Plants produce carbon dioxide all the time because
of respiration, but during the day (when there is
light), they use CO2 for
photosynthesis, and fix CO2 into other
molecules, giving as end product O2
more than CO2. This
is why it is called carbon fixation. On the other
hand, when it is dark, plants do not have an
energy source for photosynthesis, and so cannot
fix CO2 and produce O2. The
is light-independent, and
more formally known as the Calvin cycle.
During this cycle, 3 carbon
dioxide molecules are absorbed to produce the
small sugar G3P
(glyceraldehyde-3-phosphate) for the plant to use
elsewhere. Two G3P
molecules are required to make glucose.
As for comparing the amount of carbon dioxide
and oxygen between night and
day it depends on how you look at the sample
region. You can consider
anything from the block you live on to the entire
Earth. If you only
consider your block during the day, large amounts
of carbon dioxide are
produced from higher activity levels with humans,
animals, and driving of
vehicles. Additionally at this time, any living
plants are producing high
amounts of oxygen. At night, plants take in carbon
dioxide reducing it's
levels; however, humans and animals are still
When considering these fluctuations between
oxygen and carbon dioxide you
also have to consider the overall atmosphere
composition before you start
the "experiment". We start with ~21% Oxygen, and
~0.04% carbon dioxide.
Even though there is an increase in carbon dioxide
through the daytime on
your block, oxygen is much more present at the
beginning of the day.
This question really highlights the reason
climate change is occurring due
to increases in greenhouse gases (carbon dioxide
is just one of many).
Although plants do a great job of eliminating
carbon dioxide, they can't
keep up with the additions that we use during the
day. If you are
interested in looking at the changes in oxygen and
carbon dioxide an
isolated area, I'll suggest that you look into the
Biosphere 2 owned by
the University of Arizona. They have completed
some really cool research.
Lastly, addressing the idea of taking in oxygen
in the morning - since our
atmosphere is comprised mostly of nitrogen (~78%)
followed by oxygen
(~21%) and some other gases (including carbon
dioxide), we take in many
gases in the morning! If there were no oxygen,
then it would be a major
health hazard, as we require oxygen to continue
life. So overall, even
though the concentration of carbon dioxide is
increasing, it's levels are
many times lower than oxygen.
Great question! You are correct that plants
produce both CO2 and oxygen. A lot of
people don’t even understand that plants do both
cellular respiration ( sugar + oxygen → carbon
dioxide + water) and photosynthesis (carbon
dioxide + water → sugar + oxygen)
Here’s the thing, plants are doing cellular
respiration all day, too! Cells use a form of
energy called ATP that they get by doing cellular
respiration. They can’t use the energy in the
sugar they make until they break it down into ATP.
All of the cells in a plant do cellular
respiration, but not all do photosynthesis. Look
at a tree. How much of it is green? The entire
tree (except the outer bark) is living cells.
Some are deep inside the trunk or roots. They
never do photosynthesis. So where do they get
their energy? From the sugar that the leaves make.
The sugar and other stuff move around in the tree
sap. The cells that don’t do photosynthesis are
doing cellular respiration all day long to get
their energy from the sugar.
Air moves and mixes a lot too, so you won’t
actually find a difference in the CO2
concentration of the air between night and day.
But you can find a difference in CO2
concentrations in lakes that have a lot of algae.
Algae are not really plants, but they do
photosynthesis and cellular respiration. Almost
all of their cells can do photosynthesis, so they
can make a lot of oxygen during the daylight
hours. At night, they are using up the oxygen
like crazy and making a lot more CO2.
The CO2 combines with water to make
carbonic acid. Acids lower the pH of the water.
In lakes with a lot of algae, the pH can change
drastically between night and day.
Which do you think takes in more CO2
per cell, a tree that has just sprouted, or one
that is mature?
You might want to think about a career in plant
ecology if these sorts of questions interest you.
Thanks for asking,
Plants make a lot more oxygen than they produce
CO2 (remember: most of the plant's body
is made out of carbon-containing compounds. That
carbon didn't get released as CO2
during the night, but the oxygen associated with
it did get released). I don't know when the
minimum oxygen concentration is, but I guess it
would be in the early morning shortly after dawn,
as you say. Incidentally, temperature does the
same thing, for related reasons (no energy coming
in during the night...).
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