Practice question

Calcium serves as ubiquitous intracellular second messenger regulating muscle contraction, neurotransmitter release, gene expression and apoptosis, requiring precise spatial temporal control. Resting cytosolic free calcium around 100 nanomolar maintained far below extracellular 1 to 2 millimolar and endoplasmic reticulum lumen 400 to 700 micromolar, creating 10,000 fold chemical gradient plus electrical component because membrane potential negative. P-type Ca2+ ATPases accomplish low cytosolic level by active extrusion. Plasma membrane Ca2+ ATPase PMCA ejects calcium outward, while SERCA pumps into endoplasmic and sarcoplasmic reticulum refilling stores. Each cycle hydrolyzes one ATP phosphorylating conserved aspartate in DKTGT motif generating E1P with two calcium occluded high affinity facing cytosol, then conformational transition to E2P low affinity facing lumen or extracellular space releasing calcium. Activity counteracts leak channels, store operated entry and voltage gated calcium influx. Binding without transport would not lower free concentration, facilitated or passive diffusion would equilibrate toward high outside leading to toxicity. Hence these pumps actively transport calcium out of cytoplasm preserving homeostasis and signaling competence.