Final answer:
The statement regarding the extracellular Ca²⁻ concentration being approximately 104-fold higher than that in the cytosol is True. This difference is critical for cellular signaling involving Ca²⁻ as a second messenger.
Step-by-step explanation:
The statement that the extracellular concentration of Ca²⁻ is approximately 104-fold higher than the concentration of Ca²⁻ in the cytosol is True. Calcium ions (Ca²⁻) play a vital role as second messengers in cellular signaling. Inside the cell, the Ca²⁻ concentration is maintained at a very low level due to the action of ion pumps using ATP to continuously remove it, whereas extracellular fluid tends to have higher Ca²⁻ levels. Upon signaling, ligand-gated calcium ion channels allow the influx of Ca²⁻ from outside the cell or storage compartments like the endoplasmic reticulum, leading to a rise in cytosolic Ca²⁻ concentration and subsequent cellular response, such as insulin release from the pancreas or muscle contraction.