Where are most of the potassium ions (K^+) located when a muscle cell is at rest?

• A. Outside the cell (extracellular fluid)
• B. Within the nucleus
• C. Inside the cell (cytoplasm)
• D. Equally distributed inside and outside the cell

Answer: (C)

Verified step by step guidance

1. Understand the concept of resting membrane potential: Muscle cells, like other excitable cells, maintain a resting membrane potential, which is largely influenced by the distribution of ions across the cell membrane.
2. Recall the role of potassium ions (K⁺): Potassium ions are crucial for maintaining the resting membrane potential. They are actively transported into the cell by the sodium-potassium pump (Na⁺/K⁺ ATPase).
3. Learn about the sodium-potassium pump: This pump moves 3 sodium ions (Na⁺) out of the cell and 2 potassium ions (K⁺) into the cell, creating a higher concentration of K⁺ inside the cell compared to the extracellular fluid.
4. Understand the intracellular location of K⁺: At rest, most potassium ions are located inside the cytoplasm of the muscle cell, not in the nucleus or equally distributed inside and outside the cell.
5. Connect this to the resting state of the muscle cell: The high concentration of K⁺ inside the cell contributes to the negative charge inside the cell relative to the outside, which is essential for the cell's ability to respond to stimuli.