On the basis of cellular organisation in living organisms, two types of cells are recognized prokaryotic cells with no organised nuclei and eukaryotic cells having organised nuclei with nuclear envelops. The prokaryotes include bacteria, mycoplasma and blue-green algae(cyanobacteria. while the eukaryotes include higher organisms. However, another group of living organisms called the archea was discovered in 1977, which were described as archaebacteria.
So the prokaryotes are further grouped as eubacteria and arehaebacteria, But now, it is seen that the group archea is distinct from prokaryotes and eukaryotes. So, the living cells are now divided into three distinct groups: bacteria, archea and eukarya.
Prokaryotic cell (Gr.Proprimitive, karyon-nucleus)
The prokaryotic cell is the simplest kind known to biology. Fossil record reveals that they came into existence about 3.0-3.5 billion years ago. These cells are very small in size,that varies from 0.1-0.25 um (micrometer) among the mycoplasmas , a few micrometers in length in bacteria and a bit larger in cyanobacteria. The living portion of the prokaryotic cell is bounded externally by a ‘plasmamembrane’ outside of which a more or less rigid cell wall and a jelly like mucilaginous, capsule or sheath are present. The composition of the cell wall varies with the particular prokaryotic species.
The bacterial cell contains lipids, carbohydrates and complexes of mucopeptides derived from amino acids and amino sugars, while those of cyanobacteria tend more towards, eukaryotic cell in that it incorporates some cellulose.
The cellular contents constitute a dense cytoplasm and a less electron dense nuclear area, but no organised nucleus. The chromosome is a molecule of naked deoxyribonucleic acid (DNA). The basic protein, histone, which is characteristically associated with eukaryotic chromosome, is absent. The cyanobacteria and some other bacteria, possess layered membranes that are involved in photosynthesis and that appear to be derived from unfolding of the plasmamembrane.
The photosynthetic pigment in bacteria is bacteriochlorophyll; the cyanobacteria contain chlorophylla and phycocyanin. Small, rounded cytoplasmic structures responsible for cellular protein synthesis are also found in prokaryotes. True vacuoles and all other organelles are absent.
No streaming movement is observed in the cytoplasm. Some species possess flagella but their internal structure is quite different from that of an eukaryotic flagellum. The bacteria divide by simple fission and both bacteria and cyanobacteria form resting spores under adverse conditions.
Eukaryotic cell (Gr.Eu-good, karyon-nucleus)
The eukaryotic cell, found in higher forms of plants and animals, is a far more elaborately structured and partitioned larger unit than the prokaryotic one, from which it is presumably derived. The exterior of the cell is bounded by a plasmamembrane. But, in case of plants, a rigid, porous cellwall, made up of cellulose and other materials, is present outside it. The hereditary material is enclosed in a membrane-bound nucleus in the form of complex nucleoprotein bodies called chromosomes.
The cytoplasm contains a number of membrane bound organelles like mitochondria, plastid, Golgi complex, endoplasmic reticulum, centrosome, lysosome and peroxisome which play definite roles in the life of eukaryotic cells. Only the ribosomes, chromosomes, microtubules and microfibrils are non-membranous in nature.