Photosynthesis captures energy for life on Earth. Many chemicals are made to allow life processes to occur in plants. These chemicals can move in and out of cells by the process of diffusion. Osmosis is a specific type of diffusion.
By 1796, after further research, Ingenhousz was able to write the first equation for photosynthesis, which was:
Light is not directly usable in photosynthesis.
As per the "Walstad Method" (basically, a twist on what was known by old timers as the "German Method"), this method depends upon the plants (along with fish wastes) to do most of the work for maintaining the aquarium chemistry and water quality (as far as the Nitrogen Cycle ONLY) while the aquarium keeper provides circulation, some ferts and mineral cations that are not supplied or kept up with adequately by fish food, and cleanings ONLY when necessary.
This is an often misunderstood aspect of aquarium plant keeping.
There is so much anecdotal information that is out of date with more current aquarium/plant biochemistry information. Not only do plants need many of the minerals found in GH, but, just as important, potentially dangerous upward pH swings can occur if your GH is much below 50 ppm during peak plant photosynthesis.
For example, I observed a pH of 6.8 in the morning and then a pH of 7.4 in the afternoon in a tank where GH was low or almost non-existent.