B i o p h y s i c s D e mys tifie D in .NET framework

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B i o p h y s i c s D e mys tifie D
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Figure 11-10 A channel is a type of transport protein that folds in a way so as to form a hydrophilic channel or tunnel through the phospholipid bilayer
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Gated Ion Channels
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Earlier in this chapter we pointed out that lipid bilayers are very much not permeable to ions In almost all cases, transport of ions across the membrane requires the existence of an ion channel, which is a transport protein as we described above with a hollow channel lined with hydrophilic residues (One exception is vesicle transport discussed later in this chapter) In some cases the native state of the membrane transport protein involves two or more native conformations In at least one of these conformations the transport channel is blocked, preventing the movement of ions into the channel through either steric or hydrophobic forces Such a channel is said to be gated because the conformation of the protein acts as a gate that can open and close Depending on the particular transport protein and its specific purpose, various factors can act to trigger the conformational change to open or close the gate For example, binding of a protein or other molecule to a specific receptor on the surface of the membrane may trigger the gate to open or close Or the presence of a certain voltage difference across the membrane may trigger the conformational change that opens or closes the ion channel This latter situation is referred to as a voltage gated ion channel Voltage gated ion channels are common in nerve cells (neurons) and other excitable tissue (muscle cells) Typically there is a very specific voltage at which the gated ion channels of a cell open in a highly cooperative manner, so that once one channel is open, the other channels along the length of the neuron open rapidly in succession, creating an impulse that travels down the length of the neuron
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chapter 11 M e M B r a n e B i o p h y s i c s
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Active transport always involves the use of energy to drive the transport of a molecule or ion across the membrane There are two reasons why energy may be needed to drive the transport of a molecule or ion across the membrane The first reason is that there may be an otherwise unfavorable Gibbs energy change just to get the molecule into the hydrophobic interior of the bilayer This is the case with ions and other charged or large and highly polar molecules The overall Gibbs energy change to get the ion from one side of the membrane to the other may be small or even negative (favorable), but there is an energy hump, or barrier, to get over in order to get the ion into the bilayer This energy barrier is illustrated in Fig 11-11 In most cases, if the overall Gibbs energy change is
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Distance into bilayer
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Figure 11-11 Gibbs energy change for passing a positive ion into a lipid bilayer, as a function of distance into the bilayer The Gibbs energy change is initially negative as the ion approaches the negatively charged phosphate head groups if the concentration of the ion is smaller on the other side of the bilayer, then the overall Gibbs energy change will be negative (favorable) But as the positive ion passes the lipid head groups and into the hydrophobic interior of the bilayer the Gibbs energy rises sharply
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B i o p h y s i c s D emys tifie D
negative, then the cell can solve the energy barrier problem using a passive ion channel, as described previously However, in some cases, especially those which involve relatively large molecules, the cell may make use of active transport to move the molecule into the bilayer and across The second and more common reason active transport is needed is because the cell requires a higher concentration of some substance on one side of the membrane Purely passive transport would tend to equalize the concentrations, until the concentration is the same on both sides of the membrane Gibbs energy changes are typically negative for moving a molecule from a region of high concentration to a region of low concentration (primarily for entropic reasons) So the Gibbs energy change favors equalizing the concentrations This is why moving a molecule from a region of low concentration into a region of high concentration requires the input of energy to go against the concentration gradient An example of this is the uptake of glucose from digested food The glucose has to move from the hollow of the small intestine (where its concentration is low) into the cells that line the walls of the small intestine The cells lining the wall of the intestine need to concentrate the glucose on their insides and then pass the glucose into the bloodstream Without active transport, as soon as the concentration of glucose in the cells reached the level in the hollow of the intestine, no more glucose would flow into the cells A significant amount of glucose in the digested food would be lost There are two types of active transport Both are mediated by transport proteins that are part of the cell membrane Primary active transport takes energy from cleaving a high energy phosphate bond and uses that energy directly to transport a molecule or ion across the membrane Secondary active transport utilizes energy from concentration gradient to drive the active transport; that is, the favorable Gibbs energy of some molecule moving from high concentration to low concentration (a passive transport) is coupled with actively pushing some other molecule across the membrane The term secondary active transport comes from the fact that the cell expended energy, via primary active transport, to create the concentration gradient in the first place Active transport that involves transporting more than one type of molecule or ion is called cotransport Transport proteins that mediate the simultaneous transport (cotransport) of more than one type of molecule or ion are called cotransporters Cotransporters that move the different molecules in the same direction across the membrane are called symporters Cotransport proteins that move different molecules in opposite directions across the membrane are called antiporters Most examples of cotransport that have been studied involve only
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