Let’s talk about something unimaginably complex! Here, take a look at a single cell on a maple leaf. Inside, there are a number of microscopic blobbies called “chloroplasts.” Inside those are wrinkly stacks of “thylakoids.” Embedded in the thylakoid membranes are a host of protein complexes, which are, you should know, nothing more than teensy weensy molecular wind-up gadgets. Look! Here’s a protein complex called “Photosystem II.” You can see how it is wrapped in a formation of chlorophyll, xanthophyll, and carotene molecules held in place by the proteins of a “light harvesting complex.” Inside this sheath of chemical antennae (for that is what the chlorophylls and xanthophylls and carotenes amount to) is the “reaction centre” of Photosystem II. At the heart of the reaction centre lie two more cozy chlorophylls of a specific configuration that scientists like to call P680 for some boring reason.
But now direct your attention to a single photon of light zipping through the woods. Perhaps it passes through a cell wall on a maple leaf, and through that cell’s innards, through its chloroplast membranes and on to the thylakoid. It would zip right on through the thylakoid too, except—hi!—it encounters one of those antenna molecules. And there things get downright loopy.
The antenna acts as a sort of photon net, and it scoops the photon up. But the photon was going so fast that the netting jostles an electron out of place. The scientists call this excited electron an exciton. Now watch—you can’t! The exciton is in multiple places at once! (Seriously, this is the conclusion of the latest science on the matter. We’re deep in the spooky world of quantum mechanics, where particles are waves, and waves are particles, and things can be both up and down at the same time.) The exciton bounces around the light harvesting complex randomly until it finds its way to one of the P680 chlorophylls, but it does so by trying all the ways at once and then taking the shortest way that it found so as to be most efficient.
Don’t worry, I don’t get it either. And neither do the scientists, really, but apparently the math works out.
So then once the exciton has made it to a central chlorophyll, it settles down a bit into an electron again, but not before it pops off and scuttles away with some philandering pheophytin molecule, leaving the chlorophyll minus one electron. And then the chlorophyll discovers that its neighbour has just experienced the very same kind of infidelity, and now we have a Photosystem II in its wound-up state, with the two chlorophylls of P680 in a jealous rage for want of electrons.
And yet there is justice. Water molecules, hapless as they are, have the unfortunate habit of getting stuck to Photosystem II in pairs at a most conveniently accessible place for P680. Snicker snack! The chlorophylls rip some electrons away for themselves, and as a result the water molecules fall to pieces, leaving four hydrogen atoms floating around with two oxygens. The oxygens quickly bond and float away in the form of O2.
Oh hey! That’s how plants make oxygen!
But what of those hydrogens and those promiscuous electrons that ran off with the pheophytins? I wish I had the space to tell you because their journeys are truly astounding; in fact, they yield the foundational source of food for almost all of life on earth. But for now, in this autumnal season, there are a couple other things that you simply must know.
About those antenna molecules: all of them, because of their shapes and constituent parts, find certain wavelengths of light a bit slippery. Oranges and reds just bounce off of carotenes and carry on their way; for xanthophylls it’s yellows, and for chlorophylls, greens. Since the vast majority of antenna molecules in such leaves as maples are chlorophylls, what you end up with is a green-looking leaf, because that is the main colour of light that gets away from the the light harvesting complexes of Photosystem II. (And Photosystem I, as it happens, but, again, there’s not enough space.)
Alas, all chlorophylls break down and need to be replenished. When the days get cooler and shorter, many trees shut off the flow of nutrients required to build new chlorophylls. Why? Nobody really knows, although there are some hypotheses about energy conservation in preparation for winter. In any case, eventually all that remains in the Photosystem complexes is a rearguard of xanthophylls and carotenes that have no problem soaking up green light, but they sure do let slip a glorious array of yellows, reds, and oranges.
Enjoy the colours, everyone, and happy Thanksgiving!