Final answer:
In electrochemical equilibrium, only potassium ions can be in equilibrium across the cell membrane, while sodium ions are regulated and maintained at non-equilibrium concentrations.
Step-by-step explanation:
In a cell, the membrane is normally impermeable to sodium ions (Na+), but it is permeable to both potassium ions (K+) and sodium ions (Na+). However, in electrochemical equilibrium, only potassium ions (K+) will be at equilibrium across the membrane.
During the resting state of a cell, the diffusion of potassium ions (K+) and chloride ions (Cl-) creates layers of positive charge on the outside of the membrane and negative charge on the inside. This results in a voltage or potential difference across the cell membrane. On the other hand, the movement of sodium ions (Na+) is regulated by channels and pumps in the membrane to maintain the resting potential.
Therefore, in electrochemical equilibrium, only potassium ions (K+) can be in equilibrium across the cell membrane, while sodium ions (Na+) are regulated and maintained at non-equilibrium concentrations.