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Unformatted text preview: BIO311C (2008) Introductory Biology I Lecture Review 05 (02/25-02/27) Membrane potential, cell communication 1. Membrane transport (1) Permeability of naked bilayer ¡ Lipid-only membranes have natural permeability ¡ Proteins will be involved in modifying this permeability ¡ A great deal of cellular energy is involved in moving around small ions and molecules ¡ Normal condition: [Na + ] in < [Na + ] out [K + ] in > [K + ] out [Ca 2+ ] in < [Ca 2+ ] out (2) Purposes of ion transport ¡ Maintain water balance ( Osmolarity of cell) ¡ Cause rapid responses: Even a small increase in ion number can be a huge concentration increase ¡ Keep productive work: e.g. Synthesis of ATP 2. Gradient across the membrane (1) Chemical gradient: A concentration gradient of ions forms across the membrane (2) Electrical gradient: Electric charge difference across the membrane Æ Electro chemical gradients 3. Membrane potential (1) Membrane potential is the voltage difference across the membrane and caused by differences in anion and cation numbers. (2) Every cell has a membrane potential as a result of the electrochemical gradient maintained by the cell (3) Negative membrane potentials indicated more negative charge in cytosol (4) Positive membrane potentials indicate more negative charge outside cell (5) Continued ion pumping can cause membrane potential: Movement of ions establishes an electric potential difference (6) Changing permeability changes membrane potential 1 Æ Opening channels causes ions to move down their electrochemical gradient, changes potential 4. Coupled carriers (1) Active transport (2) Movement of one solute down its concentration gradient provides energy for unfavorable movement of another solute (3) Uptake of Na + can help drive the import of many other molecules (such as glucose)...
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