The concentration of calcium inside the cell is ______细胞内钙的浓度______

Question

BlackTom AI · Accepted Answer

Intracellular free Ca2+ is kept at very low concentrations (on the order of 10^-7 M) compared with extracellular fluid (about 10^-3 M), so the correct choice is a. kept at low levels.

1) Relevant knowledge points and concepts
- Resting cytosolic free Ca2+ concentration in most cells is ≈100 nM (1.0 × 10^-7 M). Extracellular Ca2+ is ≈1.0–2.5 mM (≈1.0–2.5 × 10^-3 M). 
- A large transmembrane Ca2+ gradient (≈10^4-fold) is essential so Ca2+ can act as a rapid intracellular second messenger when channels open.
- Maintenance mechanisms: plasma membrane Ca2+ ATPases (PMCA), Na+/Ca2+ exchangers (NCX), SERCA pumps into endoplasmic/sarcoplasmic reticulum, mitochondrial uptake, and cytosolic Ca2+-binding proteins.

2) Step-by-step reasoning
- Compare typical concentrations: extracellular ≈1.2 × 10^-3 M, intracellular free ≈1.0 × 10^-7 M.
- Cells actively remove Ca2+ from the cytosol to maintain this low baseline using ATP-driven pumps and exchangers and by sequestering Ca2+ in organelles or binding proteins.
- The low resting level allows small influxes to produce large relative changes that trigger signaling pathways.

3) Calculation (intermediate steps)
- Extracellular / intracellular ratio ≈ (1.2 × 10^-3 M) / (1.0 × 10^-7 M) = 1.2 × 10^4 ≈ 12,000-fold difference.
- This quantifies “kept at low levels” relative to outside.

4) Why the correct answer is correct
- The physiology described above directly supports that intracellular Ca2+ is maintained at low levels to permit rapid, regulated signaling and to avoid toxic cytosolic Ca2+ elevations.

5) Why other options are incorrect
- b. equal to extracellular fluid: incorrect — there is a large gradient; intracellular is much lower.
- c. greater than extracellular fluid: incorrect — intracellular is far lower, not higher.
- d. constantly moving across the membrane: misleading — Ca2+ fluxes occur during signaling, but the defining fact is active maintenance at low cytosolic concentration, not continuous uncontrolled movement.

6) Conclusion
The correct answer is a. kept at low levels, which matches the provided answer.

The concentration of calcium inside the cell is 细胞内钙的浓度Single choice

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