( ( Cell wall ) ( ( is ( ( the ( rigid structure ) ) ( that ( surrounds ( the ( plant cell ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Cell) (NN wall)) (VP (VBZ is) (NP (NP (DT the) (JJ rigid) (NN structure)) (SBAR (WHNP (WDT that)) (S (VP (VBZ surrounds) (NP (DT the) (NN plant) (NN cell))))))) (. .)))
Cell wall is the rigid structure that surrounds the  plant cell.
(ROOT (S (NP (DT A) (NN cell) (NN wall)) (VP (VBZ is) (RB not) (ADJP (JJ present) (PP (IN in) (NP (NN animal) (NNS cells))))) (. .)))