Friday, April 11, 2014 • 12:01 PM Comments ()

Calvin cycle

posted by Caleb Rounds

My onions have completed the first arc of their migration. They begin, at least for me, as seeds that magically arrive in my mailbox. I plant them in soil blocks then put them under lights in the basement. Once the weather has warmed enough I carry them out to the cold frame for some real light. They are happy there for a while but eventually begin to outgrow their little soil blocks.

Last weekend I put them in the garden. After only a few days their happiness, or whatever happiness an onion can have, is apparent: they’re growing.

Later, they will get harvested, dried then carried back to the basement for winter storage – a cycle – sort of.

There are actually several cycles going on here, the biggest of which is the carbon cycle. My onions are getting bigger because they’re putting together ingredients from several sources: mostly the air.

For centuries it was thought that plants grew by eating soil. The misconception persists: if you ask, as I have, a group of college biology students whether a tree gets it’s dry weight from the soil, the water, the light or the air, they will almost invariably answer tahe soil (I’ve asked a few hundred over the years).

In the early 1600’s Jan Baptista Helmont conducted a now famous experiment to test this hypothesis using a willow tree. He planted a five pound in a large container filled with 200 pounds of soil. Over five years he added nothing to the container but rain water or distilled water. When he harvested the tree he found that it had grown to 169 pounds, but the soil had only lost a few ounces. He concluded that the tree had gained mass from the water alone. Even though he was wrong, it was not a bad experiment really, given his time and understanding.

Ironically Helmont was one of the first to infer the existence of the actual source of the dry matter: carbon dioxide. In a separate experiment Helmont burned some charcoal in a closed container and noticed that the charcoal lost the majority of its weight leaving no other solids. He concluded that the mass must have become something he called a “gas” or wild spirit.” In this he was right.

Helmont also is famous for his mouse recipe: “underwear soiled with sweat together with some wheat in an open mouthed jar.” After a 21 day incubation, mice will emerge. I’m guessing he didn’t check the progress of the experiment daily. Perhaps that one wasn’t so well thought out.

Thankfully, others followed up on his photosynthesis research. Because of Melvin Calvin and Hans Adolf Krebs among others we now know that my onions (and other plants) are adding mass mostly by taking carbon dioxide out of the air and turning it into various organic molecules (the Calvin cycle) – that is molecules containing carbon to carbon bonds.

All the rest of us parasites (that is non-plants) exist by oxidizing these molecules – essentially a process of incremental combustion of organic molecules (the Krebs cycle). We turn them back into carbon dioxide. As long as we stick to combusting the things we can readily plant, we do fine. My onions will suck in a bunch of carbon dioxide, then I’ll eat them and breathe it out. It’s when we dig up the stuff that went through the Calvin cycle a million years ago that we get into trouble.

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