Saturday, October 31, 2009

Eukaryotic Cells: Size Matters

Every living thing is either a cell or made up of cells. The fact is, every living thing starts life as a single cell. This divides and divides again as the body is constructed cell by cell.

Lewis Thomas in his famous essay “Lives of a Cell said: “ The uniformity of the earth's life, more astonishing than its diversity, is accountable by the high probability that we derived originally from a single cell.”

The cell is the basic building block of biology. It can be all there is to an organism as many creatures are single-celled, including all bacteria. For humans and other multi-celled creatures one cell can be a tiny fraction of our body, yet each and every cell in our bodies carries our entire genetic identity.

The cells of every living thing are unique. They all contain DNA molecules, the molecules of heredity, that are unique to that particular organism and nothing else. That's why my body can detect the cells of other organisms and produce antibodies that mark these alien cells for destruction by my immune system.

It was about two billion years ago I recall, when something very important happened and a new type of cell developed from bacterial cells. This new type of cell we call Eukaryotic.

If you are old enough to remember, at that time there were no plants, no animals, only microscopic critters called bacteria. Lots and lots of bacteria, many different kinds of bacteria: bacteria that ate iron sulphate and produced sulphur and bacteria that ate sulphur and produced hydrogen sulphide, bacteria that produced methane and bacteria that ate methane. And while there was great diversity in the different metabolic processes that bacteria could do there wasn't much diversity in the size and shape of bacteria.

Why would diversity of size and shape matter? Think about what a world of nothing but bacteria would look like... We're talking soup, slime, and ooze, and PU, what a smell. Enough to literally kill you - not a particularly attractive place to raise your kids. And that was what life on Earth was like for maybe three billion years. Until eukaryotic cells came along, that is.

OK, now what does life look like? There's grass, human beings, seaweed, eagles, redwood trees, moss, elephants, sharks and whales. There's microscopic single-celled sea creatures with fantastic glass houses called diatoms, there's turtles and squids and giant clams and periwinkles. Talk about different shapes and sizes and temperments. Better than 60 degrees of slime any day.

Eukaryotic cells, which is what plants, animals, fungi, and diatoms are made of, are bigger and more complex than prokaryotic (bacterial) cells. They have more parts than bacterial cells and much more DNA, 1000 times more. Eukaryotic cells have more membranes that separate and protect all the numerous parts of the cell.

Eukaryotic cells can do more things, they can specialize and link up to other cells forming organs and entire bodies. They have structures and scaffolding that allow the cells to move about like amoeba or link together like bones, skin, nerves and muscles or layers and fibers in a tree. They can secrete shells of calcium carbonates or silicates to surround and protect themselves.

Bacteria just don't have it in themselves to do any of these things. Their masters of the slime universe but what do they do besides consuming and polluting and exchanging genetic calling cards?

Let's not be disrespectful to our elders now. After all it was out of bacteria that the eukaryotic cell evolved. And that's the amazing thing that I'd like you to contemplate, because if we're looking at the “tree of life” then bacteria aren't in the branches and they aren't part of the trunk. They are in the roots.

Charles Darwin conceived of evolution as a tree with successive species as successively smaller branches, as old species went extinct and new species came into being. New species developed by inheriting new characteristics that eventually separated them from the old species.

Darwin saw the mechanism behind the generation of new species as the competitive struggle to survive. But note: the roots of a tree don't compete with each other. They each extract nutrients from the ground and send them into the tree.

Just so, the first eukaryotic cell evolved not from bacterial competition but from bacterial cooperation. That's something that Darwin didn't anticipate. Tune in next week as I describe how this symbiosis came about.

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