The postulation of the cell theory by schleiden and schwann generated more interest in biologist to investigate all aspects of cell and consequently , the knowledge about cells became rich ;more facts emerged will respect to cells .I n light of the available facts,Rudolph Virchow [1858] made another biological generalization that new cells can arise only from pre-existing cells,i.e., ominis cellula e cellula , leading to the establishment of the law of lineage .This made it clear that the succession of cells is continuity of life and the vital function of the body such as metabolism , growth , inheritance , diesease , aging , death , etc ., are varied aspects of the funxctioning and the behavour of the cells .These facts modified the cell theory into modern cell doctrine .The main postulates of the doctrine include : 1.The cell is the structural and functional unit of all living things. 2.It is the unit of development of all living things . 3. All cells come from pre-existing cells. 4. They are hereditary units 5. The cell is the unit of pathology. Two types of Cells The cells of organisms belong to two basic categories : the prokaryotic cells and the eukaryotic cells .The prokaryotic cells lack the membrane bound nucleus.Nor they have mitochondria ,golgi body and lysosomes .Their so called nucleus has just a single chromosome , consisting of a DNA molecule , without histone and lying exposed in the cytoplasm Their enzymes involved in the protein synthesis and respiration , are associated with the cell membrane .Cell of bacteria and cyano bacteria [blue green algae ] are prokaryotic cells . The eukaryotic cells are true cells of animals from from protozoa to mammalia , and plants from algae to angiousperms .These cells have a distinct nucleus ,bounded by a nuclear membrane .The nucleus has nucleolus and varying number of chromosomes.
