The Transport of Molecules through Membranes

Passive diffusion directly through the cell membrane highly depends on the solubility of the molecule in the lipid bilayer, which will be the focus of the next section. Some barriers in the body are specially modified to enhance or limit the transport of molecules through particular barriers. For example, relatively large gaps between cells in the small intestinal wall and fenestrations in kidney capillaries both increase the rate of transport across these barriers for many molecules. Conversely, capillaries in the central nervous system (GNS) have especially thick walls and extremely small gaps between cells, which limits the passage of molecules into the GNS and represents some of the factors responsible for the blood-brain barrier. 

The applications of this law are numerous, but only a few examples will be given here. Take first our body as an example. Our lungs are made up of very thin cells (less than a millionth of a meter) and have a large effective area (about 100 m ) that faciUtates the exchange of gases, which depends on the diffusion and the solubility of the fluid in the membrane of the lungs. The transport of molecules through the pores and channels in the membrane of our cells as well... 

Every cell possesses a plasma (or cell) membrane which isolates its contents from its surroundings. This membrane consists of a double layer of phospholipid molecules with proteins attached or dispersed within. The uneven distribution of proteins and their ability to move in the plane of the membrane led to the description of this structure as a fluid mosaic (Figure 1.2) (Chapter 5). Some of these proteins facilitate the transport of molecules and ions through the membrane, while others are receptors for extracellular molecules which provide information about conditions in adjacent cells, blood and elsewhere in the body. Physical or chemical damage to these membranes can render them leaky so that, for example, Na and Ca ions, the concentrations of which are much higher in the extracellular fluid, can enter the cell causing damage. On the outer surface of... 

Peppas and Reinhart have also proposed a model to describe the transport of solutes through highly swollen nonporous polymer membranes [155], In highly swollen networks, one may assume that the diffusional jump length of a solute molecule in the membrane is approximately the same as that in pure solvent. Their model relates the diffusion coefficient in the membrane to solute size as well as to structural parameters such as the degree of swelling and the molecular weight between crosslinks. The final form of the equation by Peppas and Reinhart is... 

Permeability across epithelial cells can be affected by the presence of influx or efflux transporters (saturable integral membrane proteins that catalyze the transfer of molecules through a biological membrane). For example, in the gastrointestinal... 

The transport of molecules across the plasma membrane of a cell occurs by three major mechanisms simple diffusion across the membrane, facilitated diffusion through channels or carriers, and active transport of molecules by carriers and pumps. The latter form of transport requires energy from the cell and is divided into two major types primary active and secondary active (1,2). 

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