Monday, November 9, 2009

Eukaryotic Cells: A Symbiotic Journey

Symbiosis, is everywhere around us if we know what to look for. Lichens, those scraggly little things that grow on rocks, are half algae half fungi. Fungi and Algae are two very distant families The algae provides the ability to photosynthesize and the fungi provides the physical structure and the ability to gather nourishment from rocks. Neither of the two species that makes up the lichen can exist anymore without the other.

Coral is a creature that forms all the coral reefs in the warm waters of the oceans. Coral is both an animal and an algae. The algae is what gives coral it's colour the greens, reds, and yellows. The animal, called a polyp, is what secretes the calcium carbonate or rock hard body of the coral. If the water stays too warm for too long it kills the algae. If the algae die, the coral dies, because the animal part cannot survive without the energy it gets from the photosynthesizing algae.

The largest structure made from living creatures is the Great Barrier Reef in Australia. This is a series of coral reefs 2600 kilometers long and 344,000 square kilometers. This huge living structure is bleaching out and dying because the algae part of the coral organism cannot tolerate the warmer waters.

Trees are in symbiotic relationships with soil fungi that live on their root tips. The fungi extract minerals and chemicals from the soil and feed them to the tree and the tree is able to feed the fungi with sugars manufactured through photosynthesis in it's leaves.

Termites, which can eat wood, digest the wood with the help of specialized bacteria in their stomachs. Cows and other grass eating herbivores are able to digest the cellulose in grass because of bacteria in their stomachs. Without the bacteria, the cow would not be able to digest the grass. Without the cow, the bacteria wouldn't have access to so much fresh grass,

There are bacteria in our intestines that help us process our waste, making it easier for our intestines to absorb Vitamins B12 and K. Without these vitamins we get blood disorders. When both creatures benefit this is called symbiosis.

There is, in every cell in our body a thing called a mitochondria. Much smaller than a cell, the are about the size of a bacterial cell. The mitochondria take oxygen and sugars and join them to phosphorus creating molecules that store energy. They're like little generators inside your cell. And there can be anywhere from one to a thousand or more of these little mitochondria in each cell, especially in cells that do a lot of work like muscle cells.

In fact, mitochondria are in every type of eukaryotic cell, the type of cell that makes up just about every living thing we know about . But they do not exist in any prokaryotic cells, which is what bacteria are. That means that mitochondria originated when eukaryotic cells originated, two billion years ago, after the proportion of oxygen had increased in the atmosphere. Bacteria could live without oxygen, but eukaryotic single celled creatures with their bigger size and more complex structures couldn't have done without the extra energy that oxygen provided.

One of the most interesting things about mitochondria is that they have their own DNA. Not only that, they appear to be one of the few organelles like plastids in plants and algae, that divides by itself.

Microbiologist Lynn Margulis was not the first scientist to suggest that mitochondria and plastids were actually forms of ancient bacteria, but her version was the first to gain acceptance in the biological community. This was largely because when it became technically feasible to analyze the DNA of mitochondria and plastids separately from the cell's nucleus, scientists discovered that the DNA of these organelles was more closely related to the DNA of ancient bacteria then to the DNA in the cell's nucleus.

Something happened two billion years ago. An oxygen breathing bacteria got swallowed by or invaded a non-oxygen breathing bacteria. Instead of harming each other they benefitted each other. The new cell couldn't survive without the oxygen breathing bacteria so they too were passed on whenever the cell divided. Over time the oxygen breathing bacteria lost some of it's independence, until it too could not survive outside the cell.

All algae, all plants, all animals, all fungi, all single-celled eukaryotes came from this symbiotic combination of bacteria two billion years ago. And the evidence for this caper is in every one of our cells.

One of the reasons it took so long to accept this theory of cell symbiosis is because this didn't quite fit the Darwinian picture. Natural selection was supposed to be imperceptively gradual and stepwise. But here we have the biggest revolutionary change in biology next to the origin of life, the evolution of eukaryotic cells from procaryotic cells, occurring through the fusion of two or more types of ancient bacteria.

This wasn't a gradual series of changes in the cells' characteristics. This was a relatively sudden jump in the structure and functioning of a cell due to the joining together of two or more distinct species. It was not the slow gradual time frame that Darwin had suggested.

The evolution of dinosaurs and human beings was child's play compared to the evolution of the eukaryotic cell. And it couldn't have happened without symbiosis.

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