[littlestymaar]

Edit: this is wrong, see comments below.

The correct answer is water though, not air. Air only accounts for the Carbon content of the tree, which is a big part, but still a minority.

First, a live tree contains a lot of water (30-50%), that's why you need to let wood dry a lot before burning it.

And there's something else:

> Next we might look at water, but water is only made of hydrogen and oxygen, and the oxygen is released.

Not all oxygen is released. All oxygen contained in a plants molecule (mostly cellulose and hemicellulose, which are polymers of glucose, and lignin) comes from water. And there's a lot of it: there's as much Oxygen as Carbon in a glucose molecule.

[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...