Which ion movement contributes to more negative charges on the inside during resting membrane potential?

• A. Na+ entering the cell
• B. K+ leaving the cell
• C. Ca++ entering the cell
• D. Cl- leaving the cell

Answer: (B)

Potassium leak out of the cell sets the resting membrane potential to be negative. The membrane is more permeable to K+ at rest, so K+ ions move down their concentration gradient from inside to outside. As positive charge leaves, the inside becomes more negative. Moving positively charged ions into the cell, like sodium or calcium, would depolarize and make the inside less negative, while removing chloride from inside would reduce the negative charge inside. The Na+/K+-ATPase helps maintain gradients but its direct electrogenic effect is smaller than the continuous K+ leak. Therefore, the outflow of potassium is the main driver of the more negative interior at rest.