lecture 2 notes - 1 Human Physiology NROSCI/BIOSCI 1250...

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1 Human Physiology NROSCI/BIOSCI 1250 Lecture 2, 2008 Membranes and the Movement of Molecules Throughout this course we will emphasize the importance of maintaining the constancy of the internal fluids of the body. In a sense, we can consider all of the processes that contribute to this at the level of movement of molecules through and between different compartments in the body. In order to appreciate this, we need to discuss the basic ways in which molecules move. Diffusion Molecules in solution are in constant movement. If a high concentration of molecules of substance X were placed in the center of a beaker of water, they would tend to move away from the area of high concentration to the area of low concentration. (Slide 4) This is called diffusion . Diffusion is the net movement of molecules of a specific substance from an area of higher concentration to an area of lower concentration. Given enough time, the molecules will equally distribute in the space. Consider the movement of molecules from one side of a chamber to the other. At equilibrium, there is no net movement of molecules from one side to the other, but molecules are still moving. At equilibrium the flux one way is equal to the flux the other way; influx (f i ) = area x conc o whereas efflux is the exact opposite and net flux = f i - f o or F=A(Co-Ci) [don’t worry about this formula, but it is important to understand the meaning of net flux] Diffusion is this net flux or movement of molecules. Diffusion has the following properties: (Slide 5) Net movement of molecules down a concentration gradient No energy (other than the concentration gradient itself) is required; diffusion is passive What factors influence diffusion? (Slide 6) temperature mass of the molecule - the smaller the molecule the faster it moves (Stokes-Einstein relation). (Note: water moves 1500mph whereas glucose moves at 500 mph) surface area the medium - more rapid in gas than liquid (due to density of molecules and therefore frequency of collisions; might be better to think about Jell-O versus water.) 1
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2 Fick’s first law (slide 7): the rate of diffusion is proportional to surface area and the difference in concentration Speed is also a critical issue (slide 8), especially in physiology. Diffusion times increase in proportion to the square of the distance. For example, glucose, a rather small molecule, takes 3.5 sec to approach 90% of diffusion equilibrium 10 um away, whereas it would take years to go a few centimeters. (Consider the importance of this for how close cells need to be to the circulatory system.) Diffusion through membranes Let us separate two compartments with a membrane, and look at diffusion in and between these compartments. (Slide 9) Let’s start with the membrane being a simple lipid bilayer. One possibility is that our molecule of interest cannot permeate the membrane - the membrane, being lipid will be impermeant to charged or polar molecules (i.e., ones that cannot dissolve in lipid); in this case, the membrane would serve as a barrier to diffusion. Many organic molecules that the cell needs - e.g.,
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