The Cell Membrane

The external membrane or plasmalemma of a neuron (Fig. 23) has the general composition of all biological membranes: a bimolecular layer of opposing phos-pholipid molecules, with hydrophilic phosphorylated heads at the external and internal membrane surfaces and hydrophobic chains in the water-free interior. Cholesterol, proteins, glycoproteins, and glycolipids are interspersed in the phospholipid bilayer. The lipids form a barrier to prevent ions and water-soluble molecules from entering and to retain the cytoplasm. They also provide a fluid matrix in which protein molecules lie in a fluid mosaic manner. The proteins permit selective passage of certain materials through pores, as carrier enzymes, antigens, and receptor sites for bonding neuroactive substances. The apparent uniformity of the plasmalemma is misleading. Regional peculiarities in the distribution of membrane proteins and lipids are known. The apical, lateral, basolateral, and basal domains of the columnar epithelial cell noted previously are good examples.

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