Practice question

Sea urchin sperm are kept quiescent in gonads where pH is acidic and carbon dioxide high, maintaining intracellular pH around 7.2. Upon spawning into seawater pH 8.0, sodium-hydrogen exchangers extrude protons driven by sodium gradient, raising internal pH to 7.6. Alkalization activates dynein ATPase, increases mitochondrial oxygen consumption, enhances flagellar beat frequency, and sensitizes guanylate cyclase to chemotactic peptides. Acidifying agents inhibit motility, while artificial alkalization with ammonium chloride activates motility without egg factors. Thus increased internal pH, not decreased pH or mere external pH change without internal effect, initiates forward motility and chemosensory competence.