Final answer:
The voltage difference across a cell membrane is referred to as the membrane potential, which is key for nerve and muscle cell function.
Step-by-step explanation:
The voltage difference across a membrane of a cell is called the membrane potential. Due to active transport and the selective permeability for ions, a distribution of charge across the cell membrane generates this potential difference. Typically, this electrical potential has a value of around -70 to -90 mV in an unstimulated or resting state, but can change upon stimulation, such as during the firing of a nerve signal. This difference in electrical charge across the membrane is crucial for the function of nerve and muscle cells and a significant amount of a cell's energy is devoted to maintaining this potential.