Friday, January 16, 2009

Origins 4, Domains

Biological life used to be divided into Kingdoms, known as Plant, Animal and Monera (one-celled organisms). Now one of the most common ways, as I said in a previous post, is to divide life into three Domains: Archaea, Bacteria, and Eukaryota.

The reason organisms are divided in this way is that there are 3 very distinct cell structures. The structure is more important in this categorization than the number of cells (which is why some scientists disagree with this system). The Archaea and Bacteria are one-celled, but the Eukaryotes can be one-celled or multi-celled, as in trees and humans. The cell structure of Eukaryotes remains basically the same, although can be specialized (I just read that humans have 210 cell types).

I'll show you a few differences today and next week. For one, the Bacteria make a cell wall outside of the regular cell membrane. The membrane is the outer part of the cell, but it can be covered with the wall and other layers. The wall has a distinct chemical makeup, called peptidoglycans, which link together. The structure is shown at right. The letters stand for atoms (oxygen, nitrogen, hydrogen, carbon). Almost all Bacteria, and only Bacteria, have the wall.

Another difference is between Archaea and the other two Domains. All cell membranes have what are called phospholipids. They keep many of the cell molecules inside and let only certain things go through either direction. The arrangement of atoms are distinctly different in the Archaea, as seen in the picture at left. (Again, the letters stand for atoms, and a clearer picture can be found at the Wikipedia entry "Archaea"). The phospholipid layer is made by cell machinery (proteins called enzymes). Three of the enzymes which make the different link in Archaea are reported by Koga et al. in Microbiology and Molecular Biology Reviews to be different from any known enzymes, meaning their structure is unique.

Next week I will show you other differences between the 3 Domains. These are important, because they are the reason that many scientists have concluded that they are not ancestors to each other. It was thought that archaea evolved to bacteria which in turn evolved to eukaryotes, but because they are so different, it would not be mathematically possible for the different proteins to have developed by this route. Instead they all need a previous ancestor to make the case that there was only one organism which formed by random movements of atoms and molecules.

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