Cellular Classification

▶Eubacteria: The Eubacteria are one of two subdivisions of the prokaryotes. Prokaryotes are the simplest living cells, typically 1–10 _m in diameter, and are found in all environmental niches from the guts of animals to acidic hot springs. Classically, they are defined by their structural organization. They are bounded by a cell (plasma) membrane comprising a lipid bilayer in which are embedded proteins that allow the exit and entry of small molecules. Most prokaryotes also have a rigid cell wall outside the plasma membrane which prevents the cell from swelling or shrinking in environments where the osmolarity differs significantly from that inside the cell. The cell interior (cytoplasm or cytosol) usually contains a single, circular chromosome compacted into a nucleoid and attached to the membrane , and often plasmids [small deoxyribonucleic acid (DNA) molecules with limited genetic information], ribonucleic acid (RNA), ribosomes (the sites of protein synthesis) and most of the proteins which perform the metabolic reactions of the cell.

Some of these proteins are attached to the plasma membrane, but there are no distinct subcellular organelles as in eukaryotes to compartmentalize different parts of the metabolism. The surface of a prokaryote may carry pili, which allow it to attach to other cells and surfaces, and flagella, whose rotating motion allows the cell to swim. Most prokaryotes are unicellular; some, however, have multicellular forms in which certain cells carry out specialized functions. The Eubacteria differ from the Archaea mainly in their biochemistry. The eubacterium Escherichia coli has a genome size (DNA content) of 4600 kilobase pairs (kb) which is sufficient genetic information for about 3000 proteins. Its molecular biology has been studied extensively.The genome of the simplest bacterium, Mycoplasma genitalium, has only 580 kb of DNA and encodes just 470 proteins. It has a very limited metabolic capacity.

▶Archaea: The Archaea, or archaebacteria, form the second subdivision of the prokaryotes and tend to inhabit extreme environments. Structurally, they are similar to eubacteria. However, on the basis of the evolution of their ribosomal RNA(rRNA) molecules, they appear as different from the eubacteria as both groups of prokaryotes are from the eukaryotes and display some unusual biochemical features, for example ether in place of ester linkages in membrane lipids. The 1740 kb genome of the archaeon Methanococcus jannaschii encodes a maximum of 1738 proteins. Comparisons reveal that those involved in energy production and metabolism are most like those of eubacteria while those involved in replication, transcription and translation are more similar to those of eukaryotes. It appears that the Archaea and the eukaryotes share a common evolutionary ancestor which diverged from the ancestor of the Eubacteria.

▶Eukaryotes: Eukaryotes are classified taxonomically into four kingdoms comprising animals, plants, fungi and protists (algae and protozoa). Structurally, eukaryotes are defined by their possession of membrane-enclosed organelles with specialized metabolic functions. Eukaryotic cells tend to be larger than prokaryotes: 10–100 _m in diameter. They are surrounded by a plasma membrane, which can have a highly convoluted shape to increase its surface area. Plants and many fungi and protists also have a rigid cell wall. The cytoplasm is a highly organized gel that contains, in addition to the organelles and ribosomes, an array of protein fibers called the cytoskeleton which controls the shape and movement of the cell and which organizes many of its metabolic functions. These fibers include microtubules, made of tubulin, and microfilaments, made of actin . Many eukaryotes are multicellular, with groups of cells undergoing differentiation during development to form the specialized tissues of the whole organism.

▶Differentiation: When a cell divides, the daughter cells may be identical in every way, or they may change their patterns of gene expression to become functionally different from the parent cell. Among prokaryotes and lower eukaryotes, the formation of spores is an example of such cellular differentiation. Among complex multicellular eukaryotes, embryonic cells differentiate into highly specialized cells, for example muscle, nerve, liver and kidney. In all but a few exceptional cases, the DNA content remains the same, but the genes which are transcribed have changed. Differentiation is regulated by developmental control genes. Mutations in these genes result in abnormal body plans, such as legs in the place of antennae in the fruit fly Drosophila. Studying such gene mutations allows the process of embryonic development to be understood. In multicellular organisms, co-ordination of the activities of the various tissues and organs is controlled by communication between them. This involves signaling molecules such as neurotransmitters, hormones and growth factors which are secreted by one tissue and act upon another through specific cell-surface receptors.