Final answer:
The capacitor in the electrical equivalent of a cell membrane symbolizes the membrane's ability to store charge, analogous to charge buildup in a parallel-plate capacitor, which is why it is represented in parallel with other resistances.
Step-by-step explanation:
In the electrical equivalent of a cell membrane, the capacitor is symbolized and set parallel to resistances because it represents the membrane's ability to store charge. The cell membrane behaves like a capacitor due to its insulating properties and its capacity to separate charges on either side. Positive and negative charges accumulate on the two sides of the membrane, creating a voltage across it. This configuration is parallel because the voltage across each component (resistances and the capacitor) is the same, which is consistent with the parallel connection in circuits.
The capacitor symbolizes the charge accumulation at the cell border, providing a mechanism to represent how the membrane temporarily stores ionic charge as a voltage. This is akin to how a parallel-plate capacitor works, where an electric field resides between the plates, and the field strength is directly proportional to the surface charge density. In physiological terms, the semipermeable cell membrane allows for different ion concentrations inside and out, which can lead to a buildup of charges and an electric potential, much like a capacitor storing charge in an electric circuit.