3b B130_03b_Membrane_Transport_

Intracellular and extracellular fluid are isotonic if

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intracellular and extracellular fluid are isotonic if total solute concentration changes on either side, net movement of water will toward fluid with higher concentration of solutes movement of water into / out of cells has consequences for the cell … 8
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Tonicity (Mammalian Red Blood Cell) 9 `
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Osmotic Effects on Cells with Walls? 10 plant cell animal cell turgor pressure: pressure of cell contents against cell wall in plant and bacterial cells
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Note that it is solute that CANNOT cross the membrane that draws the water across ... 11 1. water can cross but large solute cannot; next flux of water will be into upper chamber 2. at equilibrium , concentration of large solute (and water) is equal, and water moves across in both directions no NET flux 2 water 1 large solute semi-permeable membrane initially at equilibrium
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Strategies for Maintaining Osmotic Balance extrusion come single-celled eukaryotes have contractile vacuoles that periodically pump out water become iso-osmotic with one’s environment marine organisms adjust their internal salt concentrations to match seawater terrestrial organisms circulate fluid through their bodies that is iso-osmotic with their cytoplasm turgor most plants are hyper-osmotic to their environment
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Paramecium : hyper-osmotic to pond water
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Major Functions of Membrane Proteins
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Membrane Transporter Proteins 15 carrier protein (shuttle) channel protein (tunnel) in common: passageways for particular molecules or classes of molecules most are multi-pass proteins in contrast: channels mostly detect size and charge, as long as channel open, anything with that size / charge can pass carriers require that molecule fit a particular binding site; one molecule at a time can be transported
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Ion Channels (Pores) functions: allow movement (‘conductance’) of Na+, K+, Ca2+, Cl- down concentration gradient critical in many cell activities regulation of cell volume formation and propagation of nerve impulses secretion of substances into extracellular space muscle contraction features: usually highly selective much faster than carriers (1000x)
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‘chemical’ gradient: concentration inside versus outside ‘electrical’ gradient: whether it is being attracted across membrane (by oppositely charged molecules) or repelled (by like-charges) combined: ‘electrochemical’ gradient considering only concentration differences acting on ion considering both concentration and electrical forces driving ion movement What are the forces driving diffusion of ions?
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18 voltage-gated: sensitive to Δs in charge across the membrane ligand-gated – shape depends on binding of specific molecule (ligand), which is usually NOT the solute to be shipped through the channel
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