There are shapely cell walls in independently existed unicellular
organisms (bacteria, paramecia, infusoria, euglena, etc.), as well as in some
cells of the higher plant and animal organisms. A clear cytological picture of
cell wall, plasmalemma and plasmodesma can to be seen in Figure 2–56.
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Figure 2–56. Cell wall, plasmalemma and plasmodesma in a root of Allium cepa (After Frey-Wissling and Mühlethaler, 1965)
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Figure 2–57. Cross-section of plasmic membrane of human erythrocyte (From Yost, 1975).
In some cells (mainly animal) participating in tissues and organs of
multicellular organisms or strictly specialized, the outer cell wall is absent or it is not
shapely. Plasmic membrane of human erythrocyte, where outer cell wall is not
observed, is shown in Figure 2–57. At present, the liquid-mosaic hypothesis on
plasmic membrane structure is the most appreciated one. According this hypothesis the
membrane is built of a double lipid layer and some amount of protein molecules built-in. The
lipid molecules are facing with their hydrophobic ends. Diagrammatically this is shown in Figure
2–58. Besides, the membrane contains glycolipids, sterines, aliphatic acids, etc. The
lipids participating in the double membrane layer are labile compounds and their molecules
change the position and mutual location continuously.