“How does grass make sheep?” We were walking across a field in Northern England recently, and the child-like question popped up, unbidden and unexpected, like similar ones sometimes do: “How do raindrops know to fall straight down and redwoods to grow straight up?” “How does my brain make sense of tiny squiggles on a sheet of paper or screen (as you’re now doing)?” The sheep-grass query led, inevitably, to: “How do you get grass from air and water?”
You probably learned the answer at school, as I’m sure I did, but long forgotten. In case you’re as fuzzy as I was until I started researching this column, here’s a quick primer on photosynthesis. This is the process by which most plants and simple organisms, such as algae (collectively known as photoautotrophs), synthesize carbohydrates directly from water and carbon dioxide using light as energy, with oxygen as a byproduct. Typically, photosynthesis leads to “hexoses,” 6-carbon simple sugars:
6CO2 + 6H2O + light →
C6H12O6 (carbohydrate) + 6O2
The most common hexose is glucose, the energy source for most living organisms and the key ingredient of a slew of organic building blocks, including RNA and DNA, lipids, proteins and cellulose. Extra glucose in plants is stored as starch, the most common carbohydrate in our diet, in the form of staples such as wheat, corn, potatoes, rice and cassava.
The efficiency of photosynthesis is only about 5 percent (compared to domestic solar panels that convert sunlight into electricity with an efficiency of around 20 percent) but there’s a lot of it going on. According to one estimate, the total global energy captured by photosynthesis is eight times the total human energy consumption.
All of which raises the question: Where does the light that powers the whole enterprise come from? That would be the sun, of course. Every second, about 700 tons of hydrogen in the core of our neighborhood star fuses into 695 tons of helium. The “missing” 5 tons emerges as energy, per Einstein’s E = mc2, derived in 1905 as a consequence of Special Relativity. That 5 tons worth of energy radiates out from the sun’s surface as visible and ultraviolet light. Along with the other planets and moons, Earth intercepts a tiny fraction of the sun’s output — a mere half-billionth — but that’s still equivalent to 173 terawatts of power, or 1,361 watts per square meter, a figure called the solar constant.
This represents about 10,000 times the world’s total energy use or, in a frequently cited meme, Earth receives more solar energy in an hour than humans use in a year. Worldwide, only about 4.5 percent of our energy comes from solar. If only we could figure out how to harness it as effortlessly as grass does!
And that’s just for starters. How do you get sheep — and us, from grass? I’m saving that for another column.
Barry Evans (he/him, barryevans9@yahoo.com, planethumboldt.substack.com) often wonders if the sea is alive.
This article appears in Klamath River Ecosystem Booming One Year After Dam Removal.