Channel Proteins.docx

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Biomedical Engineering
BMEN 515
William Huddleston

Channel Proteins  Ion channels are made from just these sorts of membrane-spanning protein molecules.  Typically, a functional channel across the membrane requires that 4-6 similar protein molecules assemble to form a pore between them  One important property of most ion channels, specified by the diameter of the pore and the nature of the R groups lining it, is ion selectivity.  Another important property is gating. o Channels can be opened and closed by changes in the local microenvironment of the membrane o Ion Pumps  Other membrane-spanning proteins come together to form ion pumps  Ion pumps are enzymes that use energy released by the breakdown of ATP to transport certain ions across the membrane The Movement of Ions  Ionic movements through channels are influenced by two factors: diffusion and electricity  Diffusion o Ions and molecules dissolved in water are in constant motion. o This temperature-dependent, random movement will tend to distribute the ions evenly throughout the solution o There will be a net movement of ions from regions of high concentration to regions of low concentration  diffusion o Diffusion will cause ions to be pushed through channels in the membrane  For example, NaCl is dissolved in fluid on one side of a permeable membrane  The Na and Cl ions will cross until they are evenly distributed in the solutions on both sides  The difference in concentration on both sides is called a concentration gradient  Thus, it is said that ions flow down a concentration gradient o Driving ions across the membrane by diffusion happens when: 1) The membrane possess channels permeable to the ions, and 2) There is a concentration gradient across the membrane  Electricity o Another way to induce a net movement of ions in a solution is to use an electrical field o Since opposite charges attract and like charges repel, there will be a net movement of Na toward the negative terminal and of Cl toward the positive terminal. o The movement of electrical charge is called electrical current (I) and is measured in amperes o Two important factors determine how much current will flow: electrical potential and electrical conductance o Electrical potential (voltage – V) is the force exerted on a charged particle and it reflects the difference in charge between the anode and the cathode  More current will flow as this difference is increased o Electrical conductance (G) is the relative ability of an electrical charge to migrate from one point to another  Depends on the number of particles available to carry electrical charge and the ease with which these particles can travel through space  Electrical resistance (Ω) is simply the inverse of conductance and it is the relative inability of an electrical charge to migrate  There is a simple relationship known as Ohm’s Law and it is I = gV.  If the conductance is zero, no current will flow even when the potential difference is very large The Ionic Basis of the Resting Membrane Potential  The membrane potential (V ) is the voltage across the neuronal membrane at any m moment.  Vmcan be measured by inserting a microelectrode into the cytosol o A typical microelectrode is a thing glass tube with an extremely fine tip that will penetrate the membrane of a neuron with minimal damage o This method reveals that electrical charge in unevenly distributed across the neuronal membrane  The inside of the neuron is electrically negative with respect to the outside  Equilibrium Potentials o The electrical potential differences that exactly balances an ionic concentration gradient is called an ionic equilibrium potential and it is representedion E . o There are form points that are raised in the topic of equilibrium potentials 1) Large changes in membrane potentials are caused by miniscule changes in ionic concentrations 2) The net difference in electrical charge occurs at the inside and outside surfaces of the membrane 3) Ions are driven across the membrane at a rate proportional to the difference between the membrane potential and the equilibrium potential 4) If the concentration difference across the membrane is known for an ion, an equilibrium potential can be calculated for that ion
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