Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Lipids and Membranes
Membranes have very complex functions

It is now evident that membranes perform numerous complex, dynamic functions and possess a range of remarkable biological properties; consequently, they can be viewed neither as simple, inert barriers enclosing The Cell contents, nor as static, unchanging structures. Most membranes contain Enzymes, some of which interact with substrates located on the outer side of the membrane, while others interact with substrates within the membrane-enclosed space. The inner mitochondrial membrane and the thylakoid membrane of METABOLISM/14.html">Chloroplasts harbor a sophisticated system of numerous enzymes and other Proteins. Typically, membranes also incorporate transport systems that mediate the uptake of specific organic nutrient molecules, such as glucose, allow certain inorganic ions to enter the cell, and enable Metabolic waste products to exit. By regulating the flux of substances in and out of the cell, these transport systems help maintain a stable intracellular environment. The membrane surface also bears electrically charged groups that help maintain an electrical potential difference across the membrane. This is particularly crucial in Nerve Cells, which are capable of transmitting impulses in the form of rapid, wave-like alterations in the membrane's electrical properties along the elongated cell body or axon. Cell membranes spontaneously restore their integrity: if punctured or mechanically disrupted, they automatically reseal within a short time.

The outer surface of membranes features specific recognition sites whose function is to detect particular molecular signals. For instance, it is through the membrane that certain Bacteria sense minor fluctuations in nutrient concentration, which stimulates their movement toward a food source—a phenomenon known as chemotaxis. The outer surface of animal cell membranes also possesses sites that recognize other Cells of the same type, thereby facilitating Cell-to-Cell adhesion during tissue formation. Yet another Class of recognition sites acts as specific Hormone Receptors. For example, designated sites On the surface of Liver and Muscle cells recognize and bind Hormones such as Insulin, Glucagon, and epinephrine. Upon hormone binding, these receptor sites transmit signals across the membrane into intracellular enzyme systems to modulate their activity. In addition, the cell surface contains specialized sites unique to a given individual or species, commonly referred to as histocompatibility sites.

Gangliosides appear to serve as vital components of many recognition or receptor sites in animal cell membranes. Although their concentration relative to other Membrane Lipids is very low, they are believed to be capable of localized clustering. The immense diversity of gangliosides, each bearing a distinct oligosaccharide HEAD group, has led to the hypothesis that gangliosides, along with Glycoproteins, form specific mosaic structures on the cell surface that function as receptor sites. The negatively charged polar head groups of gangliosides can act as receptors—effectively serving as cell-surface antennas that recognize specific signaling molecules, particularly hormones.

Thus, cell membranes are highly complex structures; their constituent molecular complexes form an ordered two-dimensional mosaic, which imparts biological Specificity to the membrane surface. The Molecular Organization of cell membranes remains one of the most pressing research topics in modern cell biology and biochemistry.