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Lecture 4 - Lecture 4 Gases and small-uncharged polar...

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Lecture 4 Gases and small-uncharged polar molecules (ex. Ethanol) can cross membrane. That makes sense because the bulk of the interior of the membrane is hydrophobic and once the ethanol gets beyond the polar part, it can cross very easily. Water crosses the membrane in two ways. Water can actually diffuses across the cell membrane because water is slightly polar and there are channels to help it cross the membrane. When we get to large uncharged polar molecules like sugars, they essentially cannot cross; they require carriers or pumps to cross. Membrane is impermeable to ions, require ion channels and ion pumps. Charged polar molecules (ex. AA, ATP, etc) cant cross the membrane, cross with by using pumps or carriers. Know the transporting chart Movement across membrane is saturable except for simple diffusion. Protein-mediated transport (facilitated diffusion) saturates at a certain concentration since there are a finite number of protein facilitators. Extracellular concentration when it comes to Ca++ means two places: one is outside the cell and two is inside in the ER and to a lesser extend the mitochondria. If you look at the concentration gradient of calcium, what do you see? If it could enter, it would enter explosively. GlUT1 is in erythrocytes and GLUT2 is in liver cells. Glucose by itself moves in very slowly by passive diffusion, but moves in very fast with if there are transporters. These glucose transporters are not simple machines. They are large supramolecular structures, with multiple subunits. Question: Are liposomes good models for the uptake of glucose or other molecules into cells? No, because the lipid environment of a liposome is not the same as that of the cell membrane. This answer is ambiguous because if you could make a liposome with the same lipids as the membrane, then it might be a good model. Channels are essentially gated. They can be gated by electrical currents, ligands (inside or outside), or mechanically gated (usually a sheer force on the membrane opens a channel). The channel that allows you to hear is mechanically-gated by sound waves (sound waves causes vibration in the membrane, which causes a sheer force and opens the channel). There are an enormous number of cyclic nucleotide gated channels. What they all do is they will bind a particular cyclic nucleotide and that will activate them and that activation results in some ionic flow which itself turns on a number of reactions. Now when the channel binds a cyclic nucleotide what you get is a huge amplification of the signal. With cyclic nucleotides you get an enormous amplification of the reaction caused by opening up of a whole bunch of ion channels.
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Proton pumps maintain pH of the various compartments in a cell. There are many different ion carriers scatter throughout the cellular membranes.
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