However - However movement of a population of molecules may...

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However, movement of a population of molecules may be directional. For example, if we start with a permeable membrane separating a solution with dye molecules from pure water, dye molecules will cross the barrier randomly. The dye will cross the membrane until both solutions have equal concentrations of the dye. At this dynamic equilibrium as many molecules pass one way as cross in the other direction. In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient . This spontaneous process decreases free energy and increases entropy by creating a randomized mixture. Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances. The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen. The concentration gradient represents potential energy and drives diffusion. However, because membranes are selectively permeable, the interactions of the molecules with the membrane play a role in the diffusion rate. Diffusion of molecules with limited permeability through the lipid bilayer may be assisted by transport proteins. 3. Osmosis is the passive transport of water Differences in the relative concentration of dissolved materials in two solutions can lead to the movement of ions from one to the other. The solution with the higher concentration of solutes is hypertonic . The solution with the lower concentration of solutes is hypotonic . These are comparative terms. Tap water is hypertonic compared to distilled water but hypotonic when compared to seawater. Solutions with equal solute concentrations are isotonic .
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Imagine that two sugar solutions differing in concentration are separated by a membrane that will allow water through, but not sugar. The hypertonic solution has a lower water concentration than the hypotonic solution. More of the water molecules in the hypertonic solution are bound up in hydration shells around the sugar molecules, leaving fewer unbound water molecules. Unbound water molecules will move from the hypotonic solution where they are abundant to the hypertonic solution where they are rarer. This diffusion of water across a selectively permeable membrane is a special case of passive transport called osmosis . Osmosis continues until the solutions are isotonic. The direction of osmosis is determined only by a difference in total solute concentration. The kinds of solutes in the solutions do not matter. This makes sense because the total solute concentration is an indicator of the abundance of bound water molecules (and therefore of free water molecules).
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This note was uploaded on 08/08/2009 for the course BIO 1406 taught by Professor Gomez during the Spring '09 term at Collin College.

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However - However movement of a population of molecules may...

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