Key Concepts of Section 7.1

Key Concepts of Section 7.1

The Lipid Bilayer: Composition and Structural Organization

  • Membranes are crucial to cell structure and function. The eukaryotic cell is demarcated from the external environment by the plasma membrane and organized into membrane-limited internal compartments (organelles and vesicles).

  • The phospholipid bilayer, the basic structural unit of all biomembranes, is a two-layered lipid sheet with hydrophilic faces and a hydrophobic core, which is impermeable to water-soluble molecules and ions. Proteins embedded in the bilayer endow the membrane with specific functions (see Figure 7-1).

  • Phospholipids spontaneously form bilayers and sealed compartments surrounding an aqueous space (see Figure 7-3).

  • As bilayers, all biological membranes have an internal (cytosolic) face and an external (exoplasmic) face (see Figure 7-5). Some organelles are surrounded by two, rather than one, membrane bilayer.

  • The primary lipid components of biomembranes are phosphoglycerides, sphingolipids, and sterols such as cholesterol. The term “phospholipid” applies to any amphipathic lipid molecule with a fatty acyl hydrocarbon tail and a phosphate-based polar head group (see Figure 7-8).

  • Biomembranes can undergo phase transitions from fluidlike to gel-like states depending on the temperature and the composition of the membrane (see Figure 7-9).

  • Most lipids and many proteins are laterally mobile in biomembranes (see Figure 7-10).

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    Different cellular membranes vary in lipid composition (see Table 7-1). Phospholipids and sphingolipids are asymmetrically distributed in the two leaflets of the bilayer, whereas cholesterol is fairly evenly distributed in both leaflets.

  • Natural biomembranes generally have a viscous consistency with fluidlike properties. In general, membrane fluidity is decreased by sphingolipids and cholesterol and increased by phosphoglycerides. The lipid composition of a membrane also influences its thickness and curvature (see Figure 7-11).

  • Lipid rafts are microdomains containing cholesterol, sphingolipids, and certain membrane proteins that form in the plane of the bilayer. These lipid-protein aggregates might facilitate signaling by certain plasma-membrane receptors.

  • Lipid droplets are storage vesicles for lipids, originating in the ER (see Figure 7-13).