Traditionally, living organisms have been divided into two kingdoms: Plant and Animal. However, many biologists now recognize five: Prokaryotae, Protoctista, Fungi, Plantae and Animalia. A fundamental difference separates the Prokaryotae, the bacteria and their allies, from the eukaryotic organisms in the other four kingdoms. In prokaryotic types the fine structure of the cell, as revealed by the electron microscope, is quite different from that of animals, green plants, fungi and protoctists. Thus in the bacteria there is no true, membrane-bound nucleus or chromosomes, genetic material consisting of a circular strand of DNA lying in the cytoplasm. Mitochondria and endoplasmic reticulum are missing. The cell surface membrane may be infolded to form mesosomes. Ribosomes are smaller. Flagella, if they occur, are simpler in structure, being made up of individual sub-units of a protein called flagellin. They resemble just one of the 9 + 2 microtubules of eukaryotic flagella. The striking difference between cellular organization in fungi and bacteria can be illustrated by comparing a unicellular fungus, such as the yeast Saccharomyces with a bacterium (Fig. 1.1).
KeywordsGreen Plant Germ Tube Slime Mould Apical Growth Cell Surface Membrane
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