and only requires oxygen and
sugars be supplied to the plant. The
respiration that is possible under sunlight is
called photorespiration; it is possible
because of a protein called
carboxylase/oxygenase, abbreviated RuBisCO.
RuBisCO is an enzyme that can catalyze reactions
that use oxygen, ultimately causing carbon dioxide
to be produced.
Photorespiration undercuts the results of
photosynthesis because photorespiration takes
away some of the carbon dioxide that
photosynthesis is trying to use to produce energy
for the plant. Since both processes can occur
under sunlight, a plant can then produce oxygen
(as a waste produce of photosynthesis) and carbon
dioxide (as a waste product of photorespiration).
Keep in mind that in the dark, carbon dioxide
is produced as a waste product of respiration as
well, but in this case, respiration is not
undercutting photosynthesis; it is simply a
process by which plants use their storage of
sugars to survive, and in the dark, plants do
not produce oxygen.
Plants need energy during the night when there
isn't enough light to photosynthesize. They
get this energy the same way you do: by
combining sugars with oxygen to produce
carbon dioxide and water.
During the day, however, plants can get the
energy they need from light, and use the excess
energy to combine carbon dioxide with water to
make sugars that they will burn during the night
and to build up their bodies (plant bodies are
mostly made out of polymers of sugar). This
making sugar produces oxygen as a waste
Note that getting carbon dioxide out of the air
also means losing water into the air. This means
that some plants save up energy generated
during the day to do the making of sugars at
night. This usually happens in deserts.
Plants use food, just like we do. They make
food by photosynthesis, but they use food by
respiration to get energy. Here's a definition of
"a process in living organisms involving the
production of energy, typically with the intake of
oxygen and the release of carbon dioxide from the
oxidation of complex organic substances">
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