Question

Standard reduction potential

(A) Ubiquinone/ubiquinol = + 0.045

b) NO3-/NO2- = 0.421

C.) NADP+/NADPH= -0.324

Question 5: which chemical has the least affinity for electrons?

Question 6"

If, under standard biochemical conditions, ubiquinol donated two electrons to NO₃^-, then (select any/all answers that apply):

	A.	the coupled redox reaction would be exergonic.
	B.	the coupled redox reaction would be endergonic.
	C.	the coupled redox reaction would have a negative ΔG'° value.
	D.	ubiquinol would become oxidized to ubiquinone during the coupled redox reaction.
	E.	ubiquinol would become reduced to ubiquinone during the coupled redox reaction.

QUestion 7

Which of the following chemical species has the potential to act as an electron acceptor?  Select any/all answers that apply.

	A.	ubiquinol
	B.	NO2-
	C.	NADPH
	D.	NO3-
	E.	NADP

QUESTION 8

1. Which of the following species is in its reduced form?  Select any/all answers that apply.

   	A.	ubiquinol
   	B.	NO2-
   	C.	NADPH
   	D.	NO3-
   	E.	NADP+

Answer 1

Answers:

5. (C)

Explanation: Standard reduction potentials are expressed as half reactions and written in the direction of a reduction reaction. Redox pairs with a positive reduction potentials (Eº’) have a higher affinity for electrons that redox pairs with a negative Eº’. Hence, option (C) NADP⁺/NADPH redox pair with Eº’= -0.324 has the least affinity for electrons among these chemicals.

6. (A), (C) and (D)

Explanation: Since, the standard reduction potential of ubiquinone/ubiquinol is lower than the NO₃^-/NO₂^- redox pair, electrons move from ubiquinol to NO₃^– in this coupled redox reaction and the energy is released (exergonic). The denitrifying bacteria uses this electron transport mechanism during anaerobic respiration to produce energy!

7. (B), (D) and (E)

Explanation:

An electron acceptor is an oxidising agent and is itself reduced in the redox reaction. NADP and NO₃^– both have high (+) oxidation state and hence are capable of accepting electrons. NADP is the final electron acceptor in the light-dependent reaction of photosynthesis. While, nitrate (NO₃^–) is used as the electron acceptor in anaerobic respiration.

Although, nitrite (NO₂^-) is a reduced form of nitrate (NO₃^-) but the nitrogen atom in a nitrite has an oxidation number is +3. So, it can act both as oxidising agent (electron acceptor) or reducing agent (electron donor).

8. (A), (B) and (C)

Explanation: The three redox states of Coenzyme Q₁₀ are: fully oxidized (ubiquinone), semiquinone (ubisemiquinone), and fully reduced (ubiquinol). Nitrite (NO₂^-) is the reduced form nitrate (NO₃^-). NADP⁺ gains two electrons and reduced to NADPH.

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