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

Membranes and Cell Walls

Previously, we examined the ways in which protein subunits can associate with each other to form closed oligomers and long helices. Another extremely important mode of protein and lipid packing leads to The formation of lamellar structures, or membranes [1-10], which from a molecular standpoint can be regarded as virtually limitless two-dimensional surfaces. This chapter is devoted to the Structure, chemical properties, and Functions of Introduction/36.html">Biological Membranes, as well as The Cell walls of Bacteria, Fungi, and plants.

Membranes perform A wide variety of functions within the cell. The most obvious of these is the compartmentalization of the intracellular space. Plasma Membranes, for instance, enclose the cell contents, whereas mitochondrial membranes segregate mitochondrial Enzymes and metabolites from the Cytoplasm. The semipermeability of membranes enables them to regulate the influx of both ions and uncharged molecules into Cells and cellular Organelles. The uptake of many of these substances occurs against a concentration gradient. Thus, through a process known as Active Transport, osmotic work is performed. Oxidative Phosphorylation, which takes place within the membrane structures of bacteria and Mitochondria, serves as an energy source for the Organism. The METABOLISM/14.html">Chloroplasts of green leaves contain highly folded membranes that harbor chlorophyll, capable of absorbing solar energy. The delicate membranes of retinal cells contain photoreceptor Proteins that perceive light signals, while nerve cell membranes mediate the transmission of electrical impulses.

A significant portion of our knowledge regarding membranes stems from intensive, long-standing research conducted on specific types of membranes. These include the following: 1. The myelin sheath, composed of plasma membranes formed by Schwann cells that envelop many Neurons. Schwann cells wrap around neuronal axons, squeezing out their cytoplasm to form thin yet tightly packed membrane layers that surround the axons and serve as an excellent electrical insulator. Of all known membranes, myelin exhibits the highest stability and the highest lipid content (80%). 2. The plasma membranes of human erythrocytes, which can be isolated via osmotic lysis of these cells. The resulting erythrocyte ghosts consist of approximately 1% cellular dry weight and are arguably the most thoroughly characterized of all membranes. 3. Bacterial membranes, most notably those of E. coli. 4. The outer segment of retinal receptor cells known as rods (Chap. 13), consisting of numerous orderly arranged and densely packed flattened discs, each of which in turn comprises two membranes. 5. The double-layered membranes of bovine Heart mitochondria. 6. The regularly stacked membranes (grana) of plant chloroplasts (Chap. 13).