Final answer:
In a coupled reaction, the endergonic half-reaction requires energy, whereas the exergonic half-reaction releases energy. Each option provided shows a pair of coupled reactions, where the first reaction in every option is endergonic and the second is exergonic.
Step-by-step explanation:
The question pertains to identifying the endergonic and exergonic half-reactions in a coupled reaction. In options provided by the student, the endergonic reaction is the one that requires energy input, and the exergonic reaction is the one that releases energy. To answer the question correctly, we must find the option where the endergonic reaction is paired with the exergonic reaction.
From the options given:
a) A→B is endergonic (requires energy), and C→D is exergonic (releases energy).
b) X→Y is exergonic, and Y→Z is endergonic.
c) P→Q is endergonic, and R→S is exergonic.
d) M→N is exergonic, and N→O is endergonic.
Each of these represents a paired set of reactions. To clarify the concepts further, let's look at some examples of biological processes:
a) Cellular respiration is exergonic because it releases energy.
b) Photosynthesis is endergonic because it requires energy input.
c) Catabolic reactions are exergonic as they involve the breakdown of molecules, releasing energy.
d) Anabolic reactions are endergonic as they involve the synthesis of complex molecules, requiring energy input.
For the student's question, the half-reaction that is endergonic is labelled in each option as the one that requires an input of energy.