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My main concern is why are the dark reactions (those that produce hexose) needed? As far as I understand, the sole purpose of photosynthesis is to synthesize carbohydrates, which are only used for energy storage. So, why don’t plants store energy directly as ATP (by photophosphorylation) and not as carbohydrates? (Of course, that would be too bad for other creatures.)

Elaborating my question:

  1. Plants use chlorophyll to convert light energy into chemical energy of ATP and NADPH by photophosphorylation and chemiosmosis.

  2. Then plants use the ATP and NADPH to synthesize glucose by the Calvin cycle.

So, what is the need of step 2? Why don‘t plants just use that ATP and NADPH for their metabolism?

PS: although @David has provided a nice answer to the current form of the question, the selected answer by @christian still remains selected since it answered my original query i.e. why excess ATP is not feasible for plants instead of why synthesis of carbohydrates is important. Any further answers/comments shall address the latter one, although none of them can be selected as correct since the latter query is not the original one.

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    $\begingroup$ No, photosynthesis is used to assimilate inorganic carbon in order to use it in biosynthesis (that's what "synthesis" stands for). It has little to do with energy. There are quasiphotosyntetic bacteria that only use light to harvest energy, hence they depend on first-level producers for organic carbon (organisms with functioning Calvin cycle, Arnon cycle etc). You need organic carbon to make proteins, nucleic acids and lipids. $\endgroup$
    – Eli Korvigo
    Commented Jan 1, 2016 at 13:12
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    $\begingroup$ thanks, i was actually misled to the fact that carbohydrates are only synthesized for energy storage, and totally forgot that all our body is carbon-based, which we basically get from carbohydrates & their derivatives ;-) $\endgroup$
    – another 'Homo sapien'
    Commented Jan 1, 2016 at 13:39
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    $\begingroup$ Plants are Eukaryotes and have Mitochondria, so they perform glycolysis and oxidative phosphorylation as well. Animals don't store energy in ATP any more than plants do. We store it in glycogen and fats. $\endgroup$
    – AMR
    Commented Jan 1, 2016 at 16:17
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    $\begingroup$ Carbohydrate doesn't easily release the energy, so they can be used for storage and long-distance transport. Like a piece of paper (though cellulose), fires-up only if it gets a touch of flame, though the reaction is exothermic. $\endgroup$
    – user25568
    Commented Sep 17, 2016 at 14:15
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    $\begingroup$ Possible duplicate of Why is ATP produced in photosynthesis used to synthesize glucose? or the other way round. $\endgroup$
    – WYSIWYG
    Commented Oct 7, 2019 at 8:43

2 Answers 2

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ATP is a bad medium for storing energy as detailed here.

  • ATP has a molecular weight of 507 Da
  • Glucose has a molecular weight of 180 Da, and contains the same amount of energy as 31 ATP molecules
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    $\begingroup$ This answer addresses one aspect of the question "Why store energy as carbohydrates rather than ATP?" It could have also considered the actual storage forms (polymerized hexoses) and factors like their neutrality and osmotic properties. However, it does not dispute the assertion in the question that the sole purpose of the synthesis of carbohydrate in the dark reaction is for storage. It is not, nor is it even the main purpose, as explained in my own answer. $\endgroup$
    – David
    Commented Dec 28, 2017 at 15:25
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Apology

This question has a very simple answer which was provided in a comment by @EliKorvigo (and acknowledged by the poster) two years before writing this answer. However the commenter did not post an answer so the poster, instead of withdrawing his question accepted an answer that, although not factually incorrect, is not a full answer to the question. I feel that photosynthesis is an important enough topic that a more comprehensive answer should be provided.

The Fallacy

The poster writes in his question:

“the sole purpose of photosynthesis is to synthesize carbohydrates, which is only used for energy storage”

In fact, as is emphasized in simpler accounts than bloated biochemistry text books,

“the essential purpose of photosynthesis is to synthesize the carbohydrates — and from them the other organic compounds — that constitute living organisms themselves (plants and photosynthetic bacteria)”

The Purpose of the two reactions of photosynthesis

Dark Reaction: Converts CO2 to carbohydrate compounds

Light Reaction: Converts light energy to chemical energy to enable the synthesis of the carbon–carbon and other bonds of the carbohydrates, and the NADPH needed to reduce the CO2.

(For details see e.g. Ch. 20 and Ch 19, respectively of Berg et al.)

Distribution of carbohydrate in plants and other functions

Photosynthesis occurs in those (green) tissues of plants that contain chloroplasts. The carbohydrate product must be distributed to those other tissues that do not contain chloroplasts. This transport is done through the phloem, with the glucose 6-P and fructose 6-P first converted to the neutral transport form, sucrose. (see SparkNotes for a simple summary, or Liu et al. (2012) for a more advanced treatment.)

Although much of the sucrose will be metabolized to provide the structural components that constitute the tissues (especially cellulose), some of it will be used to generate the ATP and NADPH needed for metabolism.

Storage of carbohydrates in plants

The accepted answer can be taken as accepting the fallacy in the question that the purpose of hexose production in photosynthesis is for storage, and further of implying that glucose is the storage form of carbohydrate in plants. Of course, polymeric hexoses, starch and sucrose are the main storage forms of carbohydrate in plants, together with some fats and oils, especially in seeds.

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  • $\begingroup$ It is a good answer, but I selected the earlier answer because it was to the point. I do not have any problem with the modified form of the question (which, indeed, takes it to another direction) since it is also a good question for the site, but I cannot accept your answer now because it does not answer the original query (how I came to ask this question is a long story). Anyways, +1 for the effort :) $\endgroup$
    – another 'Homo sapien'
    Commented Dec 28, 2017 at 6:53
  • $\begingroup$ @another'Homosapien' — No problem about which answer is accepted. Just came across this question by chance and felt that an answer addressing the purpose of the dark reaction was required in addition to one dealing only with energy storage. $\endgroup$
    – David
    Commented Dec 28, 2017 at 15:16

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