Chapter 40

After studying this chapter, you should be able to:

• Know that biological membranes are mainly composed of a lipid bilayer and associated proteins and glycoproteins. The major lipids are phospholipids, cholesterol, and glycosphingolipids.
• Appreciate that membranes are asymmetric, dynamic structures containing a mixture of integral and peripheral proteins.
• Know the fluid mosaic model of membrane structure and that it is widely accepted, with lipid rafts, caveolae, and tight junctions being specialized features.
• Understand the concepts of passive diffusion, facilitated diffusion, active transport, endocytosis, and exocytosis.
• Recognize that transporters, ion channels, the Na+–K+-ATPase, receptors, and gap junctions are important participants in membrane function.
• Know that a variety of disorders result from abnormalities of membrane structure and function, including familial hypercholesterolemia, cystic fibrosis, hereditary spherocytosis, and many others.

Membranes are highly fluid, dynamic structures consisting of a lipid bilayer and associated proteins. Plasma membranes form closed compartments around the cytoplasm to define cell boundaries. The plasma membrane has selective permeabilities and acts as a barrier, thereby maintaining differences in composition between the inside and outside of the cell. The selective permeabilities for substrates and ions are provided mainly by specific proteins named transporters and ion channels. The plasma membrane also exchanges material with the extracellular environment by exocytosis and endocytosis, and there are special areas of membrane structure—gap junctions—through which adjacent cells exchange material. In addition, the plasma membrane plays key roles in cell–cell interactions and in transmembrane signaling.

Membranes also form specialized compartments within the cell. Such intracellular membranes help shape many of the morphologically distinguishable structures (organelles), eg, mitochondria, ER, Golgi, secretory granules, lysosomes, and the nucleus. Membranes localize enzymes, function as integral elements in excitation-response coupling, and provide sites of energy transduction, such as in photosynthesis and oxidative phosphorylation.

Changes in membrane components can affect water balance and ion flux, and therefore many processes within the cell. Specific deficiencies or alterations of certain membrane components (eg, caused by mutations genes encoding membrane proteins) lead to a variety of diseases (see Table 40–7). In short, normal cellular function depends on normal membranes.

Life originated in an aqueous environment; enzyme reactions, cellular and subcellular processes, and so forth have therefore evolved to work in this milieu, encapsulated within a cell.

### The Body's Internal Water Is Compartmentalized

Water makes up about 60% of the lean body mass of the human body and is distributed in two large compartments.

#### Intracellular Fluid (ICF)

This compartment constitutes two-thirds of total body water and provides a specialized environment for the cell (1) to make, store, and utilize energy; (2) to repair itself; (3) to replicate; and (4) to perform cell-specific functions.

#### Extracellular Fluid (ECF)

This compartment contains about one-third of total body water and is distributed between the plasma and ...

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