Passive transport, biological

It is clearly impossible to give a comprehensive overview of this rapidly expanding field. I have chosen a few experts in their field to discuss one (class of) transport protein(s) in detail. In the first five chapters pumps involved in primary active transport are discussed. These proteins use direct chemical energy, mostly ATP, to drive transport. The next three chapters describe carriers which either transport metabolites passively or by secondary active transport. In the last three chapters channels are described which allow selective passive transport of particular ions. The progress in the latter field would be unthinkable without the development of the patch clamp technique. The combination of this technique with molecular biological approaches has yielded very detailed information of the structure-function relationship of these channels. 

Membranes play essential roies in the functions of both prokaryotic and eukaryotic cells. There is no unicellular or multicellular form of life that does not depend on one or more functional membranes. A number of viruses, the enveloped viruses, also have membranes. Cellular membranes are either known or suspected to be involved in numerous cellular functions, including the maintenance of permeability barriers, transmembrane potentials, active as well as specific passive transport across the membranes, hornione-receptor and transmitter-receptor responses, mitogenesis, and cell-cell recognition. The amount of descriptive material that might be included under the title of biological membranes is encyclopedic. The amount of material that relates or seeks to relate structure and function is less, but still large. For introductory references see Refs. 53, 38, 12, 47, 34, 13. Any survey of this field in the space and time available here is clearly out of the question. For the purposes of the present paper we have selected a rather narrow, specific topic, namely, the lateral diffusion of molecules in the plane of biological mem-branes.38,12,43,34 We consider this topic from the points of view of physical chemistry and immunochemistry. 

It should be mentioned here that, in living systems the transport of mass sometimes takes place apparently against the concentration gradient. Such uphill mass transport, which usually occurs in biological membranes with the consumption of biochemical energy, is called active transport, and should be distinguished from passive transport, which is the ordinary downhill mass transport as discussed in this chapter. Active transport in biological systems is beyond the scope of this book. 

In order to be able to distinguish between active and passive transport through biological membranes, P. Meares and H. H. Ussing (95) likewise made a study of the fluxes through a membrane under the influence of diffusion together with an electric current. They studied the influxes and the outfluxes of sodium- and chloride ions at a cation exchange resin membrane. They started from the Nemst-Planck flux equations of the type ... 

Meares and his collaborators are especially interested in transport processes across biological membranes. They wish to distinguish experimentally between the active and the passive transport of a solute. For that purpose they determined the fluxes of the sodium ions in each direction through the membrane, using the technique of radio-tracers. The ratio of these experimental fluxes was compared with the theoretical ratios. The same is done with regard to the chlorine ions. 

Osmotic pressure is vital in biology, where the cell contents has a different concentration of solutes than the surrounding medium If too much medium moves into a cell, it bursts and dies ("lysis") conversely, if too much medium moves out of a cell, it shrinks and dies these movements are called passive transport. Active transport involves proteins on the cell wall, which promote movements of nutrients and waste products despite the osmotic pressure. 

Transport across biological membranes is classified according to the thermodynamics of the process. Passive transport is a thermodynamically downhill process the species move toward the equilibrium. The driving force for the passive transport is the potential difference between the two sides of the membrane. Active transport is a thermodynamically uphill process, it is coupled to a chemical reaction and is driven by it. The following transport mechanisms have been recognized ... 

The lipid bilayer of biological membranes, as discussed in Chapter 12. is intrinsically impermeable to ions and polar molecules. Permeability is conferred by two classes of membrane xoXems, pumps and channels. Pumps use a source of free energy such as ATP or light to drive the thermodynamically uphill transport of ions or molecules. Pump action is an example of active transport. Channels, in contrast, enable ions to flow rapidly through membranes in a downhill direction. Channel action illustrates passive transport, or facilitated diffusion. 

Transport of solutes and water occurs both across and between the epithelial cells that line the renal tubules. Transport is both active (energy requiring) and passive, but many of the so-called passive transport processes are dependent upon or secondary to active transport processes, particularly those involving sodium transport. All known transport processes involve receptor or mediator molecules, many of which have now been identified and characterized using molecular biological teclmiques. The activity of many of these molecules is regulated by phosphorylation facilitated by protein kinase C or Their renal distribution has... 

Biological membranes have complex effects on passive transport by diffusion because they are structured as a mosaic of regions with distinct hydrophobic and hydrophilic properties. Consequently, polarity affects the ability of molecules to pass through biological membranes by passive diffusion. The ability of membrane proteins specifically to admit some polar molecules and exclude others dramatically affects the responses of biological membranes to concentration gradients. 

Many substances cross biological membranes according to their lipid solubility. Other polar molecules, such as amino acids and glucose, cross the membranes more rapidly than expected according to their solubUity in lipids. Cations, such as Na" and K, also cross membranes rapidly in spite of their hydrophilic nature. This passive transport of substances at higher rates than predicted from their lipid solubility is termed facilitated diffusion. That proteins are directly involved in facilitated diffusion was shown by comparison of experiments with natural membranes and synthetic membranes produced with phospholipid films. With phospholipid films all molecules, except water, diffuse according to lipid solubility and molecular size. Ions are essentially impermeable. The addition of membrane proteins, however, frequently allowed many polar and charged species to penetrate the membrane at rates comparable to natural membranes. 

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