Practice question

Cellular ion homeostasis requires continuous energy expenditure to maintain steep gradients. Sodium gradient high outside 145 millimolar low inside 12 millimolar and potassium opposite 4 outside 140 inside established by Na+/K+ ATPase hydrolyzing ATP forming phosphoenzyme E1P E2P cycling three sodium out two potassium in per ATP. Calcium gradient extreme low cytosolic 100 nanomolar versus 1 to 2 millimolar extracellular maintained by plasma membrane Ca2+ ATPase PMCA and sarcoplasmic endoplasmic reticulum Ca2+ ATPase SERCA, both P-type pumps transporting two calcium per ATP against ten thousand fold gradient and thapsigargin sensitive. These gradients underlie excitability and signaling. Chloride in contrast distribution of about 110 millimolar extracellular versus 4 to 20 millimolar intracellular in many cells largely approximates passive Donnan equilibrium governed by membrane potential and net impermeable intracellular anions. Transport achieved via KCC potassium chloride cotransporter and NKCC sodium potassium two chloride secondary active, and ClC chloride channels providing conductance, but no mammalian primary ATP dependent pump directly hydrolyzes ATP to pump chloride uphill as main mechanism. Hence chloride considered not primary actively transported in most animal cells.