( ( Transport ( of ( substances ( across membranes ) ) ) ) ( often ( ( ( requires energy ) ( to ( overcome ( a ( concentration gradient ) ) ) ) ) . ) ) )
(ROOT (S (NP (NP (NNP Transport)) (PP (IN of) (NP (NP (NNS substances)) (PP (IN across) (NP (NNS membranes)))))) (ADVP (RB often)) (VP (VBZ requires) (NP (NN energy)) (S (VP (TO to) (VP (VB overcome) (NP (DT a) (NN concentration) (NN gradient)))))) (. .)))
Transport of substances across membranes often requires energy to overcome a concentration gradient.
(ROOT (S (NP (JJ Primary) (JJ active) (NN transport)) (VP (VBZ involves) (NP (NP (DT the) (NN forcing)) (PP (IN of) (NP (DT a) (NN substance)))) (PP (IN into) (NP (NP (DT a) (NN cell)) (PP (IN against) (NP (PRP$ its) (NN concentration) (NN gradient)))))) (. .)))