Transport copy - MOVEMENT OF MOLECULES ACROSS CELL MEMBRANES(MEMBRANE TRANSPORT The Problem The plasma membrane is a barrier to insulate and

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Unformatted text preview: MOVEMENT OF MOLECULES ACROSS CELL MEMBRANES (MEMBRANE TRANSPORT) The Problem The plasma membrane is a barrier to insulate and compartmentalize the interior of the cell from the outside environment. However, some molecules must pass over the membrane and are vital to the cells survival. How are they able to do this? Lipophilic (hydrophobic) molecules can dissolve into the lipid membrane and pass through . Lipophobic cannot dissolve into the membrane and cannot pass through . Table showing plasma membrane permeabilities. The Solution Lipophilic and small polar molecules can pass through the membrane by simple diffusion . Ions and large polar molecules need mediated transport to carry them across the membrane. This mediated transport is via integral proteins called carriers and channels . Passive (Simple) Diffusion Characteristics Energy Requirements- Work is required to move mass from one place to another which requires an energy source . What is the energy source? A concentration gradient (A form of stored energy). A concentration gradient is the difference in the concentration of a species in one place vs that in another place. A molecular species will move from a high concentration (more ordered state) to a lower concentration (less ordered state) via random molecular movement. i.e., the molecules will randomize to the lowest energy state which will be the lowest overall concentration (least ordered). Random molecular motion distributes the molecules homogeneously throughout the solute. The molecular motion is random but the overall motion of the molecules has direction. Law of Passive Diffusion A species moves down a concentration gradient from a region of high concentration to a region of lower concentration. A concentration gradient is a form of stored energy and is able to perform work such as the moving of molecules. Passive Diffusion Across Membranes If there are two compartments divided by a membrane and a species is in higher concentration in one compartment than the other AND IF THE SPECIES CAN PASS OVER THE MEMBRANE then there will be a movement of molecules from one compartment to the other . We could say that the concentration gradient is like a driving force pushing the molecules from one compartment to the other. The graph above shows one way fuxes in both directions and the resultant NET FLUX....
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This note was uploaded on 04/30/2011 for the course ANATOMY 3504 taught by Professor Steffens during the Spring '11 term at St. John's.

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Transport copy - MOVEMENT OF MOLECULES ACROSS CELL MEMBRANES(MEMBRANE TRANSPORT The Problem The plasma membrane is a barrier to insulate and

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