Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Membranes and Cell Walls
Metabolism and Functions of Membranes
Fluidity of Membrane Components

The Study of time-lapse micrographs of Cells has revealed that Cell/30.html">The Plasma Membrane, much like Mitochondria and other Organelles, is in a state of constant motion. Mitochondria writhe and rotate, while the membrane surface continually undergoes wave-like undulations. Vesicles discharge their contents into the extracellular environment, expelling substances from The Cell, whereas the uptake of extracellular Materials occurs via endocytosis (Ch. 1, Sec. B.4). Furthermore, chemical analyses have demonstrated that membrane constituents are transported from The Endoplasmic reticulum to Golgi vesicles, secretory granules, and ultimately the plasma membrane. A crucial milestone in intracellular biosynthetic pathways is the attachment of carbohydrate (glycosyl) residues to protein molecules, yielding Glycoproteins and Glycolipids. The Enzymes catalyzing these reactions—Glycosyltransferases (Ch. 12)—have been localized to the Endoplasmic reticulum and Golgi vesicles. These enzymes catalyze the stepwise addition of carbohydrate units (one at a time per reaction cycle) to specific sites on Proteins, Lipids, and Other Compounds destined for cellular export. Other Enzymes mediate The addition of sulfate and acetyl groups to the carbohydrate moieties of glycoprotein molecules.

Glycoproteins and glycolipids residing on the outer surface of the plasma membrane are likewise presumed to be synthesized within the endoplasmic reticulum and Golgi vesicles. These membrane components must be translocated from the cell interior to the plasma membrane. During this transport process, newly synthesized molecules approach the inner face of the membrane, where vesicle-bound Enzymes effect the attachment of glycosyl moieties and other structural modifications. If this proposed model is correct, the plasma membrane must undergo rapid turnover and continuous replenishment with new constituents. Clearly, to balance this process, a complementary mechanism must exist to remove specific components from the membrane surface. It is highly plausible that this missing link in the "life cycle" of membrane components is supplied by the highly active endocytosis of fluid and particulate matter by cells. It has been hypothesized that eukaryotic Plasma Membranes function as molecular sieves regulating a rapid and directionally controlled flux of lipid molecules [36a]. As we shall explore further, cells rely on a diverse array of lysosomal enzymes. One of the primary Functions of these enzymes appears to be the degradation of excess membrane components, including complex Polysaccharides internalized from the outer membrane surface.

A vital property of membranes is the ability of small localized patches of their surface to fold and pinch off, forming structures approximating a spherical geometry. Electron microscopic examination of aqueous phospholipid Suspensions has revealed The formation of concentric multilamellar structures (Liposomes). Ultrasonication disrupts these structures into smaller vesicles bounded by phospholipid bilayers analogous to native membrane bilayers. Under specific conditions, these small vesicles can fuse to form larger ones. Similarly, cells occasionally fuse with one another to generate multinucleated cells, a phenomenon likely linked to enhanced membrane fluidity as well as altered orientation of phospholipid polar HEAD groups [37]. This phenomenon holds significant practical value for plant breeding and the investigation of Human Chromosomes (Ch. 15).