Figure 3.

Schematic representation of the major Ca²⁺-gradients in cells. A Ca²⁺ concentration of 2 millimolar in blood is about 20,000 times higher than the 100 nanomolar Ca²⁺ concentration in the cytoplasm of unstimulated (resting cells). Intracellular Ca²⁺ gradients can also be built up inside cells. Ca²⁺-ATPases are enzymes that reside in membranes and which can actively transport Ca²⁺ ions against the Ca²⁺ gradient. This way, the Ca²⁺ concentration in the lumen of the endoplasmic reticulum can become much higher than in the cytoplasm. Sex steroids can influence this transmembrane gradient. If more Ca²⁺ enters the cell that can be extruded by Ca²⁺-ATPases present the plasma membrane, the rough endoplasmic reticulum is triggered to remove extra Ca²⁺ along with proteins [3]. Calcareous egg shells are one of the possible outcomes of such strategy. This figure is borrowed from [3].