( It ( ( is ( thought ( that ( ( ( ( chloroplasts , ) ( ( ( the bodies ) ( in ( plant cells ) ) ) ( that ( do photosynthesis ) ) ) ) , ) ( ( were once ) ( small ( independent cells ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (VP (VBN thought) (SBAR (IN that) (S (NP (NP (NNS chloroplasts)) (, ,) (NP (NP (DT the) (NNS bodies)) (PP (IN in) (NP (NN plant) (NNS cells))) (SBAR (WHNP (WDT that)) (S (VP (VBP do) (NP (NN photosynthesis)))))) (, ,)) (VP (VBD were) (ADVP (RB once)) (NP (JJ small) (JJ independent) (NNS cells))))))) (. .)))
It is thought that chloroplasts , the bodies in plant cells that do  photosynthesis , were once small independent cells.
(ROOT (S (NP (NNS Photosynthesis)) (VP (VBZ occurs) (PP (IN in) (NP (DT the) (NN chloroplast)))) (. .)))
Photosynthesis occurs in the chloroplast.