After studying this chapter, you should be able to:
Know that biologic 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.
Describe the widely accepted fluid mosaic model of membrane structure.
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.
Be aware that a variety of disorders result from abnormalities of membrane structure and function, including familial hypercholesterolemia, cystic fibrosis, hereditary spherocytosis, among others.
Membranes are dynamic, highly fluid 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. Selective membrane molecular permeability is generated through the action of specific 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 may communicate by exchanging 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), for example, mitochondria, endoplasmic reticulum (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 in plants (chloroplasts) and oxidative phosphorylation (mitochondria).
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 in genes encoding membrane proteins) lead to a variety of diseases (see Table 40–7). In short, normal cellular function critically depends on normal membranes.
MAINTENANCE OF A NORMAL INTRA- & EXTRACELLULAR ENVIRONMENT IS FUNDAMENTAL TO LIFE
Life originated in an aqueous environment; enzyme reactions, cellular and subcellular processes 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 to (1) make, store, and utilize energy; (2) to repair itself; (3) to replicate; and (4) to perform cell-specific functions.