( ( In general ) ( , ( ( ( ( cells ( ( ( can not ) actively ) ( ( transport water ) ( against ( its gradient ) ) ) ) ) but ) ( that ( is ( what ( we ( want ( to ( accomplish ( if ( we ( are ( to ( make ( ( the urine ) ( more ( concentrated ( than ( the blood ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (PP (IN In) (NP (NN general))) (, ,) (S (NP (NNS cells)) (VP (MD can) (RB not) (ADVP (RB actively)) (VP (VB transport) (NP (NN water)) (PP (IN against) (NP (PRP$ its) (NN gradient)))))) (CC but) (S (NP (DT that)) (VP (VBZ is) (SBAR (WHNP (WP what)) (S (NP (PRP we)) (VP (VBP want) (S (VP (TO to) (VP (VB accomplish) (SBAR (IN if) (S (NP (PRP we)) (VP (VBP are) (S (VP (TO to) (VP (VB make) (S (NP (DT the) (NN urine)) (ADJP (RBR more) (VBN concentrated) (PP (IN than) (NP (DT the) (NN blood))))))))))))))))))) (. .)))
In general, cells cannot actively transport water against its gradient but that is what we want to accomplish if we are to make the urine more concentrated than the blood.
(ROOT (S (NP (JJ Primary) (JJ active) (NN transport)) (VP (VBZ involves) (NP (NP (DT the) (NN forcing)) (PP (IN of) (NP (DT a) (NN substance)))) (PP (IN into) (NP (NP (DT a) (NN cell)) (PP (IN against) (NP (PRP$ its) (NN concentration) (NN gradient)))))) (. .)))