Final answer:
Increasing the external sodium concentration to 150mM will lead to depolarization of the cell membrane, shifting the membrane potential (Vm) towards a more positive value.
Step-by-step explanation:
When the external concentration of sodium (Na+) is increased to 150mM, there are implications for the membrane potential (Vm) of a cell.
The resting membrane potential, usually around -70 mV, is heavily influenced by the concentration gradients of ions across the cell membrane, chiefly sodium and potassium ions. According to the Goldman-Hodgkin-Katz voltage equation, changes in the concentration of ions outside the cell can alter the membrane potential.
Sodium ions, due to their positive charge, can affect the electrical gradient when they enter the cell. By increasing [Na]+out to 150mM, more sodium ions are driven into the cell due to a heightened concentration gradient, leading to depolarization. This influx of sodium ions makes the inside of the cell less negative, moving Vm towards a more positive value. If enough sodium enters, the Vm may reach values like +30 mV, the typical peak of an action potential.