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The Bioelectric Cell

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The Bioelectric Cell Biophysics: An Introduction CHAPTER 10 THE BIOELECTRIC CELL A. The Nature of Bioelectricity Bioelectricity is fundamental to all of life’s processes. Indeed, placing electrodes on the human body, or on any living thing, and connecting them to a sensitive voltmeter will show an assortment of both steady and time-varying electric potentials depending on where the electrodes are placed. These biopotentials result from complex biochemical processes, and their study is known as electrophysiology. We can derive much information about the function, health, and well-being of living things by the study of these potentials. To do this effectively, we need to understand how bioelectricity is generated, propagated, and optimally measured. Bioelectricity is a cellular phenomenon. Every living cell has a membrane potential (of about-70mV), with the inside of the cell being negative relative to its external surface. The cell membrane potential is strongly linked to the cell membrane transport mechanisms in that much of the material that passes across the membrane is ionic (charged particles), thus if the movement of charged particles changes, then it will influence the membrane potential. Conversely, if the membrane potential changes, it will influence the movement of ions. Although the fluid inside and outside cells is essentially neutral, there is a difference in ion concentration that produces electricity potesial at cell boundaries. The cells which generate an electric potential creates a thin layer of negative charge on the surface of the boundary and a thin layer of positive charge on the outer surface of the membrane is the boundary. (see Figure 10.1). Figure 10.1. All living things have a net electroneutrality, meaning that they contain equal numbers of positive and negatively charged ions within their biological fluids and structure. The most fundamental bioelectric processes of life occur at the level of membranes. In living things, there are many processes that create segregation of charge and so produce electric fields within cells and tissues. Bioelectric events start when cells expend metabolic energy to actively transport sodium outside the cell and potassium inside the cell. The movement of sodium, potassium, chloride, and, to a lesser xtent, calcium and 203 Biophysics: An Introduction magnesium ions occurs through the functionality of molecular pumps and selective channels within the cell membrane. Figure 10.2. The concentrations of ions outside and inside the cell. Positive ion concentrations digrafikkan above the line and negative ion concentrations digrafikkan below the line to help describe that fluids are electrically neutral. The membrane is permeable to ions K+ and Cl- ions, which diffuses in the opposite direction as shown. Short arrows indicate that the Coulomb force continuously inhibit the diffusion of K+ ions and Cl- ions. If the membrane becomes permeable to Na+ ions, the diffusion gradient and Coulomb forces work to drive Na+ into the cell. There are many ions in the cells inside and outside fluid. Ions are important in creating the cell potentials are ions Na+, K+, and Cl-. There is a big difference in the concentration of these ions inside and outside the cells, as shown in Figure 10.2 (a). Negative ions other than Cl- indicated by A-. Note that the total charge inside and outside is zero, thereby, the fluids that are electrically neutral. To see how the cell potential is formed, consider what happens in the example of the cell membrane initially neutral and have different concentrations as shown in Figure 10.2 The cell membrane is normally impermeable to K+ and Cl- ions; membrane was approximately 100 times less permeable to Na+ and highly impermeable to other ions. Only K+ and Cl- which diffuses through the cell membrane in goodly numbers. Net diffusion direction is from the area of high concentration to areas of low concentration. 204 Biophysics: An Introduction Therefore, the ions K+ and Cl- ions diffuse in the opposite direction, as shown in Figure 10.2 (b). Ions K+ and Cl- ions have opposite charge, then there is a very strong Coulomb attractive force between the ions that cause the ions to form two thin layers of charge right at the side of the cell membrane, as shown in Figure 10.2. Diffusion of K+ and Cl- continues until the attractive force and repulsive Coulomb stop. During the charge layers are formed on the cell membrane, increasing the Coulomb forces. Tensile similar charges not work to attract ions of K+ and Cl- back into its regions with high concentration. Furthermore, the repulsive force similar charges work to maintain the ions in order not to leave the area with a high concentration. An equilibrium is quickly reached between the diffusion of high concentration to low concentration and Coulomb forces opposite. As soon as this equilibrium is reached, the cells are in a resting state (resting state). If more of the ions K+ and Cl- diffuse through the cell membrane, increasing Coulomb forces and some ions move back. Net displacement is zero so that the equilibrium is stable. Coulomb force is very strong, and only about 1 out of 100,000 ions K+ and Cl- move through the membrane. This is such a small part that does not change the overall concentration of ions. After all the fluid inside and outside the cell remains essentially neutral although some have a separate charge. However, the small charge separation is very important, because it is a source separation Bioelectricity . Potential in the cells of 70-90 mV lower than the potential outside the cell, approximately 90 mV in the nerve cells and muscle. Potential outside the cell is usually taken 0 V, resulting in the nerve cells and muscle has a resting potential of approximately-90 mV. Equilibrium between the concentration gradient and the Coulomb force is also an energy equilibrium. Proper equilibrium electric potential energy with the potential energy due to the concentration difference is, the ability to conduct business concentration gradient by moving ions against electric potential. Energy balance equation that describes this is the Nernst equation. Nernst equation gives the voltage that will be created by the difference in concentration, but this equation only for a membrane that is permeable to a single type of ion perfect and perfect is not permeable to all other ions. In this environment the Nernst equation is kT V Vin V out 2,30 logC in logC out .... (10.1) Ze where V is the potential difference (inside minus outside), Cin and Cout are the concentrations of ions to which the membrane is permeable, k is Boltzmann's constant, T is the absolute temperature, and Ze is the charge on the ion multiplied by the electron charge (Z is valence ions). The minus sign indicates that the excess positive ions can diffuse in the fluid inside the cell produces a negative voltage in the cell. The most important aspect is that the Nernst equation potential difference is proportional to the concentration difference. Using concentrations are given in Figure 10.2 (a), we can calculate how much potential that will be created by each type of membrane permeable to ions if the ion itself. For Na+ the result is a potential in the+109 mV. Since the actual potential in the nerve approximately-90 mV, indicating that the membrane is not very permeable to Na+. For K+ and Cl- respectively the result is-88 mV and-70 mV. These results approach the actual 205 Biophysics: An Introduction values in many types, but it need not imply that the membrane is permeable to K+ and Cl-. This only implies that if the membrane is permeable to K+ and Cl-, there is a slight movement under resting conditions. The structure and characteristics of the membrane are topics of research interest. Many things are not yet understood, but it can be stated that the resting potential is definitely a big influence membrane structure and characteristics. Although only 90 mV resting potential, it is at ptensial membranes which have an average thickness of 8 nm. So the voltage per meter is very large, on the order of 11 MV per meter. Voltage per meter is so big it can straighten molecules and have an influence on the pore and membrane permeability. For example, if the potential at the membrane, the membrane permeability change drastically, suddenly becomes about 1000 times more permeable to Na+ than the existing ones. No one knows exactly why the permeability change, but one reason is the potential effect on the membrane structure. Nature has to find a way to use such permeability changes to continue Bioelectricity signals. Example 10.1: Calculate the potential of the membrane to K+ ions at a temperature of 310 K, if the concentration of K+ ions inside the cell is 140 mol/m3 and outside the cell is 10 mol/m3. Boltzmaan constant k = 1.38 10-23 J / K. K+ ion charge is e = 1.60 10-19 C. Completion: We use equation (10.1) with z = 1, in order to obtain kT V V V 2,30 (logC logC ) in outZe in out 23 (1,38 10 J/K)(310K) 33 V 2,30 (log 140 mol/m log 5 mol/m ) (1)(1,60 1019 C) 4 V ( 614,9625 10 )(1,447158032) volt 3 V 88,9947921 10 volt V 89 mV. B. Neural System The use of electrical phenomenon in most living organisms found in animal nervous system. Nerve cells are also called neurons form a complex network in the body that receive, process and forward information from one part of the body to other body parts. Center is located in the brain tissue, which has the ability to store and analyze information.
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