All Things Natural: The Nature of Green

All Things Natural: The Nature of Green
A leaf close-up. Photo Credit: Roo Reynolds
Ed Kanze

January 4 2013

Not all plants have it, and if you have it, that doesn't make you a plant. Christmas wouldn't have Christmas trees without it. I refer to chlorophyll, a magical, magnesium-based molecule that has been greening our home planet for more than two billion years.

The inventors of chlorophyll, which is to say, the living things in which it first evolved, were ancient bacteria that lived in the sea. For life on earth, the advent of chlorophyll and the photosynthesis it makes possible marked a great leap forward. Now bacteria could soak up energy from the sun (ergo "photo") and stash it for their own future benefit in bonds. The bonds stitched together carbon atoms, creating (hence "synthesis") the energy-rich compounds we call sugars and carbohydrates.

Chlorophyll absorbs most wavelengths of the visible spectrum, yet green light bounces off of it. Which is why photosynthetic bacteria (they used to be called blue-green algae), seaweeds, plants, and a grab bag of other chlorophyll-driven organisms appear green rather than some other color.

Chlorophyll-employing organisms that practice photosynthesis produce a waste product called oxygen. When it first began flooding earth's primordial seas, oxygen was a toxic pollutant. In time, however, living things evolved to capitalize on the stuff. Among them were our very distant ancestors.

Bacteria are prokaryotes, which is to say, they are relatively simple organisms without the membrane-bound, chromosome-filled nuclei that typify the organisms (ourselves included) called eukaryotes.

The question of how chlorophyll found its way into the cells of multi-celled seaweeds and plants long puzzled scientists. In high school biology class forty years ago, I was taught that the chlorophyll in plant cells (seaweeds were lumped with plants in those days) was contained inside organelles called chloroplasts. Yet something funny seemed to be going on. Chloroplasts inside cells themselves seemed to be cells.

In the last quarter century, evolutionary biologists, taxonomists, and cell biologists such as the late, great Lynn Margulis (1938-2011) helped guide into mainstream science what at first seemed a far-fetched notion. This was that chloroplasts essentially are cyanobacteria whose free-living ancestors long, long ago took up residence inside the ancestors of single-celled algae. In so doing they made possible the rise of the photosynthetic seaweeds that wash up on beaches and to the green terrestrial organisms we call "plants."

The clincher, the thing that finally broke down resistance to the idea that chloroplasts are essentially symbiotic bacteria living inside the cells of seaweeds and plants, was the discovery that chloroplasts contain their own DNA, distinct from the DNA contained in the nuclei of the cells in which they reside.

Does the name "mitochondria" ring a bell? You learned about these in high school biology class, too. Like chloroplasts, mitochondria are organelles, but instead of practicing photosynthesis, they carry out other chemical reactions, especially ones that make energy available to cells. This is why mitochondria are often called the "powerhouses" of our cells.

Guess what? They also have their own DNA, which has led scientists to accept the idea that they, too, originated as ancient bacteria that took up symbiotic positions inside the cells of eukaryotes.

If you're having trouble following all this, welcome to the club. The science of figuring out how the components of organisms fit together and where they come from and how they operate cooperatively remains in its infancy. It's a wild, wondrous world out there, and it's a wild, wondrous world inside every cell, too.

Ed Kanze owns and operates The Adirondack Naturalist Company, offering full and half-day excursions into the Adirondack Mountain wilderness. He is a licensed naturalist and lives with his family in the Adirondacks. His column runs weekly in Saratoga Wire

Photo Credit: Roo Reynolds



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