( ( The ( small intestine ) ) ( ( is ( where ( ( most ( of ( the digestion ) ) ) happens ) ) ) . ) )
(ROOT (S (NP (DT The) (JJ small) (NN intestine)) (VP (VBZ is) (SBAR (WHADVP (WRB where)) (S (NP (NP (JJS most)) (PP (IN of) (NP (DT the) (NN digestion)))) (VP (VBZ happens))))) (. .)))
The small intestine is where most of the digestion happens.
(ROOT (S (NP (NP (JJS Most)) (PP (IN of) (NP (DT the) (NN digestion) (NNS reactions)))) (VP (VBP occur) (PP (IN in) (NP (DT the) (JJ small) (NN intestine)))) (. .)))