( ( Animal cells ) ( ( lack ( ( the ( rigid ( cell walls ) ) ) ( that ( characterize ( plant cells ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Animal) (NNS cells)) (VP (VBP lack) (NP (NP (DT the) (JJ rigid) (NN cell) (NNS walls)) (SBAR (WHNP (WDT that)) (S (VP (VBP characterize) (NP (NN plant) (NNS cells))))))) (. .)))
Animal cells lack the rigid cell walls that characterize plant  cells.
(ROOT (S (NP (DT A) (NN cell) (NN wall)) (VP (VBZ is) (VP (VBN found) (PP (PP (IN in) (NP (DT a) (NN plant) (NN cell))) (CONJP (CC but) (RB not)) (PP (IN in) (NP (DT an) (NN animal) (NN cell)))))) (. .)))
A cell wall is found in a plant cell but not in an animal cell.