# How is water produced in photosynthesis?

I have looked everywhere, but most of the sources say water is not produced (which is not true), and some sources, such as my biology textbook, simply say it is produced. I know for every 6CO2 put into the Calvin cycle, 6 oxygen atoms are put into 2*G3P. Where do the other 6 leave the Calvin cycle? I am assuming from there, the 12 hydrogen ions not used in NADPH (as 12 are used in NADPH and put into 2*G3P, I believe, for a total of 24H put into photosynthesis in the beginning) combine with those 6 oxygen atoms to form 6 water molecules. If this is correct, where does the oxygen leave the Calvin cycle? If this is incorrect, then the question is just the general question of how is water produced?

Gold Member
most of the sources say water is not produced

(which is not true)
Are you sure? Why?

If this is incorrect, then the question is just the general question of how is water produced?
If your (and most sources) assertion that water is not produced is correct then this question is not valid.

BoB

If your (and most sources) assertion that water is not produced is correct then this question is not valid.
Every forum where I have looked says that water is not produced. Many other sources simply do not mention that water is produced. But it is. My biology textbook says that water is produced, as well as all the sources my AP class uses.

Most sources do not mention th production of water. Yet, somehow, most sources also show the balanced equation as having 6H2O on the right hand side.

Gold Member
My biology textbook says that water is produced, as well as all the sources my AP class uses.
Can you share some context where these sources say water is produced?

All of my admittedly scant knowledge (High school bio and Wikipedia) on the subject says that CO2 + H2O is photosynthesized into carbohydrates plus O2. Water is actually consumed.

BoB

Gold Member
Is it possible you haven't finished developing the equations yet? It appears there are two simultaneous processes involved.

Wikipedia said:
The general equation for photosynthesis as first proposed by Cornelius van Niel is therefore:[14]

CO2 + 2H2A + photons → [CH2O] + 2A + H2O
carbon dioxide + electron donor + light energy → carbohydrate + oxidized electron donor + water
Since water is used as the electron donor in oxygenic photosynthesis, the equation for this process is:

CO2 + 2H2O + photons → [CH2O] + O2 + H2O
carbon dioxide + water + light energy → carbohydrate + oxygen + water
This equation emphasizes that water is both a reactant in the light-dependent reaction and a product of the light-independent reaction, but canceling n water molecules from each side gives the net equation:

CO2 + H2O + photons → [CH2O] + O2
carbon dioxide + water + light energy → carbohydrate + oxygen

https://en.wikipedia.org/wiki/Photosynthesis

Also, I notice the wiki specifies "oxygenic photosynthesis". Is this a limiting case?

BillTre
Is it possible you haven't finished developing the equations yet? It appears there are two simultaneous processes involved.

https://en.wikipedia.org/wiki/Photosynthesis

Also, I notice the wiki specifies "oxygenic photosynthesis". Is this a limiting case?
Exactly, that is what I already knew. My question is how that water on the right hand side comes about. In any form of photosynthes, not just oxygenic.

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Also, I notice the wiki specifies "oxygenic photosynthesis". Is this a limiting case?
Also, yes. Sulfur bacteria, for example, use H2S as their hydrogen source. Still, water is produced, as the water produced, I’m assuming in the light-independent redactions (though that goes along with my question here), is composed of some of the hydrogens from the initial hydrogen source (the input water for green plants) and some of the oxygen from the input carbon dioxide. The oxygen in the output water is not the oxygen from the input water in plants. This was shown by examining the water and oxygen outputs of photosynthesis when either the oxygen in the input carbon dioxide or the oxygen in the input water was marked with O-18.

Gold Member
Exactly, that is what I already knew. My question is how that water on the eighth hand side comes about. In any form of photosynthes, not just oxygenic.

It looks to me like photosynthesis as we generally know it IS oxygenic photosynthesis. The alternative seems to be:
https://en.wikipedia.org/wiki/Anoxygenic_photosynthesis

in which it looks like water is a reactant of H2S and CO2

It looks to me like photosynthesis as we generally know it IS oxygenic photosynthesis. The alternative seems to be:
https://en.wikipedia.org/wiki/Anoxygenic_photosynthesis

in which it looks like water is a reactant of H2S and CO2
No, water is a product.

Edit:
Additionally, photosynthesis refers to both oxygenic and anoxygenic photosynthesis, just oxygenic more commonly in terms of what we see in our everyday lives. But the answer to my question should remain the same, oxygenic or anoxygenic.

Mentor
End products of light reactions are Oxygen, ATP and NADPH. End product of Dark reaction is Glucose which will be further converted into starch.

What does end product mean in this context? Note my statement does not mention water. Why? Water is used up in the dark reaction. All gone. So your terminology is correct but you are confusing intermediate products with end products, I think.

So while what you are saying is correct you appear to have gotten fuzzy on what it actually means: water is not an end product. Meaning it got used up later when you consider the dark reaction - which part of photosyntesis, just like the light reaction. Will your equations balance if you put H20 in the end product section after the dark reaction? i.e., glucose + H20 That is the test you should apply.

pinball1970 and BillTre
BWV
Water has to be conserved, you cannot have had a billion+ years of plant life adding water to the planet.

pinball1970
Then why does my biology textbook state that the equation for photosynthesis is 12H2O+6CO2+light→C6H12O6+6O2+6H2O?
Additionally, look under “light-independent reactions” and it says that water is a product of the light-independent reactions:
https://en.m.wikipedia.org/wiki/Photosynthesis

BWV
That equation consumes water

That equation consumes water
No, there is a net consumption of water. There is, however, production of water, as evident by the H2O on the right side of the equation. As I have said, experiments show that different water molecules are produced from the ones consumed. In the light independent reactions, however, the equation (see the link in post 12) shows a net production of water. So water is produced. My question is how.

rootone
Perhaps it might be useful to look at it from the point of view that all of the chemistry happens in water solution.
No new water is being produced, ultimately it's getting broken into Hydrogen and Oxygen with the help of Sunlight,
This causes the assemblage of hydrocarbons and other more complex molecules than water,
However the reverse process can easily take place given the right conditions, then you get water back and some emission of heat

Gold Member
No, there is a net consumption of water. There is, however, production of water, as evident by the H2O on the right side of the equation. As I have said, experiments show that different water molecules are produced from the ones consumed. In the light independent reactions, however, the equation (see the link in post 12) shows a net production of water. So water is produced. My question is how.
(You and others have noted that photosynthesis results in a net consumption of water. Also, it's been noted that the light reactions consume water to make ATP, NADPH, and oxygen. So all that's left are the dark reactions. Starting from there...) You could go through each of the steps in the Calvin cycle individually to see what happens. In what follows, I'm only tangentially concerned with mechanistic details (and not concerned at all about ion charges, or pH, or anything like that); I'm focused mainly on the stoichiometry--i.e., where the water comes from.

In the first step, RuBisCO catalyzes the addition of a CO2 to RuBP, and a water molecule is consumed to hydrolyze the 6-carbon intermediate to two 3-carbon products (3-phosphoglycerate). So that's one water molecule consumed. The empirical formula looks like:
$$C_5H_{12}O_{11}P_2 +CO_2+H_2O \longrightarrow 2 C_3H_7O_7P$$

In the next step, a phosphate condensation reaction occurs:
$$C_3H_7O_7P + HPO_3 \longrightarrow C_3H_8O_{10}P_2$$
but this reaction can really (very very crudely) be thought of as:
$$C_3H_7O_7P + H_3PO_4 \longrightarrow C_3H_8O_{10}P_2+H_2O$$
(Side note: a "condensation" reaction almost always means water is generated as a product. Similarly, a dehydration/hydrolysis reaction means that water is consumed as a reagent.)

Since two of these reactions happen per 1 RuBP molecule, you get a net production of one molecule of water. Further, since the Calvin cycle involves 3 reactions of CO2 with RuBP, you get a net production of 3 molecules of water. If you continue in this fashion, with the rest of the Calvin cycle, you see that water is consumed/produced at multiple points along the way, generally when phosphate groups are being added or removed.

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Isaac0427 and Ygggdrasil
(You and others have noted that photosynthesis results in a net consumption of water. Also, it's been noted that the light reactions consume water to make ATP, NADPH, and oxygen. So all that's left are the dark reactions. Starting from there...) You could go through each of the steps in the Calvin cycle individually to see what happens. In what follows, I'm only tangentially concerned with mechanistic details (and not concerned at all about ion charges, or pH, or anything like that); I'm focused mainly on the stoichiometry--i.e., where the water comes from.

In the first step, RuBisCO catalyzes the addition of a CO2 to RuBP, and a water molecule is consumed to hydrolyze the 6-carbon intermediate to two 3-carbon products (3-phosphoglycerate). So that's one water molecule consumed. The empirical formula looks like:
$$C_5H_{12}O_{11}P_2 +CO_2+H_2O \longrightarrow 2 C_3H_7O_7P$$

In the next step, a phosphate condensation reaction occurs:
$$C_3H_7O_7P + HPO_3 \longrightarrow C_3H_8O_{10}P_2$$
but this reaction can really (very very crudely) be thought of as:
$$C_3H_7O_7P + H_3PO_4 \longrightarrow C_3H_8O_{10}P_2+H_2O$$
(Side note: a "condensation" reaction almost always means water is generated as a product. Similarly, a dehydration/hydrolysis reaction means that water is consumed as a reagent.)

Since two of these reactions happen per 1 RuBP molecule, you get a net production of one molecule of water. Further, since the Calvin cycle involves 3 reactions of CO2 with RuBP, you get a net production of 3 molecules of water. If you continue in this fashion, with the rest of the Calvin cycle, you see that water is consumed/produced at multiple points along the way, generally when phosphate groups are being added or removed.
Thank you very much! So, through all these reactions, there is a net production of 6 water molecules, right?