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Unformatted text preview: Membrane Dynamics / Neurons / CNS 04/02/2009 03:31:00 Diffusion Cell membranes are selectively permeable o Only certain molecules can pass Molecules such as water, oxygen, carbon dioxide, and lipids easily pass while ions, most polar molecules, and very large molecules such as proteins, enter cells with more difficulty the size and the lipid solubility of molecules are two properties that influence its movement across cell membranes o very small molecules and those that are lipid soluble can cross directly through the phospholipid bilayer o larger or less lipid soluble molecules are excluded from crossing the bilayer unless cell has a specific mechanism for transporting them across Two ways to categorize how molecules move across membranes: Passive Transport and Active Transport o Passive transport does not require the input of energy o Active transport requires the input of energy from some outside source, such as ATP Passive transport across membranes uses the kinetic energy inherent in molecules o gas molecules and molecules in solution constantly move from one place to another starting from concentrated areas to less concentrated areas until evenly distributed throughout the available space Diffusion Diffusion has seven properties o Diffusion is a passive process Does not require outside energy o Molecules move from higher concentration areas to lower concentration areas o Net movement of molecules occurs until the concentration is equal everywhere o Diffusion is rapid over short distances but much slower over long distances Time required for a molecule to diffuse from point A to point B is proportional to the square of the distance from A to B If distance doubles from 1 to 2 the time needed for diffusion increases from 1 2 to 2 2 o Diffusion is directly related to temperature At higher temperatures molecules move faster, therefore the rate of diffusion increases o Diffusion rate is inversely related to molecule size The larger the molecule the slower its diffusion o Diffusion can take place in an open system or across a partition that separates two systems Substances that are hydrophilic and dissolve in water are lipo phobic and they do not readily dissolve in lipids o Therefore the hydrophobic lipid core of the cell membrane acts as a barrier that prevents hydrophilic molecules from crossing Substances that can pass through the lipid center of a membrane move by diffusion o Diffusion directly across the phospholipid bilayer of a membrane is called simple diffusion Simple diffusion has the following properties in addition to the properties of...
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This note was uploaded on 02/25/2012 for the course CJ 2350 taught by Professor Todd during the Fall '08 term at Texas State.
- Fall '08