Practice question

Sea urchin sperm remain immotile in testes due to low pH and high CO2, then activate upon spawning into alkaline seawater. Egg jelly peptides speract and resact bind receptor guanylate cyclase on flagellar membrane, elevating cGMP, opening K+ channels, hyperpolarizing membrane, and activating Na+/H+ exchanger raising intracellular pH. Alkalization triggers CatSper channels opening causing calcium influx. Calcium pulses transform flagellar beat from symmetric low-amplitude to asymmetric high-amplitude turning events, accelerating respiration and preparing acrosomal exocytosis. Thus calcium acts as central second messenger linking chemotactic sensing to motility activation and readiness for fertilization.