Today Electrical properties of membranes Resistance Capacitance Membrane potentials Ionic current going across the membrane. Volt meter and ameter measures current which is flowing in wires, and the current there is carried by electrons. So we need to have a transition between current carried by ions and current carried by electrons. Most common way to do this is to use some silver wire or a silver plate (very pure) and you deposit chloride ions onto the surface, so you get a silver chloride electrode. And the virtue of this is that we can have two electrode reactions (anode or the cathode) which allow for the exchange of chloride ions going onto or off the electrode and electrons being liberated or deposited in the same way. The anode is positive (suck electrons away: Ag goes to Ag+ + e-, then Ag+ attracts a Cl-, which forms an insoluble salt) This is how you get the transformation between an ionic current and an electronic current. At the cathode, the opposite happens. The cathode is a source of electrons. e- + Ag+ --> Ag and Cl- leaves. So we put a voltage source (a battery) on the system and we measure the current that flows under the application of a particular voltage, and then we can measure the resistance of that particular piece of membrane. Units! 1 amp = flow of 1 coulomb/sec Symbol for current is I, and charge is Q I = dQ/dt Faraday = the number of coulombs/mol The charge on a single ion carries 1.602x10-19 coulombs of charge, so therefore 1 mol corresponds to 96,500 coulombs/mol (1.6x10^-19x6x10^23) That number is very close to 10^5 C/mole *********KNOW THIS NUMBER************* Resistivity (opposite of conductivity) See notebook notes So when you do the experiment described above, you get an Rm of 10^8 ohm*cm^2 But the real Rm = 10^3-10^4 ohms*cm^2 (this change is the result of proteins in the membrane) Different story with capacitance, though: Measured membrane capacitance = 1microFarady/cm^2 Real membrane capacitance = 1 microFaraday/cm^2 So the point is: the capacitance of the membrane is due only to the phospholipids, not the proteins. Most of today's notes are in the notebook. Consult it. --------------------------------------------- Membrane Proteins There are special proteins which get inserted into membranes. (remember pg 477 of book from Steinbock) There are a special subset of proteins that have properties such that they can be inserted into membranes, but not all proteins can do this. Soluble proteins (such as hemoglobin) are not going to be associated with a membrane unless they are designed by natural selection to be associated with a membrane. Inside the membrane there is this lipid bilayer and there are some proteins which will swim around in the phospholipid bilayer because they are just diffuse. As we have already discussed, the association between the phospholipids is non-covalent - i.e. they move around. They can move around, but they don't flip. A lot of the proteins can just wander around in the fluid sea of phospholipids. Some proteins are fixed like some of the protein molecules located at synapses, because they have to be in a particular location. They are not fixed by associations with the membrane, but rather are fixed to the cytoskeleton. Different regions of the proteins are designed by natural selection to have hydrophobic and hydrophilic properties. The word for this is amphipathic, meaning there are regions which are hydrophobic and regions which are hydrophilic (lipids are also like this). Obviously, these line up in the same way that membrane lipids do. So how do they do this? A polypeptide chain consists of groups of amino acids. So you have a chain of amino acids, joined by peptide bonds, and it is the nature of the side chains that determines hydrophilic vs hydrophobic. See table of amino acids in the book. Hydrophobic side chains contain mostly saturated carbons, and can also include sulfur. Hydrophilic side chains are those which are charged
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