|How come plants produce oxygen even though they
need oxygen for respiration?
|Question Date: 2012-03-08|
By using the energy of sunlight, plants can
convert carbon dioxide and water into
carbohydrates and oxygen in a process called
photosynthesis. As photosynthesis requires
sunlight, this process only happens during the
day. We often like to think of this as plants
`breathing in carbon dioxide and `breathing out
oxygen. However, the process is not exactly this
simple. Just like animals, plants need to break
down carbohydrates into energy. Oxygen is
to do this. Then why do the plants get rid of all
the oxygen they produce during photosynthesis?
answer is, they do not. Plants actually
a small amount of the oxygen they produced in
photosynthesis and use that oxygen to break down
carbohydrates to give them energy.
what happens at night when there is no sunlight
which is needed in photosynthesis? Interestingly,
in order to maintain their metabolism and
respiration at night, plants must absorb oxygen
from the air and give off carbon dioxide (which
exactly what animals do). Fortunately for all of
us oxygen breathers, plants produce approximately
ten times more oxygen during the day that what
they consume at night.
Plants break down sugar to energy using the
same processes that we do. Oxygen is needed to
break the sugar into carbon dioxide, releasing
energy the plants can use to stay alive.
However, plants also take in energy from the
sun(light), carbon dioxide from the atmosphere,
and water from the soil; they use all of them in
make sugar, and release oxygen. (They use the 'carbon'
in carbon dioxide to build the sugar molecule).
Since there's no sunlight at night, this gives
plants a way to stay alive, even when there's no
However, plants use sugar to build
pretty much everything! Cellulose, the hard stuff
in plants, is just a bunch of sugar molecules
linked together. We can't digest it though, but
some animals can. Similarly, plants make starch
(sugar linked together, but not as tightly) to
store energy for when it's dark. We're able to
Since the plants use the
sugar they make for more than just energy, they
produce more oxygen than they use.
Great question! Plants produce oxygen, because
when they photosynthesize, they take carbon
dioxide (CO2; a gas-form of carbon
bonded to two oxygen molecules) and water
(H2O; an oxygen bonded to two hydrogen
atoms) and combine them using light energy to
produce sugars and oxygen. This stores the energy
in chemical bonds (in the sugars) and releases
O2. The chemical equation for this
6CO2 + 6H2
The plants use those sugars
like we do when we consume them, for energy.
Plants use the sugars they make by oxidizing them
(with O2, just like us) to release the
energy stored in the bonds. They release
CO2 (just like us, when we breathe).
But, when plants are photosynthesizing, they
release more O2 during photosynthesis
than they will consume in respiration (oxidizing
the sugars they have made). They release the
oxygen through the same pores that allow the
CO2 to enter their leaf cells.
The quick answer to your question is that
oxygen is just a waste product when plants do
Plants can do two
Use energy from the sun to turn
CO2 (carbon dioxide) and
(water) into sugar
oxygen (O2) left over. This is
And they can:
down the sugar
CO2 (carbon dioxide) and
(water), but they need (O2) oxygen to
do it. This is cellular respiration.
can only do the second thing.
The first law of
thermodynamics tells us that matter cannot be
created or destroyed. It cannot come from
and it cannot disappear. So the same number of
atoms (C, H, O) have to enter and leave. Let us
write photosynthesis as a balanced equation.
Count up the number of carbon
atoms on each side of the arrow. If you have six
on one side, you need six on the other. Now
the hydrogen atoms. (6 X 2) on one side and 12
the other. How many oxygen atoms are on the left
(6 X 2) + (6 X 1) = ___. Now how
oxygen atoms are in the glucose? 6.
have oxygen atoms left over. That is where the
O2 comes from. It is the left over
material from making sugar. Just like when you
make something, the scraps you cut off do not
disappear. The plant breathes out the
oxygen, which is good for all of us animals
because we need oxygen, as you know.
there be animals without plants? Could there be
plants without animals?
Plants produce oxygen as a waste product of
making sugar using sunlight, carbon dioxide, and
water. If a plant needs energy, but doesn't have
sunlight, then it can burn the sugar that it made
back when it had sunlight, and doing so requires
Click Here to return to the search form.
Copyright © 2017 The Regents of the University of California,
All Rights Reserved.