( ( Each ( of ( these foods ) ) ) ( ( contributes ( ( calories , ) ( which ( are ( units ( of energy ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT Each)) (PP (IN of) (NP (DT these) (NNS foods)))) (VP (VBZ contributes) (NP (NP (NNS calories)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBP are) (NP (NP (NNS units)) (PP (IN of) (NP (NN energy))))))))) (. .)))
Each of these foods contributes calories, which are units of energy.
(ROOT (S (NP (DT A) (NN calorie)) (VP (VBZ is) (VP (VBN used) (S (VP (TO to) (VP (VB express) (NP (NP (DT the) (NN amount)) (PP (IN of) (NP (NN energy)))) (PP (IN in) (NP (NN food)))))))) (. .)))
A calorie is used to express the amount of energy in food.