Final answer:
The resting potential of an excitable cell's plasma membrane is primarily due to its selective permeability to potassium ions (K+), which allows these ions to move out of the cell more freely than sodium ions (Na+) can enter, leading to a net negative charge inside the cell.
Step-by-step explanation:
The resting potential of the plasma membrane of an excitable cell, such as a neuron, is principally due to the membrane's selective permeability to certain ions. This resting state is characterized by the cell's interior being negatively charged compared to the exterior. Key to this negative charge is the permeability of the cell membrane to potassium ions (K+), in contrast to its impermeability to sodium ions (Na+) at rest. Potassium ions, which are in higher concentrations inside the cell, can move out more freely than sodium ions can move in, due to a greater number of potassium leakage channels. This differential movement results in a net negative charge inside the cell. Additionally, the sodium-potassium pump helps maintain this potential by pumping out more Na+ ions than it brings in K+ ions, contributing to the overall negative charge inside relative to outside.