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I am trying to understand in a qualitative way photosynthesis. I am not a chemist so please have some patience.

I read on the Wikipedia page for photosynthesis that the way plants use $\ce{CO2}$ is through the reaction:

$\ce{energy + 6 CO2 + 6 H2O -> C6H12O6 + 6O2}$

but then for cellular aerobic respiration, I find exactly the opposite reaction:

$\ce{ C6H12O6 + 6O2 -> 6 CO2 + 6 H2O + energy}$

So, from this simplified view where only net reactions are written it looks that this mechanism can be used to transfer and store energy. But from a $\ce{CO2}$ point of view it looks that plants do not "consume" carbon dioxide since what they use via photosynthesis they give back through respiration. Clearly this cannot be true. So where is the catch ?

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  • $\begingroup$ One must be very careful of 'balanced' equations such as your Eqn 1 when considering photosynthesis. In PS, oxygen (all of it) is produced from the splitting of water, and it takes $12 H_2O$ to produce $6 O_2$. A better equation, maybe, is (discussed here): $\ce{6 CO2 + 12 H2O -> C6H12O6 + 6O2 + 6H2O \tag{1}}$ $\endgroup$ – tomd May 2 at 12:54
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Plants are able to store energy as carbohydrates, i.e. they can make more carbohydrates than they need for their metabolism when there is no sunlight, and store it for later use. So the amount of energy in your equations is not equal.

Some plants are offering this stored energy as sugars as nectar or fruits in exchange for pollination and spreading their seeds or use it as energy storage for times when they can't make enough carbohydrates for quick use, e.g. in winter or as storage for high demand, e.g. quick growth to outsmart competitors for space. In addition, quite much of this fixed CO2 goes into structures like stems, roots and leaves and thus is sequestered for some time from the atmosphere.

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  • $\begingroup$ Thanks this is useful (+1). So, just considering carbon, I understand that not all CH2O is reconverted via respiration. Some will be reconverted at a later point ( which nevertheless over time gives a zero balance ), but some is also sequestred into stems, root and leaves, or given to insects and will follow a different "chemical" path. I guess that than insects and other microorganism will produce again some CO2 when using these sugars so the calculation of the net effect of CO2 storage is a bit difficult to evaluate. Or am I missing some important mechanism that "stores" this "unused" sugar? $\endgroup$ – Thomas May 1 at 9:06
  • $\begingroup$ Thank you, Thomas. I think you got it :) Of course, most of the CO2 sequestration is temporally, but nevertheless an important sink that removes some of it from the atmosphere. In the long run, a little of this fixed carbon dioxide will be buried in the ground and finally form coal and oil again. $\endgroup$ – imalipusram May 1 at 9:10
  • $\begingroup$ Thanks again. It is a bit sad that you that "a little". That would mean that only a little of CO2 is sequestred into a long-term carbon storage by plants. Anyway I guess we should see this is as a cycle. Carbon does not get destroyed anyway ( without nuclear reactions ) and cycles into its different forms. The game would be to have not large quantities of it in its most dangerous form for a long time. So temporary sequestrations help to limit this, even if the longer is this "temporary" the better :) $\endgroup$ – Thomas May 1 at 9:31
  • $\begingroup$ ( PS: FYI I will wait for a couple of days to see if there are other contributions and than also accept your answer ) $\endgroup$ – Thomas May 1 at 9:33
  • $\begingroup$ That's fine, there should be more possible and probably better answers. The concentration of CO2 in the atmosphere has increased substantially by burning fossil fuels in the past centuries which have needed millions of years to form. It won't be an easy task to get the level down in the long run. Marine algae which will sink to the seabed can make the carbon disappear for some substantial time and in significant amounts, but also that will take time. $\endgroup$ – imalipusram May 1 at 9:47
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Biomass

The net production of $\ce{CO2}$ in all living organisms is close to zero. The argument is that if the total biomass on earth does not change over a period of time, and the net transfer from biomass to other reservoirs of carbon does not change much, the production and consumption should balance out. Wikipedia's article on biomass states:

The total live biomass on Earth is about 550–560 billion tonnes C, and the total annual primary production of biomass is just over 100 billion tonnes C/yr.

Plants only

[OP]But from a $\ce{CO2}$ point of view it looks that plants do not "consume" carbon dioxide since what they use via photosynthesis they give back through respiration.

Plants do produce some $\ce{CO2}$ through respiration, offsetting the $\ce{CO2}$ consumed through photosynthesis, but this is not a net zero balance. For an individual plant that grows, the carbon in its biomass comes from carbon dioxide, so it is a net consumer. When a plant dies and gets decomposed by bacteria, or when it gets eaten by an animal, or when it burns in a forest fire, the carbon eventually turns into carbon dioxide again. Depending on your perspective, you could count that as $\ce{CO2}$ production by the plant, or by bacteria, fire and animals.

Overall carbon distribution

For some context, here is a figure showing the carbon distribution on earth, together with the amount that exchanges from one reservoir to the next (found at http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap6.html, from cElroy, M.B., The Atmosphere: an Essential Component of the Global Life Support System, Princeton University Press):

enter image description here

This is the pre-industrial situation. The numbers in the boxes ("inventories") show the mass of carbon in $\pu{e15 g}$. The numbers outside of the boxes show the flow per year. So the vast majority of carbon is in the ocean sediments. This is shown as steady state. In the current era, the carbon in the atmosphere increases at the expense of carbon in the crust (fossil fuel).

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Your second equation (respiration) happens less than your first equation (photosynthesis) while the plant is growing.

About a third of the mass of a typical plant is cellulose, which is created by linking the glucose that is product of the first reaction. So the fact that you have a plant in front of you at all means that the plant has performed the first reaction more times than the second.

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