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BIO310H5 (13)
Chapter 4

Ch. 4: Movement of Moleucles across cell membranes

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University of Toronto Mississauga
Angela B Lange

Ch 4 Movement of Molecules across cell membranes Diffusionthe molecules of any substance be it solid liquid or gas are in a continuous state of movement or vibration and the warmer a substance is the faster its molecules moves the avg speed of this thermal motion depends upon the mass of the molecule water faster than glucose because it lighter in solutions such rapidly moving molecules undergoing millions of collisions every secondeach collision alters the direction of the molecules movement so that the path of any one molecules becomes unpredictablesince a molecule may at any instant be moving in any direction such movement is random with no preferred direction of movement the random thermal motion of molecules in a liquid or gas will eventually distribute them uniformly throughout a containerthus if we start with a solution in which a solute is more concentrated in one region than another random thermal motion will redistributethe solute from regions of higher concentration to regions of lower concentration until the solute reaches a uniform concentration throughout the solution this movement of molecules from one location to another solely as a result of their random thermal motion is known as diffusion many processes in living organisms are closely associated with diffusion For example oxygen nutrients and other molecules enter and leave the smallest blood vessels capillaries by diffusion and the movement of many substances across plasma membranes and organelle membranes occurs by diffusionMagnitude and direction of diffusioninitially glucose is present in compartment 1 at a concentration of 20mmolL and there is no glucose in compartment 2the random movements of glucose molecules in compartment 1 carry some of them into compartment 2 the amount of material crossing a surface in a unit of time is known as a flux this one way flux of glucose from compartment 1 to compartment 2 depends on the concentrations of glucose in compartment 1if the number of molecules in a unit of volume is doubled the flux of molecules across each surface of the unit will also be doubled since twice as many molecules will be moving in any direction at a given time net flux the net flux between the compartments at any instant is the difference between the two oneway fluxes It is the net flux that determines the net gain of molecules in compartment 2 and the net loss from compartment 1 eventually the concentrations of glucose in the two compartments become equalthen the net flux becomes 0system has now reached diffusion equilibriumthree fluxes can be identified at any surfacethe two one way fluxes occurring in opposite directions from one compartment to the other and the net flux the difference between the previous two net flux proceeds from regions of higher concentration to regions of lower concentration the greater the difference in concentration between any two regions the greater the magnitude of the net fluxother effects on magnitude of net flux
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