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human phys lec 2 - Membrane others

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Membrane Permeability A membrane that allows passage of a given b t i id t b bl substance is said to be permeable A membrane that does not allow passage of a given substance is said to be impermeable Cell membranes are selectively permeable : permeable to some substances but impermeable to others
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What properties influence permeability? Relative solubility of a molecule in lipid Highly lipid soluble: uncharged and non polar molecules can readily permeate Highly lipid soluble: uncharged and non polar molecules can readily permeate Not lipid soluble: charged and polar molecules are much less permeable. Size of the molecule All other things being equal, smaller molecules permeate membranes more effectively. Specialized membrane transport proteins are necessary for transport of many physiological substances Channels Carriers Pumps Even with permeability, there still needs to be a thermodynamic driving force – something needs to “push” the particle across the membrane A concentration gradient An electrical charge gradient Chemical energy from ATP hydrolysis
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Types of membrane transport Simple Diffusion – Lipophilic molecules only Membrane Transport Facilitated diffusion Active transport Secondary active transport Vesicular transport
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Types of membrane transport Simple Diffusion – Lipophilic molecules only Membrane Transport Facilitated diffusion Active transport Secondary active transport Vesicular transport
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Diffusion The passive movement of molecules from an area of higher concentration to an area lower concentration The random thermal (Brownian) movement of molecules causes them to gradually spread out until they are evenly distributed throughout the available space. At the end of the process, entropy is maximized. The idea that diffusion is simply the Brownian motion of many particles was first proposed by Albert Einstein in his PhD thesis.
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Brownian motion of a single particle – the “random walk” The particle tends to spend a lot of time “exploring” a small volume but on rare events moves long distances. This is why diffusion is very fast for short distances (nm) and slow over longer distances ( μ m). This is a consequence of completely random behavior.
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Properties of Diffusion Molecules move passively from an area of higher concentration to an area of lower concentration Concentration gradient (d C /d x ) Net movement occurs until the concentration is equal everywhere (equilibrium) Diffusion is faster over shorter d ( ) d distances (nanometers) and at higher temperatures. It is slow over distances of tens of micrometers.
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Fick’s First Law of Diffusion Fick s First Law of Diffusion J = D d c / d x where J is the diffusion flux, D is a diffusion coefficient, d c /d x is the concentration gradient, c is concentration of the solute, and x is distance.
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