<span>The correct answer is cGMP.</span>
<span>cGMP, during the dark (when there are no stimulations of photoreceptor cells), reaches high intracellular levels. When the light stimulation occurs cGMP levels decrease because it is converted to GMP by phosphodiesterase. Phosphodiesterase is activated by transducin which is activated by excited rhodopsin (contains photopigment).</span>
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Low levels of cGMP block the cGMP gated ion channels and that prevents the entry of Na and Ca. Because of that membranes becomes more negative, hyperpolarization occurs and this creates a signal for the next cell.</span>
Question 1: B: rapid water drainage
