L10 Channels new 2011 - COPYRIGHT Mammalian Physiology...

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1/13 COPYRIGHT Prof. Beyenbach Mammalian Physiology BIOAP 4580 2011 MEMBRANE TRANSPORT: CHANNELS 1) Diffusion. Diffusion is the most fundamental transport mechanism in non-living as well as living things. The proximal cause of diffusion is the concentration difference; the ultimate cause (as far as we know) is energy in the form of heat that causes molecular motion (Brownian movement). Viewed from the perspective of molecular motion and hence the likelihood of molecular collisions, diffusion is also a statistical phenomenon. The frequency of one molecule colliding with another molecule decreases with decreasing concentration. Thus, a molecule heading in the direction of low concentration is less likely to hit another molecule blocking its path than a molecule moving in the opposite direction. Accordingly: On average, more molecules pass from the region of high collision frequency (high concentration) to a region of low collision frequency (low concentration). Statistical integration of the likelihood of collisions will yield the rate of diffusion. a) Diffusion through lipid. Many molecules can diffuse through the lipid phase of the membrane: O 2 , CO 2 , NH 3 , and other lipid-soluble molecules. Since these solutes dissolve in lipids, they are able to diffuse through the lipid matrix of the membrane. They do not require special transport routes. Even water can diffuse through lipid, albeit at low rates. b) Diffusion through special transport routes. Solutes and solvents that cannot pass through lipid barriers at high rates require specialized pathways. One such pathway is aquaporin, the water channel (a protein complex) that spans the cell membrane. Other transport mechanisms for mediating fast transport across cell membranes are carriers, pumps and the mechanisms of pinocytosis and exocytosis. Pinocytosis, endocytosis and exocytosis are mechanisms of transport where water and dissolved and non-dissolved solutes are engulfed and surrounded by a membrane, forming vesicles which then are transported through the cell in container-like transport. Container contents may be digested in the Golgi apparatus (in case of foreign solutes) or excreted by cells, as in epithelial cells excreting toxins. “Container-transport” provides not only a transport pathway for lipid-insoluble molecules, but more importantly, for solutes (macromolecules) too large for permeation through channel proteins embedded in the membrane. 2) Ionic mobility. The absolute mobility of an ion in a solution is defined as the velocity of movement (cm/s) in an electric field of 1V/cm (Table 1). What surprises is that the ionic mobility of halide metal ions in water increases with increasing crystal radius. The opposite is expected: the larger the ion, the greater the frictional interaction with water, and the slower to rate of migration. The solution to this paradox is found in the fact that ions electrically attract water. The smaller the ion, the more water it attracts and the greater its “hydrated
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This note was uploaded on 04/09/2011 for the course BIOAP 4580 taught by Professor Beyenbach,k. during the Spring '11 term at Cornell University (Engineering School).

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L10 Channels new 2011 - COPYRIGHT Mammalian Physiology...

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