So now is probably a pretty good time to talk about why leaves change color in the Fall. Back in the Spring we never had to talk about why flowers bloom or leaves grow; reproduction and sustenance are pretty obvious reasons. But in the Fall it’s not quite as clear. Why do leaves change color? Why don’t they just fall off? If you’ve ever hacked a branch off a tree and left it lying around for a week or two before taking to the dump, you probably noticed that the green leaves didn’t start turning to some spectacular red or gold color- they just shrivel up and die.
But to understand why leaves change color in the Fall, we should probably look at why they’re green in the Spring and Summer- something we skipped over back in the Spring.
Part Where I Go Overboard On Detail But It Turns Out to Be Way Worth It
Complex, or Eukaryotic cells, the kind found in all complex multicellular organisms- moose, birds, trees, us- contain various structures inside the cell body called organelles. There are lots of organelles that do important things in cells, but the 2 most significant are mitochondria and chloroplasts.
Tangent: “Simpler” cells, that don’t contain organelles, are called Prokaryotic cells. Bacteria are a good example of prokaryotic cells, and eukaryotic cells almost certainly evolved from prokaryotic cells.
Mitochondria are found in the vast majority of eukaryotic cells, in both plants and animals, and their primary- but not only- function is power generation: they generate the adenosine triphosphate (ATP) that we talked about when we were looking at brown fat and staying warm.
Chloroplasts (microscopic pic right) on the other hand, are only found in plants, and their primary function is food (specifically sugar) production. Within each chloroplast are a bunch of still smaller bodies, or sub-organelles, called thylakoids, and these little guys are where the action happens.
Evolution Tangent #1: The origin of both mitochondria and chloroplasts is one of the most complicated and fascinating stories of evolution. It is believed that eukaryotic cells came about because some prokaryotic cells symbiotically evolved with, and eventually engulfed, other prokaryotic cells. The engulfing cells became the eukaryotic cells, and the engulfed cells became the organelles within. With chloroplasts, what got engulfed was cyanobacteria, which are basically photosynthetic bacteria commonly known as “Blue-Green Algae.”
Evolution Tangent #2: In the world of Algae, which is way out of scope for this post, the story gets even more complicated and exciting. In some types of Algae, cells symbiotically engulfed existing photosynthetic eukaryotic cells which already had their own “engulfed” chloroplasts, thereby evolving chloroplasts through a “secondary” symbiotic event. And it’s even possible that some other Algae did this trick a third time, evolving chloroplasts through a “tertiary” symbiotic event. Primary, secondary or tertiary, the chloroplasts in all plants and green algae appear to be monophyletic, meaning they descended from a common "engulfing" event, some ~1.5 billion years ago. But the chloroplasts in Red Algae and plants/green algae appear to be paraphyletic, meaning they evolved from a separate, and possibly earlier "engulfing" event.
Nested Tangent: Red Algae, Rhodophyta, are a group of between 5,000 and 10,000 species of marine alage and seaweeds, some 200 of which occur in freshwater, the rest in the sea. If you're familiar at all with any of them, it's probably either as a seaweed, or one of the various micro-critters that builds coral reefs.
I explained paraphylteic and monophyletic groups back in June when we talked about Wyethia, Balsamroots and my Uncles John and Peter.
OK, these thykaloids are where you find the actual chlorophyll, and specifically it’s found bound to the outer membranes of the thykaloids. We’ve looked at a lot of complicated organic molecules this summer- the irritants in Poison Ivy, the photosensitizer in Cow Parsnip, the toxins in Black Widow venom, and the various poisons in milkweed and Larkspur. But Chlorophyll (molecular diagram right) is the honking monster molecule of them all. It comes in one of 5 main variants, only 2 of which are found in plants (the other 3 occur in algae and cyanobacteria.) The most common one, “Chlorophyll a” has 137 atoms in each molecule (C55H72O5N4Mg) Each of the five chlorophyll types has a different number of carbon, hydrogen and oxygen atoms, but always 4 nitrogen and a single magnesium atom.
Chlorophyll is the key component of photosynthesis, the chemical process where energy- in the form of light- is combined with carbon dioxide and water to produce sugars. Chlorophyll needs to absorb sunlight to effect this process, but it absorbs red and violet wavelengths better than greenish wavelengths. The greenish wavelengths tend to be reflected by chlorophyll, which is why leaves appear green.
Chlorophyll, like a lot of big molecules, is pretty unstable, and it breaks down rather quickly in sunlight, but during the Spring and Summer plants constantly produce new chlorophyll, keeping the leaves green.
Next Up: Lies Your Teacher Told You, And I Finally Get To The Point Of This Post