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Figure 2–58. Three-dimensional view of a small section of a cell membrane (After Alberts et al., 1989)
Specialized protein complexes are located in the liquid layer of the lipid
membranes. The lipoproteins are in the lipid phase and are fixed through
hydrophobic bonds (integral proteins). Hydrophilic proteins are connected
by means of electrostatic bonds with inner and outer surface of the
membrane and interact with hydrophilic heads of the polar lipids. The main
role in the membrane formation is played by the hydrophilic bonds: lipid—
lipid, lipid—protein, protein—protein. The thickness of plasmalemma does
not exceed 6—10 nm.
Proteins also participate in plasmalemma. They act as enzymes,
pumps, carriers, ionic channels, and also as regulator proteins and
structural proteins. The labile structure of the cell membrane enables it to
perform different functions — barrier, transport, osmotic, electric, energetic,
biosynthetic, secretory, receptor-regulating, etc.
Nucleus
Cell nucleus is an obligatory and irreplaceable constitutive organelle of
eukaryotic cells. Its shape can be different — spheric, elliptic, prolonged,
leaf-like, etc. Sometimes it follows the form of cell. To considerable extent
the size of nucleus depends on the size of cell, its functions and
specialization. In lower unicellular eukaryotes it can be of the order of 1—2
μm, and in higher organisms varies from 4 to 40 μm. Nuclei of animal and
plant cell are shown in Figure 2–59 A, B.