Sodium and potassium ions do not diffuse in equal numbers through ligand-gated cation channels. Why?

• A. Na^+ and K^+ have identical electrochemical gradients, so they diffuse equally.
• B. Ligand-gated cation channels are only permeable to Na^+ and not to K^+.
• C. The concentration of K^+ outside the cell is much higher than that of Na^+.
• D. The electrochemical gradient for Na^+ is steeper than that for K^+, resulting in more Na^+ entering the cell.

Answer: (D)

Verified step by step guidance

1. Understand the concept of electrochemical gradients: An electrochemical gradient is the combined effect of the concentration gradient (difference in ion concentration across the membrane) and the electrical gradient (difference in charge across the membrane). These gradients drive the movement of ions across the membrane.
2. Analyze the electrochemical gradients for Na⁺ and K⁺: Sodium (Na⁺) has a much higher concentration outside the cell compared to inside, while potassium (K⁺) has a higher concentration inside the cell compared to outside. This creates different gradients for each ion.
3. Consider the steepness of the gradients: The electrochemical gradient for Na⁺ is steeper than that for K⁺ because the concentration difference for Na⁺ across the membrane is larger, and the electrical gradient also favors Na⁺ entry into the cell.
4. Examine the permeability of ligand-gated cation channels: These channels are permeable to both Na⁺ and K⁺, but the steeper electrochemical gradient for Na⁺ results in more Na⁺ ions diffusing into the cell compared to K⁺ ions diffusing out.
5. Conclude why Na⁺ and K⁺ do not diffuse equally: The unequal diffusion is due to the difference in the steepness of their electrochemical gradients, with Na⁺ having a stronger driving force for entry into the cell than K⁺.