MCB Chapter 12 notes

MCB Chapter 12 notes - Ch. 12: Membrane Transport (October...

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Ch. 12: Membrane Transport (October 22 nd , 26 th , and 29 th Lecture Notes) Principles of membrane transport o membranes are very selective in what they let through, require a lot of energy at times o The ion concentrations inside a cell are very different from those outside the osmotic and ionic balances must be maintained by a cell Na + is high inside the cell, K + is high outside the cell…very important in signaling (action potentials) o Lipid Bilayers are impermeable to solutes and ions simple diffusion rates of a solute depend on their size and solubility small, nonpolar molecules diffuse easily across next easiest = small hydrophobic solutes large polar and ions (small and charged) will not diffuse at all was measured using synthetic lipid bilayers…rates of diffusion were measured permeability coefficients (P) are proportional to the likelihood of diffusion for a molecule (its tendency to partition into a lipid bilayer)
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transport proteins facilitate the regulation of the osmotic balance within a cell, since ions cannot diffuse readily across the lipid bilayer of a membrane cells must maintain osmotic balance and electrical neutrality despite large differences in solute concentrations across cell membranes ions and hydrophobic molecules have low permeabilities… need to be transported across somehow o Membrane transport proteins fall into two classes: carriers and channels very large and diverse protein family…large number of transporters lots of molecules need to be transported (sugars, amino acids, nucleotides, ions, atp, protons) differ from membrane to membrane according to membrane function universal features: alpha-helical, multispan proteins that have a hydrophobic interior and a hydrophilic exterior carrier proteins bind their solute with great specificity, facilitates transport only for those solutes that fit specifically to their binding site, conformational changes occur that allow the release and uptake of solute into/out of the cell channel proteins are far less specific and discriminate on the basis of solute size and charge o Solutes cross membranes by passive or active transport active transport carries solutes against their concentration gradient, is carried out only by special carrier proteins that can harness an energy source
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passive transport moves solutes down their concentration gradient without the expenditure of energy (aka facilitated diffusion) Carrier Proteins and their functions o change conformation after solute binds unit = a protein dimer intermediate may be an occluded state (halfway between open and closed) binding site is responsible for conformational change transport can occur in both directions, although the binding sites
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MCB Chapter 12 notes - Ch. 12: Membrane Transport (October...

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