There are two basic types of cell: prokaryotic and eukaryotic. (Viruses are not cellular and most biologists consider them to be biological entities of course, but as not being alive in the generally accepted sense of the word. Do not make the common mistake of thinking of viruses as being cells, they are not! ) Prokaryotic cells do NOT have a discrete membrane bound nucleus that contains chromosomes, this is the defining feature of a prokaryotic cell. Prokaryotic cells have a none membrane bound single circular DNA chromosome (properly speaking it should not be referred to as a chromosome, but it in practice is often called a chromosome). ONLY the Archaea, Cyanobacteria and the Eubacteria are prokaryotic, ALL other cellular life forms are eukaryotic. Prokaryotes are absolutely and fundamentally defined on the basis of NOT having a nucleus, the region where their circular DNA macromolecule exists is called the nucleoid. Unlike eukaryotic DNA, bacterial DNA is not associated with special cationic proteins called histones. Eukaryotic cells DO have a membrane bound genome - consisting of more than one LINEAR chromosome, bounded by a double layered nuclear membrane. Fungal, protist, plant and animal cells are eukaryotic. Prokaryotic cells are much smaller than eukaryotic cells and though they can assemble together to form masses or chains, prokaryotic organisms are NOT multicellular in the accepted sense, which is that multicellularity involves association of different cell types in a discrete organism, and that the cells undertake division of labour, they have differing functions, this is seen only in a very simple way in some bacteria, which are fundamentally UNI-cellular. A typical size for a bacterium is that of a cell of Escherichia coli, about 1 um (one micron or one micrometer) diameter. A typical yeast cell (a fungus) might have a diameter of 6-8 um or more, and contains much more cytoplasm. In terms of “internal architecture” prokaryotic cells are far simpler than eukaryotic cells, as we discuss below, they lack all of the complex internal membrane based structure of eukaryotic cells. Actually, there are some recent reports that do indicate the presence of some simple membrane bound vesicles in some bacteria, so it is no longer an absolute axiom to state that bacteria do not have membrane bound organelles (MBO’s), but nonetheless, bacteria are still much less structurally complex, internally, than eukaryotic cells. The lack of complex internal structure in bacteria should not lead you into believing that they are also biochemically simple, bacteria have complex biochemical systems - metabolism in other words, they just do not operate their biochemical events within specialized “walled off” membrane bound organelle structures.
Nearly all prokaryotes have a rigid cell wall, which determines the shape of the cell, and in most cases the wall contains peptidoglycan, a unique and complex acidic polysaccharide that lends rigidity to the wall. The bacterial cell wall serves to resist the very high internal osmotic pressure that would burst (lyse) the cell unless the wall was present. The cell wall is metabolically inert, it does not participate actively in transport processes in and out of cells, it is there as a structural strengthening agent – to provide rigidity and shape. Penicillins kill bacteria by interfering with the synthesis of peptidoglycan in cell walls as it is being formed during bacterial reproduction, this weakens the cell wall structure and the bacterial cell bursts. There are a few bacterial genera that do not have cell walls, Mycoplasma is a classic example, they are parasites of animals such as cattle and are able to exist at the same osmotic pressure as the animal cells that they live in and among. Archaebacteria DO have cell walls, but peptidoglycan is not a part of their cell wall structure. Most Gram negative bacteria have a tiny but definite space between their cell walls and the cell membrane, this is called the periplasmic space. This space is of significance in pathological behaviour, it is metabolically active in many bacteria, it contains enzymes and transport proteins and other factors that mediate the ability of the bacterium either to cause disease in hosts or to evade destruction, enzymes in this space can, for instance, destroy antibiotics before they can enter the bacterium. A periplasmic space is present in some Gram positive bacteria, but it is much rarer. From now on, unless I say differently, ALL of my discussion about prokaryotic structure and function should be taken as referring specifically to eubacteria – these are the types of bacteria that cause human disease problems although the vast majority of the eubacteria are not pathogenic to man and in fact, many eubacteria are beneficial to man, either as symbiotic components of the digestive tract, as bacteria involved in food manufacture, or as components of our ecosystem that synthesize new biomass by photosynthesis or that fix nitrogen into soils for use by plants.