organ system , organ system coordinating together to form an organism . Shape The shape of cell vary greatly from organism to organism and within the organism itself. In bacteria, cell shape vary from round ( cocci ) to rectangular ( rod ). In virus, shape of the envelope varies from round to hexagonal or ‘ T ’ shaped.
In fungi, globular to elongated cylindrical cells and the spores of fungi vary cm = / metre mm = / metre = / cm µm = / , metre = / , cm nm = / , , , metre = / , , cm A º = / , , , metre = / , , cm or m = cm = mm = µm = nm = A º m = metre cm = centimetre mm = millimetre µm = micrometre nm = nanometre A º =Angstrom these are organisms intermediate between pro and eukaryotes. These contains well organized nucleus with nuclear membrane and the DNA is organized into chromosomes but without histone protein components divides through amitosis similar with prokaryotes. Certain Protozoa like Noctiluca , some phytoplanktons like Gymnodinium , Peridinium and Dinoflagellates are representatives of mesokaryotes. .
. Eukaryotes Those organisms which have true nucleus are called Eukaryotes ( Eu – True; karyon – nucleus). The DNA is associated with histones forming the chromosomes. Membrane bound organelles are present.
Few organelles may have risen by endosymbiosis which is a cell living inside another cell. The Organelles like mitochondria and chloroplast well support this theory. Origin of Eukaryotic cell: Endosymbiont theory: Two eukaryotic organelles believed to be the descendants of the endosymbiotic prokaryotes. The ancestors of the eukaryotic cell engulfed a bacterium and the bacteria continued to function inside the host cell.
greatly in shape. In plants and animals cells vary in shape according to cell types such as parenchyma, mesophyll, palisade, tracheid, fiber, epithelium and others (Figure . ). .
. Types of cells On the basis of the cellular organization and the nuclear characteristics, the cell can be classified into •