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Cell Membrane Lecture Part 2_1

Cell Membrane Lecture Part 2_1 - Membrane Structure and...

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Membrane Structure and Function Chapter 7
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What establishes direction of traffic across the membrane? What determines whether a substance will enter or exit? What mechanisms drive molecules across the membrane?
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Passive Transport Passive transport is the movement of molecules across a membrane without energy investment. Diffusion is the movement of molecules of any substance across a membrane into available space Individual particles move randomly Net movement into available space Movement of molecules continues until concentrations on both sides of the membrane, or dynamic equilibrium is reached.
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Diffusion Substances diffuse from areas where it is more concentrated to areas where it is less concentrated, or the concentration gradient. Diffusion is an example of passive transport , No energy input required. Concentration gradient drives diffusion Each substance diffuses down its own concentration gradient.
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Figure 7.13a Molecules of dye Membrane (cross section) WATER (a) Diffusion of one solute Net diffusion Net diffusion Equilibrium
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Figure 7.13b (b) Diffusion of two solutes Net diffusion Net diffusion Net diffusion Net diffusion Equilibrium Equilibrium
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Rate of Diffusion Because membranes are selectively permeable, interactions of molecules with membrane affects how rapidly the molecule can diffuse through Size Temperature Polarity Ions
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Osmosis Osmosis is the diffusion of water across a selectively permeable membrane. Passive transport (down concentration gradient) Water moves from areas of lower solute concentration to areas with higher solute concentration. Think in terms of free water concentration Water molecules cluster around hydrophilic solutes These are unable to move, effectively taking water out of the solution.
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Figure 7.14 Lower concentration of solute (sugar) Higher concentration of solute Sugar molecule H 2 O Same concentration of solute Selectively permeable membrane Osmosis
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Importance of Water Balance Solute concentration and permeability affects tonicity , the ability of a surrounding solution to cause a cell to gain or lose water.
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