( ( ( The energy ) ( required ( to ( start ( the reaction ) ) ) ) ) ( ( is ( called ( activation energy ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN energy)) (VP (VBN required) (S (VP (TO to) (VP (VB start) (NP (DT the) (NN reaction))))))) (VP (VBZ is) (VP (VBN called) (NP (NN activation) (NN energy)))) (. .)))
The              energy required to start the reaction is called activation energy.
(ROOT (S (NP (NP (DT The) (NN amount)) (PP (IN of) (NP (NP (NN energy)) (VP (VBN required) (S (VP (TO to) (VP (VB begin) (NP (DT a) (JJ chemical) (NN reaction))))))))) (VP (VBZ is) (VP (VBN known) (PP (IN as) (NP (DT the) (NN activation) (NN energy))))) (. .)))