Relative to extracellular fluid, the interior of a neuron has a negative charge because __________.

• A. potassium diffuses out of the cell
• B. potassium is actively pumped out of the cell
• C. potassium is actively pumped into the cell
• D. chloride ions are actively pumped into the cell
• E. sodium is actively pumped into the cell

Answer: (A)

Verified step by step guidance

1. Understand the concept of membrane potential: Neurons maintain a difference in charge between the inside and outside of the cell, known as the membrane potential. This is crucial for the function of neurons in transmitting signals.
2. Identify the role of ions in establishing membrane potential: The movement of ions such as potassium (K+), sodium (Na+), and chloride (Cl-) across the cell membrane contributes to the membrane potential.
3. Focus on potassium ions: Potassium ions play a significant role in maintaining the negative charge inside the neuron. The cell membrane is more permeable to potassium ions than to sodium ions.
4. Explain diffusion of potassium: Potassium ions tend to diffuse out of the cell due to their concentration gradient, which is higher inside the cell than outside. This diffusion contributes to the negative charge inside the neuron.
5. Consider active transport mechanisms: While diffusion is a passive process, active transport mechanisms like the sodium-potassium pump also play a role in maintaining the membrane potential by pumping sodium out and potassium into the cell, but the diffusion of potassium out is key to the negative charge.