As a result of a complex set of biochemical reactions, the cell mem- brane of a nerve cell pumps ions (Na+ and K+) back and forth across itself, thereby maintaining an electrostatic potential difference from the inside to the outside of the membrane. Modications of the con- ditions can result in changes in those potentials. Part of the process can be modeled by treating the membrane as if it were a simple electric circuit consisting of batteries, resistors, and a switch. A simple model of the membrane of a nerve cell is shown in Figure 5. It consists of two batteries (ion pumps) with voltages V1 = 90 mV and V2 = 45 mV. The resistance to ow across the membrane is represented by two resistors with resistances R1 = 10 k0 and R2 = 100 k0. The variability is represented by a switch, SW1. Four points on the circuit are labelled by the letters a-d. The point b represents the outside of the membrane and the point d the inside of the membrane. A. What is the voltage difference across the membrane (i.e., be- tween d and b) when the switch is open? B. When the switch SW1 is closed, what is the current owing around the loop, and the voltage drops across R1 and R2? C. When the switch SW1 is open, what is the current owing around the loop, and the voltage drops across R1 and R2? D. What is the voltage difference across the membrane (i.e.,. be- tween d and b) when the switch is closed? And, if the locations of resistances R1 and R2 were reversed would the voltages across the cell membrane be different? outside inside Figure 5: Circuit model of the cell membrane of a nerve cell