This may turn out to be a silly question, but...

Not being able to wait for Spring and driven by a bad case of cabin fever, I
recently made some containers to experiment with hydroponics. It's going
surprisingly well I think. The spinach is not happy, but the lettuce and
tomatoes are looking good after 3 weeks.

I have some understanding of how a plant works, but what I'm wondering is
where does the actual mass (tissue, cells) come from? Does a plant
literally create some amount of mass out of "thin air" through
photosynthesis? Does carbon from carbon dioxide make up a significant part
of the mass? Hypothetically, if you grew a plant in a closed system and
measured the amount of water and nutrients that were consumed, would that
exactly equal the weight of the plant, or would the plant weight more. I'm
thinking it would weigh more, and if that's so, what's the breakdown in
percentages of mass from carbon, nutrients and water?

High school biology is a long way gone. :-)

-Keith