The concentration of calcium inside the cell is ______

Question

BlackTom AI · Accepted Answer

Intracellular free Ca2+ concentration is kept at very low levels (roughly 10^-7 M, ~100 nM) compared with the extracellular fluid (roughly 10^-3 M, ~1–2 mM).

1) Relevant knowledge points and concepts
- Calcium is a ubiquitous second messenger: tiny rises in cytosolic Ca2+ trigger many signaling pathways (muscle contraction, secretion, enzyme activation).
- To function as a signal, cytosolic free Ca2+ must be maintained at a low resting level so that transient influxes produce a large relative change.
- Cells achieve low cytosolic Ca2+ by active transport (PMCA, SERCA), exchangers (Na+/Ca2+ antiporter), sequestration into organelles (ER, mitochondria), and buffering by proteins (calmodulin, parvalbumin).

2) Step-by-step reasoning
- Resting cytosolic free Ca2+ ≈ 100 nM (10^-7 M). Extracellular Ca2+ ≈ 1–2 mM (10^-3 M).
- Compute the gradient: (extracellular)/(intracellular) ≈ 10^-3 / 10^-7 = 10^4. This shows the intracellular level is about 10,000-fold lower.
- Maintenance mechanisms: PMCA and SERCA use ATP to pump Ca2+ out of the cytosol; Na+/Ca2+ exchanger uses the Na+ gradient; organellar uptake and buffering keep free cytosolic Ca2+ low.
- Functional consequence: a small absolute Ca2+ influx during signaling produces a large fold-increase in cytosolic Ca2+, enabling rapid and specific responses. High sustained cytosolic Ca2+ would be toxic (activate proteases, nucleases, cause mitochondrial dysfunction).

3) Why the correct answer is correct
- Because of the signaling role and toxicity of Ca2+, cells actively maintain very low cytosolic Ca2+ compared to extracellular fluid; therefore “kept at low levels” is accurate.

4) Why the other options are incorrect
- “Equal to extracellular fluid”: false—there is a huge gradient with extracellular much higher.
- “Greater than extracellular fluid”: false—intracellular free Ca2+ is far lower.
- “Constantly moving across the membrane”: misleading—while Ca2+ fluxes occur during signaling, the net resting state is active maintenance of low intracellular levels, not a state of equalizing movement.

Conclusion: The intracellular concentration of calcium is kept at low levels, matching the provided correct answer.

The concentration of calcium inside the cell is ______单项选择题

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