Trees 3

I had always thought of photosynthesis in a fairly simple way: A plant acts as a factory for converting water and carbon dioxide into glucose and oxygen. The plant then converts that glucose into cellulose, starches, and all that other good stuff we get from plants. But now that I needed to know where the mass of cellulose comes from, I had to look more closely at how this factory works.

The weight ratio between single atoms of carbon, hydrogen and oxygen are (more or less) 12 to 1 to 16, respectively. So, for example, in a molecule of water (H₂O), almost all of the weight comes from the oxygen. If you’re trying to figure out where the weight comes from in tree cellulose, you can almost ignore the hydrogen — it just doesn’t weigh very much. So the real question is: When H₂O and CO₂ combine to make C₆H₁₂O₆ (glucose), does the oxygen come more from the water, or from the carbon dioxide?

Well, it turns out that a plant is actually two factories. The first factory is in the business of converting photons (which provide energy) and water into hydrogen. All living things have molecules called ADP, which act as batteries. When sunlight hits its leaves, a plant charges up these batteries, pulling the hydrogen out of the water, and adding it to the ADP, which then turns into another molecule called ATP.

Basically, ADP is a microscopic uncharged solar cell, and ATP is the same solar cell, all charged up.

While this first factory needs to be bathed in sunlight (or some other light source), the second factory doesn’t need any light at all. This second factory takes in ATP (those already-charged batteries) and carbon dioxide. From the ATP it gets those charged up hydrogen atoms, and combines them with the carbon dioxide. One of the two oxygen atoms in each CO₂ molecule is released into the atmosphere, and the other one is used to make glucose — C₆H₁₂O₆.

So it turns out that none of the oxygen from the water actually gets used to make glucose (and therefore cellulose) — the glucose contains only the oxygen from the carbon dioxide in the atmosphere.

When you add up the numbers, it turns out that only about six percent of the weight of wood cellulose (the hydrogen) comes from the roots — the other ninety four percent of that mass comes from carbon dioxide.

Which means that the wood of even the most massive tree comes almost entirely from pure air.