Absorptive cells of the small intestine. This electron micrograph shows the apical portion of absorptive cells with microvilli. x29,000. Inset. Higher magnification of the area within the circle. Note that at this magnification the plasma membrane displays its characteristic appearance, showing two electron-dense lines separated by an electron-lucent intermediate layer, x95,000.

microscope (TEM), it appears as two electron-dense layers separated by an intermediate, electron-lucent (nonstaining) layer (Fig. 2.2). The total thickness of the plasma membrane is about 8 to 10 nm.

The plasma membrane is composed of amphipathic lipids and two types of proteins

The current interpretation of the molecular organization of the plasma membrane is referred to as the modified fluid-mosaic model (Fig. 2.3). The membrane consists primarily of phospholipid, cholesterol, and protein molecules. The lipid molecules form a lipid bilayer with an amphipathic character (it is both hydrophobic and hydrophilic). The fatty acid chains of the lipid molecules face each other, making the inner portion of the membrane hydrophobic (i.e., having no affinity for water). The surfaces of the membrane are formed by the polar head groups of the lipid molecules, thereby making the surfaces hydrophilic (i.e., having an affinity for water).

In most plasma membranes, protein molecules constitute approximately half of the total membrane mass. Most cholesterol molecule hydrophobic fatty acid chain

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