Which statement best describes the intracellular/extracellular distributions of potassium and sodium?

Which statement best describes the intracellular/extracellular distributions of potassium and sodium?

• A. Potassium is more concentrated inside and sodium more concentrated outside.
• B. Sodium is more concentrated inside and potassium more concentrated outside.
• C. Potassium and sodium have equal distribution across the membrane.
• D. Potassium is more concentrated outside and sodium inside.

Answer: (A)

The key idea is that ions are maintained in distinct concentrations across the cell membrane, with potassium kept high inside the cell and sodium kept high outside. This gradient is set and maintained primarily by the sodium–potassium pump, which actively moves Na+ out and K+ in, using ATP. The membrane is also more permeable to potassium at rest, so K+ tends to leak out, helping keep the interior negative relative to the outside. In numbers close to typical physiology, intracellular potassium is about 140 mM while extracellular potassium is around 4–5 mM; intracellular sodium is about 10–15 mM while extracellular sodium is about 140 mM. Because of these gradients, the statement that potassium is more concentrated inside and sodium more concentrated outside is the correct description. The other descriptions invert or nullify these well-established gradients.

The key idea is that ions are maintained in distinct concentrations across the cell membrane, with potassium kept high inside the cell and sodium kept high outside. This gradient is set and maintained primarily by the sodium–potassium pump, which actively moves Na+ out and K+ in, using ATP. The membrane is also more permeable to potassium at rest, so K+ tends to leak out, helping keep the interior negative relative to the outside. In numbers close to typical physiology, intracellular potassium is about 140 mM while extracellular potassium is around 4–5 mM; intracellular sodium is about 10–15 mM while extracellular sodium is about 140 mM. Because of these gradients, the statement that potassium is more concentrated inside and sodium more concentrated outside is the correct description. The other descriptions invert or nullify these well-established gradients.