2. The resting concentration of calcium ions, Ca* (valence Z=2 for calcium), is about Cn mole/liter in the extracellular space but is very low (Cout =10-5 mole/liter) inside muscle cells. Determine the membrane potential AV for calcium. Suppose diffusion temperature T = 27 °C. 10-3 BIOELECTRICITY. MEMBRANE POTENTIALS 1. Electrical charge. There are only two types of charge, which we call positive (proton) and negative (electron). Charge of electron and proton is e =1.6x10-" C. Mass of electron 9.1x10" kg and mass of proton 1.67x10-" kg. Then, electrical charge is q = Ne, here N amount of charged particles. 2. The electric field E is defined to be E = where F is the Coulomb or electrostatic force exerted on a small positive test charge q . E has units of N/Č. The magnitude of the electric field E created by a point charge q is F = k, where r is the distance from q . The electric field E is a vector and fields due to multiple charges add like vectors. 3. The potential difference between points A and B, Vn – VA , defined to be the change in potential energy of a charge q moved from A to B, is equal to the change in potential energy divided by the charge, Potential difference is commonly called voltage, represented by the symbol AV: AV = APE 4. In uniform electric field the potential difference is AV = Ed, where E is electric field and d'is the distance from A to B, or the distance between the plates. 5. A capacitor is a device used to store charge. The amount of charge q a capacitor can store depends on two major factors - the voltage applied and the capacitor's physical characteristics, such as its size. The capacitance C is the amount of charge stored per volt, or C = . units Farad (F). 6. Capacitance of a parallel plate capacitor c = "ad, where to = 8.85 · 10-12 F/m is called the permittivity of free space, e is the dielectric constant of the material, A is area of plates and d is distance between plates. 7. Capacitors are used in a variety of devices, including defibrillators, microelectronics such as calculators, and flash lamps, to supply energy. The energy stored in a capacitor can be expressed in three %3D AV ways: Energy = gåv CAV of the capacitor(F). 8. Electric current l is the rate at which charge flows, given by / = the amount of charge passing through an area in time t. ,where q is the charge (C), AV is the voltage (V), and Cis the capacitance units Amperes (A). Here q is Mombrane 9. Membrane potential is potential difference between inner and outer surface of biological membrane. The semipermeable membrane of a cell has different concentrations of ions inside and out. Diffusion moves the K' and CI ions in the direction shown, until the Coulomb force halts further transfer. This results in a layer of positive charge on the outside, a layer of negative charge on the inside, and thus a voltage across the cell membrane. At rest state, the membrane is normally impermeable to Na'. If Inside Couomb frce Difusion Outside Difion Couomb loro Diuion membrane is permeable only to one ion type, the membrane potential is determined by equation: AV = In S, here R = 8,31J/(mole K) is gas constant; T is the temperature of diffusion (K); F = 96500 C/mole is Faraday's constant, Z is valence of ion (Z = 1 for K*, Na*, cl-), Cin is the ions concentration inside the cell (mole/m) and Cout is the ions concentration outside the cell (mole'm'). If membrane is permeable simultaneously to three ion types, then the membrane potential is found by equation: RT PRCA + PNACM + PctCSut Cin. Cout ZF In AV = - ZF" PRCout + PxaCNa + PciC here Px, PNa, Pct are permeability of the membrane to corresponding ion type C, CNa, c are ions concentrations inside the cell (mole/m') and Cut, C, Csut are coresponding concentrations outside the cell (mole/m').