( ( ( ( ( ( ( receptors , ) ( second messengers ) ) , ) and ) ion ) channels ) ; )
(ROOT (NP (NP (NNS receptors)) (, ,) (NP (JJ second) (NNS messengers)) (, ,) (CC and) (RB ion) (NP (NNS channels)) (: ;)))
receptors, second messengers, and ion channels;
(ROOT (S (NP (NNP Calcium) (NNP ion)) (VP (VBZ is) (NP (NP (DT an) (NN example)) (PP (IN of) (NP (DT a) (JJ second) (NN messenger))))) (. .)))
Calcium ion is an example of a second messenger.