All of us already knew the basics when we started our dinner conversation about where trees get their huge mass of cellulose. In particular, we knew that water comes up through the roots, and carbon dioxide comes in through the leaves. Photosynthesis uses these ingredients, together with energy from light, to make the simple sugar glucose — which the tree can then further process to make cellulose and other carbohydrates.
Because there is more oxygen in water and carbon dioxide than there is in glucose, photosynthesis has the nice side effect of releasing oxygen into the air (which is quite fortunate for those of us who like to breath the stuff).
We even all knew the formulas when we started talking:
Water + Carbon Dioxide → Glucose + Oxygen
Or to be more precise:
H2O + CO2 → C6H12O6 + O2
You’ve probably already noticed that there are a lot more atoms on the right side of that equation than on the left side. That’s why it actually takes six water molecules and six carbon dioxide molecules to produce a single molecule of glucose:
6 H2O + 6 CO2 → C6H12O6 + 6 O2
Now everything adds up: Both sides of the equation have 12 hydrogen atoms, 18 oxygen atoms and 6 carbon atoms.
But now we were stumped. Clearly all of the carbon comes from the air, and all of the hydrogen comes from the ground water pulled in by the roots. But what about the oxygen? Does that come from the air, from the water, or some combination? Until we answer that question, we can’t really say how much of a tree’s cellulose mass comes from the air, and how much comes from ground water.
Maybe all of the oxygen comes from the water, and maybe all of the oxygen comes from the air, or maybe it’s some mixture of these. How can we definitely answer this?
More tomorrow.