( ( Some substances ) ( ( ( ( cross ( the membrane ) ) ( by ( ( ( simple diffusion ) , ) others ) ) ) ( by ( ( complex ( transport mechanisms ) ) ( requiring ( metabolic energy ) ) ) ) ) . ) )
(ROOT (S (NP (DT Some) (NNS substances)) (VP (VBP cross) (NP (DT the) (NN membrane)) (PP (IN by) (NP (NP (JJ simple) (NN diffusion)) (, ,) (NP (NNS others)))) (PP (IN by) (NP (NP (JJ complex) (NN transport) (NNS mechanisms)) (VP (VBG requiring) (NP (JJ metabolic) (NN energy)))))) (. .)))
Some substances cross the membrane by simple diffusion, others by complex transport mechanisms requiring metabolic energy.
(ROOT (S (NP (NNP Passive) (NN transport)) (VP (VBZ occurs) (SBAR (WHADVP (WRB when)) (S (NP (NNS substances)) (VP (VBP cross) (NP (DT the) (NN plasma) (NN membrane)) (PP (IN without) (NP (NP (DT any) (NN input)) (PP (IN of) (NP (NN energy))))) (PP (IN from) (NP (DT the) (NN cell))))))) (. .)))