( ( Ion channels ) ( ( act ( as ( pathways ( for ( ions ( across ( cell membranes ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Ion) (NNS channels)) (VP (VBP act) (PP (IN as) (NP (NP (NNS pathways)) (PP (IN for) (NP (NP (NNS ions)) (PP (IN across) (NP (NN cell) (NNS membranes)))))))) (. .)))
Ion channels act as pathways for ions across cell membranes.
(ROOT (NP (NP (NP (NNP Ion) (NNP channels) (NNP facilitate)) (PRN (-LRB- -LRB-) (PRP s) (-RRB- -RRB-))) (NP (NP (DT the) (NN diffusion)) (PP (IN of) (NP (NP (NNS ions)) (PP (IN across) (NP (DT a) (NN membrane)))))) (. .)))