How do the extracellular concentrations of Ca and Mg compare?

The correct selection highlights that the concentrations of calcium (Ca) and magnesium (Mg) are notably higher outside of cells (in the serum) compared to the concentrations found within the cytoplasm of the cells.

This relationship is important in various physiological processes. For calcium, it plays critical roles in cell signaling, muscle contraction, and neurotransmission; hence, maintaining higher extracellular calcium levels is essential for these functions. Magnesium, while less abundant than calcium, also serves crucial roles in enzyme function, energy production, and DNA synthesis, and it’s similarly regulated to ensure adequate availability for physiological needs.

The context of the question involves understanding that ions such as Ca and Mg generally exist in much higher concentrations in the extracellular fluid compared to the intracellular fluid. This difference is a key aspect of cellular and physiological function, providing gradients necessary for various cellular activities.

Other options do not align with the known biological principles regarding the distribution of these ions, which reinforces why the selected answer is accurate.