Final answer:
In a cell with a membrane selectively permeable to Na+, sodium ions would flood in due to a concentration gradient and electrical forces, causing depolarization. This is key to action potential generation and cell communication.
Step-by-step explanation:
In a hypothetical cell with a membrane permeable only to Na+, the sodium ions would enter the cell quickly due to the large concentration gradient. With higher concentration outside the cell, sodium ions rush in, driven by both diffusion and the Coulomb force. As the sodium cations enter, the membrane potential shifts from its resting state of -70 mV, causing the inside of the cell to become less negative in a process known as depolarization. This shift toward a positive charge inside the cell, which can peak around +30 mV, is critical in generating an action potential in neurons. Following the depolarization, other mechanisms, such as the closing of Na+ channels and the opening of K+ channels, help to repolarize the cell, returning it to its resting potential in a process essential for cellular communication and function.