Photons and Trees

Some of this article is interesting, but this isn’t very consequential.

Solar energy arrives on Earth and becomes mass in the form of green leaves, creating food we can eat and use as fuel for thought.

This is not wrong, exactly, but very, very, very little mass in plants comes from photons. It’s vanishingly tiny. Something like 99.9999% of the mass of a plant comes from carbon in the air, water, and nutrients in the soil and definitely not from photons (energy).

It’s surprisingly difficult to find out how many watt-hours of energy for instance an average tree actually absorbs per year, so let’s just try to get order of magnitude estimate going on. Assume a small tree absorbs (not receives) 100,000 watt-hours of energy per year. How much of this will be converted to mass?

One watt-hour of energy absorbed is roughly equal to 40 picograms of mass gained. Therefore, 100,000WH x 40pg = 4,000,000pg.

Thus, one small tree would gain only 0.000004 of a gram of mass from solar radiation per year. That’s 4 millionths of a gram of mass.

Do you know why plants gain mass from solar radiation? Yes, yes, mass-energy equivalence, but it’s not hard if you think about how photosynthesis occurs. But that’s a tale for another time.