Cell Structure of Archaea
Archaea are tiny, usually less than one micron long (one one-thousandth of a millimeter). Even under a high-power light microscope, the largest archaeans look like tiny dots. Fortunately, the electron microscope can magnify even these tiny microbes enough to distinguish their physical features. You can see archaean images below, made using a variety of micrographic techniques.
You might think that organisms so small would not have much variety of shape or form, but in fact archaeal shapes are quite diverse. Some are spherical, a form known as coccus, and these may be perfectly round or lobed and lumpy. Some are rod-shaped, a form known as bacillus, and range from short bar-shaped rods to long slender hair-like forms. Some oddball species have been discovered with a triangular shape, or even a square shape like a postage stamp!
Basic Archaeal Shapes : At far left, Methanococcus janaschii, a coccus form with numerous flagella attached to one side. At left center, Methanosarcina barkeri, a lobed coccus form lacking flagella. At right center, Methanothermus fervidus, a short bacillus form without flagella. At far right, Methanobacterium thermoautotrophicum, an elongate bacillus form.Structural diversity among archaeans is not limited to the overall shape of the cell. Archaea may have one or more flagella attached to them, or may lack flagella altogether. The flagella are hair-like appendages used for moving around, and are attached directly into the outer membrane of the cell. When multiple flagella are present, they are usually attached all on one side of the cell. Other appendages include protein networks to which the cells may anchor themselves in large groups.
Like bacteria, archaeans have no internal membranes and their DNA exists as a single loop called a plasmid. However, their tRNAs have a number of features that differ from all other living things. The tRNA molecules (short for "transfer RNA") are important in decoding the message of DNA and in building proteins. Certain features of tRNA structure are the same in bacteria, plants, animals, fungi, and all known living things - except the Archaea. There are even features of archaeal tRNA that are more like eukaryotic critters than bacteria, meaning that Archaea share certain features in common with you and not with bacteria. The same is true of their ribosomes, the giant processing molecules that assemble proteins for the cell. While bacterial ribosomes are sensitive to certain chemical inhibiting agents, archaeal and eukaryotic ribosomes are not sensitive to those agents. This may suggest a close relationship between Archaea and eukaryotes.
As with other living things, archaeal cells have an outer cell membrane that serves as a barrier between the cell and its environment. Within the membrane is the cytoplasm, where the living functions of the archeon take place and where the DNA is located. Around the outside of nearly all archaeal cells is a cell wall, a semi-rigid layer that helps the cell maintain its shape and chemical equilibrium. All three of these regions may be distinguished in the cells of bacteria and most other living things, but when you take a closer look at each region, you find that the similarities are merely structural, not chemical.
In other words, Archaea build the same structures as other organisms, but they build them from different chemical components. For instance, the cell walls of all bacteria contain the chemical peptidoglycan. Archaeal cell walls do not contain this compound, though some species contain a similar one. Likewise, archaea do not produce walls of cellulose (as do plants) or chitin (as do fungi). The cell wall of archaeans is chemically distinct.
Basic Archaeal Structure : The three primary regions of an archaeal cell are the cytoplasm, cell membrane, and cell wall. Above, these three regions are labelled, with an enlargement at right of the cell membrane structure. Archaeal cell membranes are chemically different from all other living things, including a "backwards" glycerol molecule and isoprene derivatives in place of fatty acids. See text below for details.The most striking chemical differences between Archaea and other living things lie in their cell membrane. Their are four fundamental differences between the archaeal membrane and those of all other cells: (1) chirality of glycerol, (2) ether linkage, (3) isoprenoid chains, and (4) branching of side chains. These may sound like complex differences, but a little explanation will make the differences understandable. The header for each explanation is color-coded to match the relevant portion of the diagram below.
(1) Chirality of glycerol : The basic unit from which cell membranes are built is the phospholipid. This is a molecule of glycerol which has a phosphate added to one end, and two side chains attached at the other end. When the cell membrane is put together, the glycerol and phosphate end of the molecules hang out at the surface of the membrane, with the long side chains sandwiched in the middle (see illustration above). This layering creates an effective chemical barrier around the cell and helps maintain chemical equilibrium.
