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I don't know, if it's a physics question, biology or chemistry question but anyways here it is:

I have been taught that for making one molecule of glucose in photosynthesis 18 ATP molecules are used up but in respiration, oxidation of the same glucose molecule releases 38 ATP, I just can't seem to wrap my head around it. Where are these extra 20 ATP molecules coming from (or in other words extra 20 units of energy)

I know there's a whole lot of reactions occurring in the cell that utilize energy as the internet puts it but the reactions of photosynthesis and respiration are just complete reverse of each other and I don't think the path to make or break glucose actually matters with respect to the amount of energy used or released in the processes; glucose always remains glucose so of course writing the same reactions in reverse orders shouldn't change the energy used/given out in the reaction.

So in my opinion these two processes do seem to violate the law of conservation of energy, so please help!

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    $\begingroup$ Photosynthesis requires the sun for a reason. $\endgroup$
    – DKNguyen
    May 6, 2022 at 14:09
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    $\begingroup$ The 18ATP in photosynthesis are produced from the light reactions, in other words the sun, these 18ATP are then used in Calvin cycle to make glucose $\endgroup$
    – Chahak
    May 6, 2022 at 14:13
  • $\begingroup$ en.wikipedia.org/wiki/Cellular_respiration and ncbi.nlm.nih.gov/pmc/articles/PMC3360099/…. may help $\endgroup$
    – Jon Custer
    May 6, 2022 at 14:28
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    $\begingroup$ You'll probably get a better answer on biology stack exchange. $\endgroup$
    – The Photon
    May 6, 2022 at 14:28
  • $\begingroup$ An analogy would be that energy from 1 barrel of oil can be used to refine (or drill up) more than 1 barrel of oil. Or it takes less energy to hunt something than the energy you get by eating it. If it did not, there is no point. $\endgroup$
    – DKNguyen
    May 6, 2022 at 14:41

2 Answers 2

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I have been taught that for making one molecule of glucose in photosynthesis 18 ATP molecules are used up but in respiration, oxidation of the same glucose molecule releases 38 ATP

In the Calvin cycle of photosynthesis (the so-called dark reactions because they do not use light), 18 ATP molecules are converted to ADP, but there are also 12 molecules of NADPH that are converted to NADP+. This is where the remainder of the “missing” energy comes from.

the reactions of photosynthesis and respiration are just complete reverse of each other

No, they really are not.

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Photosynthesis is driven by absorbing the energy of the light, whereas in non-photosynthesizing organisms the energy comes with food - i.e., the glucose is not resynthesized, but imported from elsewhere.

In other words, photosynthesis can be though of as solar panels or wind energy, whereas providing energy for respiration is just like adding gas to the tank of a car.

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