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While reading through a book I came across the equation for photosynthesis:

$$ \ce{6CO2(aq) + 12H2O(l) -> C6H12O6(aq) + 6O2(aq) + 6H2O(l)}\tag{1} $$ in the presence of sunlight + chlorophyll.

For some reason it seems counter-intuitive to me. I always thought that $\ce{CO2}$ would be present in the gaseous state. Similarly, $\ce{O2}$ is released, so I always thought that it would be in the gaseous state. I'm not able to understand why there are aqueous labels next to $\ce{O2}$ and $\ce{CO2}$. Does carbon dioxide get converted upon reaching the cell or something?

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  • $\begingroup$ It's about reactions in the tissues - that's what aq reflects - all this happens in solution! $\endgroup$
    – Mithoron
    Jan 16 at 16:28
  • $\begingroup$ I want to know exactly how all this happens. CO2 is taken in as a gas and as you've said the reaction occurs in the tissues. Can somebody elaborate on how the aqueous O2 turns into gaseous O2 as it is expelled? And whether writing the states as gas would be correct. $\endgroup$
    – entropy
    Jan 16 at 16:46
  • $\begingroup$ In this case even respiration and all other reactions that occur in our body should have (aq) and (l) states, right? Pls answer. $\endgroup$
    – entropy
    Jan 16 at 16:46
  • $\begingroup$ en.wikipedia.org/wiki/Respiratory_system#Plants $\endgroup$
    – Mithoron
    Jan 16 at 16:48
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    $\begingroup$ This synthesis is a long series of chemical and physical reactions. The given reaction is a summary of some of them, but not all. There is at least one reaction occurring before it, and one after it. Before the written reaction, the following physical phenomena (which is not a chemistry reaction) should happen : $$\ce{CO2 (gas) -> CO2 (aq)}$$ And, at the end of the written reaction, the following phenomena has to occur : $$\ce{O2 (aq) -> O2(g)}$$ although it is not a chemical reaction. $\endgroup$
    – Maurice
    Jan 16 at 16:50

3 Answers 3

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The formula you were given is greatly simplified from the full Calvin-Benson photosynthesis cycle. If you want to know the details, it is a multi-step process, involving energy transfer among many molecules. "[I]t takes six turns of the Calvin cycle to fix six carbon atoms from CO2. These six turns require energy input from 12 ATP molecules and 12 NADPH molecules in the reduction step and 6 ATP molecules in the regeneration step."

Not only that, but the process differs amongst different organisms. There are C3 and C4 plants, cacti have yet another path, and some bacteria yet another!

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I always thought that CO2 would be present in the gaseous state. Similarly, O2 is released, so I always thought that it would be in the gaseous state.

As the other answer states, this is a net reaction (or a black box with inputs and outputs). Depending where you draw your box, different physical states are appropriate.

If you look at an entire leaf, you would provide carbon dioxide in the gas phase, and release oxygen in the gas phase. If you look at a single chloroplast inside a plant cell, the aqueous physical state would be appropriate. In aqueous solution at pH~8 (in the stroma), carbon dioxide would mostly be in the hydrogen carbonate form (see Bjerrum_plot), but it seems to bind to the relevant enzyme, Rubisco, in the form of carbon dioxide (doi: 10.3390/catal11070813).

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The CO2 and water never come into contact. All this rather poor statement about photosynthesis means is that so many CO2 and H2O molecules form carbohydrate and oxygen.

There are two main complex sets of reactions, light and dark (Calvin Cycle). In the light reaction one of many chlorophylls (chl) in a protein chl complex absorbs a photon and transfers the energy to a 'special pair' of chl molecules from where electron transfer occurs (in the reaction center). The electron released ultimately is used to provide the energy reduce CO2 to carbohydrate. The 'special pair' (now positively charged) drags an electron off a water molecule via a metal complex, thus making water the fuel and the process is repeated.

Most of the structures of the complexes used are now known via x-ray diffraction of the proteins. Ultra-fast (picosecond/femtosecond) visible spectroscopy has determined the electron pathways and rate constants for many of these events. There is a summary here https://www.rsb.org.uk/images/15_Photosynthesis.pdf

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