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Unformatted text preview: Cell Membrane Structure • Membrane phospholipids are AMPHIPATHIC – polar (hydrophilic) and nonpolar (hydrophobic) regions • Polar regions of amphipathic molecules form hydrogen bonds with water. • When mixed with water, phospholipids spontaneously assemble to form Micelles or Bilayers • Cholesterol = steroid lipid; important in membrane structural and mechanical stability • Peripheral Proteins – Attach to either inner or outer face of membrane • Integral Proteins – Protrude partly or all the way across membrane – Include channel proteins and carrier proteins Cell Membrane Function • Cell membrane has property of selective permeability • Cell obtains nutrients and removes wastes via cell membrane transport mechanisms * Simple and Carrier-Mediated (Facilitated) Diffusion * Active mechanisms require ATP Active transport (primary and secondary); endocytosis, exocytosis Physics of Simple Diffusion (Passive) • First, look at diffusion in general (then look at cells) • Diffusion = movement of molecules as a result of random thermal motion Simple Diffusion between Compartments (Passive) • Terms to understand: – FLUX - movement of solute in a direction – NET flux - the net of flux (diffusion) – Equilibrium - when net flux = 0 • Net flux is from HIGH to LOW concentration What factors affect the magnitude of net flux? • Magnitude of concentration gradient • Temperature- increase temperature = increase flux • Mass of molecule • Surface area for diffusion 1- increase surface area = increase flux • Medium through which molecules are moving Driving force? The driving force behind diffusion is the difference in concentration gradient Diffusion Rate vs. Distance • Diffusion times increase in proportion to the distance squared * 10 micrometers vs. 10 centimeters * Few seconds vs. 11 years – Thickness of the cell membrane? < .01 micron Diffusion is only favored over a short distance Diffusion across a boundary • When considering diffusion between compartments, must also take into account the permeability of the boundary in addition to the other factors affecting rate + magnitude of diffusion • The cell membrane...
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- Spring '12