Question

The following half reactions play important roles in metabolism. \\[ \begin{array}{c} ^{1} / 2 \mathrm{O}_{2}+2 \mathrm{H}^{+}+2 e^{-} \rightarrow \mathrm{H}_{2} \mathrm{O} \\ \mathrm{NADH}+\mathrm{H}^{+} \rightarrow \mathrm{NAD}^{+}+2 \mathrm{H}^{+}+2 e^{-} \end{array} \\] Which of these two is a half reaction of oxidation? Which one is a half reaction of reduction? Write the equation for the overall reaction. Which reagent is the oxidizing agent (electron acceptor)? Which reagent is the reducing agent (electron donor)?

Short answer

The first reaction is reduction, and the second is oxidation. The overall reaction is \( \frac{1}{2} \text{O}_{2} + \text{NADH} + \text{H}^{+} \rightarrow \text{H}_{2}\text{O} + \text{NAD}^{+} \). The oxidizing agent is \(\text{O}_{2}\), and the reducing agent is \(\text{NADH}\).

Step-by-step solution

Identify the Oxidation and Reduction Reactions

Oxidation involves the loss of electrons, whereas reduction involves the gain of electrons. Analyze each half-reaction to determine which involves electron loss and which involves electron gain.

Analyze the First Half Reaction

The first half-reaction is \(\frac{1}{2} \text{O}_{2}+2 \text{H}^{+}+2 e^{-} \rightarrow \text{H}_{2}\text{O}\) which depicts the gain of electrons by \(\text{O}_2\). This is a reduction reaction because \(\text{O}_2\) gains electrons.

Analyze the Second Half Reaction

The second half-reaction is \(\text{NADH}+\text{H}^{+} \rightarrow \text{NAD}^{+}+2 \text{H}^{+}+2 e^{-}\) which depicts the loss of electrons by \(\text{NADH}\). This is an oxidation reaction because \(\text{NADH}\) loses electrons.

Write the Overall Reaction

Combine the half-reactions to form the overall reaction: \(\frac{1}{2} \text{O}_{2} + 2 \text{H}^{+} + 2 e^{-} + \text{NADH} + \text{H}^{+} \rightarrow \text{H}_{2}\text{O} + \text{NAD}^{+} + 2 \text{H}^{+} + 2 e^{-}\). Simplify to get \(\frac{1}{2} \text{O}_{2} + \text{NADH} + \text{H}^{+} \rightarrow \text{H}_{2}\text{O} + \text{NAD}^{+}\).

Identify the Oxidizing Agent

The oxidizing agent is the substance that is reduced. Therefore, \(\text{O}_{2} \) is the oxidizing agent as it accepts electrons.

Identify the Reducing Agent

The reducing agent is the substance that is oxidized. Therefore, \(\text{NADH}\) is the reducing agent as it donates electrons.

Key concepts

oxidation

Oxidation is a fundamental concept in redox reactions and metabolism. It involves the loss of electrons by a molecule, atom, or ion. In the context of the given exercise, the oxidation process can be illustrated by examining the half-reaction for NADH. Here, NADH loses electrons to become NAD⁺. This can be represented by the equation:

\(\text{NADH} + \text{H}^{+} \rightarrow \text{NAD}^{+} + 2 \text{H}^{+} + 2 e^{-} \)

This transformation shows that NADH loses electrons and is oxidized. Recognizing the loss of electrons will help you identify oxidation in various metabolic processes.

reduction

Reduction is another key element in redox reactions, representing the gain of electrons by a molecule, atom, or ion. In the given exercise, the reduction process is exemplified by the half-reaction involving oxygen. Oxygen gains electrons in the formation of water. The half-reaction is:

\( \frac{1}{2} \text{O}_{2} + 2 \text{H}^{+} + 2 e^{-} \rightarrow \text{H}_{2}\text{O} \)

Here, oxygen molecules gain electrons and undergo reduction to form water. Understanding how molecules gain and lose electrons will enhance your grasp of redox processes in metabolism.

oxidizing agent

An oxidizing agent is a substance that accepts electrons and gets reduced in a chemical reaction. It plays a crucial role by facilitating the oxidation of another species. In the exercise, oxygen (\text{O}_2) acts as the oxidizing agent. The relevant half-reaction:

\( \frac{1}{2} \text{O}_{2} + 2 \text{H}^{+} + 2 e^{-} \rightarrow \text{H}_{2}\text{O} \)

Here, \text{O}_2 accepts electrons and is reduced to water, enabling the oxidation of NADH. Identifying oxidizing agents is important for understanding how electron acceptors drive metabolic reactions.

reducing agent

A reducing agent is a substance that donates electrons and gets oxidized in a chemical reaction. It allows the reduction of another species. In the exercise, NADH is the reducing agent:

\( \text{NADH} + \text{H}^{+} \rightarrow \text{NAD}^{+} + 2 \text{H}^{+} + 2 e^{-} \)

NADH donates electrons and is oxidized to NAD⁺, driving the reduction of \text{O}_2. Recognizing reducing agents helps you understand how electron donors contribute to various metabolic pathways.

electron transfer

Electron transfer is the movement of electrons from one molecule to another and forms the basis of redox reactions. In metabolism, these transfers are key to energy production. In the exercise, electrons are transferred from NADH to \text{O}_2. This process is evident when you combine the half-reactions:

\( \frac{1}{2} \text{O}_{2} + 2 \text{H}^{+} + 2 e^{-} + \text{NADH} + \text{H}^{+} \rightarrow \text{H}_{2}\text{O} + \text{NAD}^{+} + 2 \text{H}^{+} + 2 e^{-} \)

Simplifying, we get:

\( \frac{1}{2} \text{O}_{2} + \text{NADH} + \text{H}^{+} \rightarrow \text{H}_{2}\text{O} + \text{NAD}^{+} \)

Recognizing the role of electron transfer in redox reactions aids in understanding energy flow and biochemical processes.