( ( Like ( a mitochondrion ) ) ( , ( it ( ( is ( surrounded ( by ( two membranes ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN Like) (NP (DT a) (NN mitochondrion))) (, ,) (NP (PRP it)) (VP (VBZ is) (VP (VBN surrounded) (PP (IN by) (NP (CD two) (NNS membranes))))) (. .)))
Like a mitochondrion, it 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))))) (. .)))