Chapter 12

# Chapter 12 - Membrane bilayer made by intrinsic properties...

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1 Membrane bilayer made by intrinsic properties of lipids Creates border between outside & inside water compartments That is, cell must have flow of material across this membrane barrier – HOW? To survive, a cell must exchange nutrients and waste products with environment. Membrane Transport Property of lipid dictates what may go through a synthetic lipid bilayer: Simple diffusion depends on: • Brownian motion of molecules • Lipid solubility of a molecule Hydrophobicity & charge • Size of molecule (small better) BUT membrane is a major barrier to most of our essential molecules: Solution? Use membrane proteins to 1. create Channels (holes) in membrane or 2. use Transport proteins.

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2 FIRST a brief look at diffusion: Remember: molecules are ALWAYS moving - Brownian motion. Not in textbook A B C BUT can consider the net movement (flux) of the whole population of molecules. e.g. sugar into coffee … Each individual molecule moves around randomly But as a POPULATION, sugar ends up going from high conc to low conc area a NET DIFFUSION Concentration gradient drives net movement upwards Diffusion, NET movement (= FLUX) of the whole population of molecules is described by … Net flux = J = - DA dc/dx a -’ve sign because you always go DOWN concentration gradient. D = diffusion coefficient - how well can molecule diffuse - innate property of molecule. A = area in which diffusion can occur dc/dx = the change in concentration of molecule over distance - i.e. the concentration gradient. Fick s Law Not in textbook
3 Net flux = J = - DA dc/dx BUT for membranes dx is small (~ 5 um), and “constant,” therefore we approximate flux as: Net flux membrane = J = - PA (C1-C2) P = Permeability Co-efficient of a molecule across a membrane, depends on its: Size (& shape) Charge Polar or non-polar (C1 - C2) = concentration difference across membrane Passive diffusion must go DOWN concentration gradient . Not in textbook Rate of diffusion depends on size, charge, polar/non-polar: RAPID SLOWER IMPERMEABLE small or large, these are too polar to go through & these are charged = NO WAY Some things get through membrane because they are not charged and/or small Water is actually very very slow for synthetic lipids, but with plasma membranes higher due to presence of (aqua)porin - very large channel protein. ? Channels & Carriers

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4 Channels & Carriers – the solution. High specificity - discriminates between related molecules (e.g. different sugars or amino acids) lower conductance rate (1 or a few molecules at a time) Variable specificity - permissive or discriminatory. Very fast conductance rates (100’s to 1000’s per second) Molecule usually small - IONS Molecules still PASSIVELY diffuse DOWN concentration gradient.
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## This note was uploaded on 04/02/2008 for the course BIO 2600 taught by Professor Vanburkem during the Winter '08 term at Wayne State University.

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Chapter 12 - Membrane bilayer made by intrinsic properties...

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