Lecture 2-Diffusion and Respiration 1

Molecule in solution to move a given linear distance

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molecule in solution to move a given linear distance  if diffusion is the only process  acting T=x2/2Dm Conclusions 
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Although diffusion is adequate for the movement over short distances, diffusion times  are unrealistically long for movement over small and modest distances For large multicellular organism (all vertebrates) diffusion alone is inadequate for  ensuring delivery of nutrients Rely on bulk flow (via cardiovascular system) to transport nutrients No cell is more that 10 micrometers fro ma capillary Design constraint imposed by the physics of diffusion in physiologically relevant fluids Influences design of vertebrate cardiovascular systems Diffusion Across a Membrane Fick’s First Law of Diffusion Factors to consider that affect the net rate of diffusion of an atom/ion/molecule across a  permeable membrane Concentration gradient Membrane permeability P Surface area of the membrane A Molecular weight of the substance MW Diffusion distance = membrane thickness  Δ X   Rate of diffusion Q Q =  Δ C*P*A/MW* Δ Measure of flux Diffusion rate over surface area Q/A = P* Δ C P (membrane permeability) is complex Depends on: the nature of the membrane Thickness Composition Solubility of molecule Size of molecule Fick’s Law Flux (J) = (P)(A)( Δ C)
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As the concentration gradient goes to 0 the flux goes to 0 Physiological systems hardly ever let he concentration gradient reaches 0
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