( It ( ( ( is not ) ( ( ( the formation ) ( of ( hydrogen sulfide ) ) ) ( that ( is ( toxic ( to ( ( the cell ) ( but ( ( the process ) ( of ( reducing ( ( the sulfur ) ( that ( results ( in toxicity ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (RB not) (NP (NP (DT the) (NN formation)) (PP (IN of) (NP (NN hydrogen) (NN sulfide))) (SBAR (WHNP (WDT that)) (S (VP (VBZ is) (ADJP (JJ toxic) (PP (TO to) (NP (NP (DT the) (NN cell)) (PP (CC but) (NP (NP (DT the) (NN process)) (PP (IN of) (S (VP (VBG reducing) (NP (NP (DT the) (NN sulfur)) (SBAR (WHNP (WDT that)) (S (VP (VBZ results) (PP (IN in) (NP (NN toxicity)))))))))))))))))))) (. .)))
It is not the formation of hydrogen sulfide that is toxic to the cell but the process of reducing the sulfur that results in toxicity.
(ROOT (S (NP (NNP Hydrogen) (NNP sulfide)) (VP (VBZ is) (NP (NP (DT a) (ADJP (JJ noxious) (CC and) (JJ toxic)) (NN gas)) (VP (VBN produced) (PP (IN from) (S (VP (VBG decaying) (NP (JJ organic) (NN matter)))))) (SBAR (WHNP (WDT that)) (S (VP (VBZ contains) (NP (NN sulfur))))))) (. .)))