Photosynthesis

A tree's mass comes mostly from thin air because photosynthesis builds sugar out of carbon dioxide gas pulled from the atmosphere, not from soil. The soil supplies water and minerals, but the carbon atoms that make up wood, leaves, and bark were CO2 molecules before the plant grabbed them. Photosynthesis happens in two linked stages. The light reactions, in the chloroplast's thylakoid membranes, absorb sunlight and use that energy to split water, release oxygen, and produce energy carriers (ATP and NADPH). The Calvin cycle then uses those energy carriers to fix carbon dioxide into sugar, building glucose six carbons at a time since CO2 brings in one carbon per molecule.

Chlorophyll drives the light reactions, and it absorbs light strongly near 450 nm (blue) and 680 nm (red), while reflecting most light around 550 nm (green). That reflection is exactly why leaves look green: we see the color chlorophyll fails to absorb. So a grower using pure green LEDs wastes most of that light, since little of it gets captured to power the light reactions.

The two stages depend on each other, so either can become the bottleneck. At low light, raising light intensity speeds up photosynthesis because the light reactions cannot supply enough ATP and NADPH. Past some intensity, the rate plateaus: the Calvin cycle cannot fix carbon any faster because CO2 supply is now the bottleneck. Raising CO2 then lifts the plateau higher, revealing CO2 as the new limiting factor.