( ( Each mitochondrion ) ( ( is ( surrounded ( by ( two membranes ) ) ) ) . ) )
(ROOT (S (NP (DT Each) (NN mitochondrion)) (VP (VBZ is) (VP (VBN surrounded) (PP (IN by) (NP (CD two) (NNS membranes))))) (. .)))
Each mitochondrion is surrounded by two membranes.
(ROOT (S (NP (CD Two) (NNS membranes)) (VP (VBP form) (NP (NP (DT the) (NN mitochondrion) (NN structure)) (PP (IN of) (NP (DT a) (NN cell))))) (. .)))