Biologists have divided cells into two types: prokaryotic and eukaryotic. The differences between these two types of cells are mainly based upon the structure of their nuclei. Eukaryotes have a very well defined nucleus, in which nuclear material (chromosomes or DNA) is enclosed in double nuclear membrane. In prokaryotic cells, however the genetic material (DNA) is without any nuclear membrane covering and is directly submerged in the cytoplasm. Organisms possessing prokaryotic cells are called prokaryotes and those possessing eukaryotic cells are called Eukaryotes include all other unicellular or multicellular organisms such as animals, plants, fungi and protista.
Diagrams of prokaryotic and eukaryotic cells are given in fig 4.2, 4.3 and 4.17 prokaryotic cells generally lack many of the membrane bounded structures found in eukaryotic cells. For example, mitochondria endoplasmic reticulum, chloroplasts and Golgi apparatus are absent in prokaryotic cells. Since there is no nuclear membrance a cytoplasm. Prokaryotes have small sized ribosome’s 70s compared to eukaryotes 80s. In prokaryotes mitosis in missing and the cell divides by binary fission.
Because of their simpler structure, it was widely accepted for a long time that prokaryotic cells represent a more primitive stage of evolution than eukaryotic cells. Perhaps the most distinctive feature of the prokaryotic cell is its cell wall, composed of polysaccharide chains bound covalently to shorter chains of amino acids forming peptidoglycan or murein. The entire cell wall is often regarded as a single huge molecule or molecular complex called sacculus the cell wall of plants is generally made up of cellulose and is differently structured than that of a bacterium.
Fig.4-17. Generalized prokaryotic cell.