( ( ( ( ( ( The cathode ) ( is ( ( ( the ( negative electrode ) ) , ) ( where ( ions ( gain electrons ) ) ) ) ) ) , ) and ) ( thus ( , ( reduction ( occurs ( at ( the cathode ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (DT The) (NN cathode)) (VP (VBZ is) (NP (NP (DT the) (JJ negative) (NN electrode)) (, ,) (SBAR (WHADVP (WRB where)) (S (NP (NNS ions)) (VP (VB gain) (NP (NNS electrons)))))))) (, ,) (CC and) (S (ADVP (RB thus)) (, ,) (NP (NN reduction)) (VP (VBZ occurs) (PP (IN at) (NP (DT the) (NN cathode))))) (. .)))
The cathode is the negative electrode, where ions gain electrons, and thus, reduction occurs at the cathode.
(ROOT (S (NP (NP (DT The) (NN electrode)) (SBAR (WHPP (IN at) (WHNP (WDT which))) (S (NP (NN reduction)) (VP (VBZ occurs))))) (VP (VBZ is) (VP (VBN called) (NP (DT the) (NN cathode)))) (. .)))