The plasma membrane (cell membrane) is a dynamic structure that actively participates in many physiologic and biochemical activities essential to cell function and survival. When the plasma membrane is properly fixed, sectioned, stained, and viewed on edge with the transmission electron 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 an amphipathic lipid layer containing embedded integral membrane proteins with peripheral membrane proteins attached to its surfaces.
The current interpretation of the molecular organization of the plasma membrane is referred to as the modifed fuid–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., they have an affinity for water). Lipids are distributed asymmetrically between the inner and outer leaﬂets of the lipid bilayer, and their composition varies considerably among different biologic membranes.
In most plasma membranes, protein molecules constitute approximately half of the total membrane mass. Most of the proteins are embedded within the lipid bilayer or pass through the lipid bilayer completely. These proteins are called integral membrane proteins. The other types of protein— peripheral membrane proteins—are not embedded within the lipid bilayer. They are associated with the plasma