BIOL 121 Lecture Notes - Lecture 13: Semipermeable Membrane, Extracellular Fluid, Osmotic Concentration

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31 Aug 2020
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Sometimes substances are polar and too large for simple diffusion, or channel diffusion. Sometimes substances must move against their concentration gradient. Movement of these kind require a (protein) mediated transport. Requires binding to a trans-membrane protein (transporter) Diffusion flux increases linearly with concentration gradient, but mediated transport will eventually reach a maximum and level off. Limited by the amount of binding of the transporter (saturation of transporters) Once all binding sites are saturated, they cant move the molecules any faster. Downhill movement (high to low) requiringa transporter, no energy required. Always proceeds from high to low concentration. Net movement will continue until concentrations are equal on both sides of the membranes. Flux will be affected by saturation of transporters. Uphill movement (low to high) requiringa transporter, requires energy. Moves a substance against its concentration gradient. Energy indirectly from ion gradient established by primary active transport. Used the downhill movement of an ion to provide a chemical gradient for the substance.

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