[OrbitRock]

I’m pretty sure the oxygen comes from CO2.

Plant carbohydrates are made directly from CO2 in the Calvin Cycle.

The Calvin Cycle is the source of all of the carbohydrate building blocks in the plant, and I’m pretty sure that oxygen from H2O does not enter the equation, just CO2 and some enzymes and cofactors.

If so, the vast majority of the mass is from the air.

(H might originate from water but its mass is pretty trivial compared to C and O)

[philipkglass]

That's correct.

"Water Is the Source of the Oxygen Produced by Photosynthesis" (and CO2 is the source of the oxygen in carbohydrates)

http://www-plb.ucdavis.edu/courses/bis/2A/bis2A-F11/Photosyn...

[littlestymaar]

I stand corrected, I had somehow built-up a wrong understanding of it (looks like those high school classes are long lost).

But where does all that excess oxygen go then? (the O/C ratio is 2 in C but ~1 in carbohydrates).

[jedimastert]

It gets released into the air through the leaves. It's the animal respiration cycle in reverse

[littlestymaar]

No. It was my first understanding as well, but it's not what happens. See this link from a sibling comment: http://www-plb.ucdavis.edu/courses/bis/2A/bis2A-F11/Photosyn...