Chapter 23, Problem 23.89EP

Indicate whether each of the following changes represents oxidation or reduction.

1. a. CoQH₂ → CoQ
2. b. NAD⁺ → NADH
3. c. Cyt c (Fe²⁺) → cyt c (Fe³⁺)
4. d. Cyt b (Fe³⁺) → cyt b (Fe²⁺)

Interpretation Introduction

Interpretation:

Whether the change ${\text{CoQH}}_2\to \text{CoQ}$ represents oxidation or reduction has to be identified.

Concept Introduction:

Electron transport chain is a sequence of biochemical reactions in which electrons and hydrogen atoms from the citric acid cycle are transferred to various intermediate carriers and finally reacts with molecular oxygen to form a water molecule.

There are four complexes associated with the electron transport chain that is present in the inner mitochondrial membrane. The four complexes that help in the electron transfer in the electron transport chain are:

Complex I: $\text{NADH-coenzyme}\text{Q reductase}$

Complex II: $\text{Succinate-coenzyme}\text{Q reductase}$

Complex III: $\text{Coenzyme}\text{Q-cytochrome c reductase}$

Complex IV: $\text{Cytochrome c oxidase}$

An overview of the electron transport chain is as follows:

Redox reactions involve oxidation and reduction reaction occurring simultaneously so that one species is oxidized and the other one is reduced. The species that gain hydrogen or electron is known as reduced form and the species that loss hydrogen or electron is known as oxidized form. The general representation of the redox reaction is,

  $\text{A}+{\text{H}}^{+}+{\text{e}}^{-}\to \text{AH}$

Here A is oxidized form and AH is reduced form.

Answer to Problem 23.89EP

The change ${\text{CoQH}}_2\to \text{CoQ}$ represents oxidation as there is a loss of hydrogen in the reaction.

Explanation of Solution

$\text{Coenzyme Q}$ is used to transfer the electrons from the complex I, II to complex III. $\text{Coenzyme Q}$ exists in two forms: ${\text{CoQH}}_2$(reduced form) and $\text{CoQ}$(oxidized form).  The change of ${\text{CoQH}}_2$ to $\text{CoQ}$ represent the oxidation reaction as hydrogen atoms are lost.  The reaction of the reduction of ${\text{CoQH}}_2$ is:

  ${\text{CoQH}}_2\to \text{CoQ}+2{\text{H}}^{+}+2{\text{e}}^{-}$

Interpretation Introduction

Interpretation:

Whether the change ${\text{NAD}}^{+}\to \text{NADH}$ represents oxidation or reduction has to be identified.

Concept Introduction:

Electron transport chain is a sequence of biochemical reactions in which electrons and hydrogen atoms from the citric acid cycle are transferred to various intermediate carriers and finally reacts with molecular oxygen to form a water molecule.

There are four complexes associated with the electron transport chain that is present in the inner mitochondrial membrane. The four complexes that help in the electron transfer in the electron transport chain are:

Complex I: $\text{NADH-coenzyme}\text{Q reductase}$

Complex II: $\text{Succinate-coenzyme}\text{Q reductase}$

Complex III: $\text{Coenzyme}\text{Q-cytochrome c reductase}$

Complex IV: $\text{Cytochrome c oxidase}$

An overview of the electron transport chain is as follows:

Redox reactions involve oxidation and reduction reaction occurring simultaneously so that one species is oxidized and the other one is reduced. The species that gain hydrogen or electron is known as reduced form and the species that loss hydrogen or electron is known as oxidized form. The general representation of the redox reaction is,

  $\text{A}+{\text{H}}^{+}+{\text{e}}^{-}\to \text{AH}$

Here A is oxidized form and AH is reduced form.

Answer to Problem 23.89EP

The change ${\text{NAD}}^{+}\to \text{NADH}$ represents reduction as there is a gain of hydrogen in the reaction.

Explanation of Solution

In complex I, electrons are transferred from the $\text{NADH}$ molecule produced in the citric acid cycle. Initially $\text{NADH}$ interacts with complex I and gets oxidized to form ${\text{NAD}}^{+}$. The reaction of the reduction of ${\text{NAD}}^{+}$ is:

  ${\text{NAD}}^{+}+2{\text{H}}^{+}+2{\text{e}}^{-}\stackrel{\text{Reduction}}{\to }\text{NADH}+{\text{H}}^{+}$

The change of ${\text{NAD}}^{+}$ to $\text{NADH}$ represent the reduction reaction as hydrogen atoms are gained in the reaction.

Interpretation Introduction

Interpretation:

Whether the change $\text{cyt}\text{c}\left({\text{Fe}}^{2+}\right)\to \text{cyt}\text{c}\left({\text{Fe}}^{3+}\right)$ represents oxidation or reduction has to be identified.

Concept Introduction:

Electron transport chain is a sequence of biochemical reactions in which electrons and hydrogen atoms from the citric acid cycle are transferred to various intermediate carriers and finally reacts with molecular oxygen to form a water molecule.

There are four complexes associated with the electron transport chain that is present in the inner mitochondrial membrane. The four complexes that help in the electron transfer in the electron transport chain are:

Complex I: $\text{NADH-coenzyme}\text{Q reductase}$

Complex II: $\text{Succinate-coenzyme}\text{Q reductase}$

Complex III: $\text{Coenzyme}\text{Q-cytochrome c reductase}$

Complex IV: $\text{Cytochrome c oxidase}$

An overview of the electron transport chain is as follows:

Redox reactions involve oxidation and reduction reaction occurring simultaneously so that one species is oxidized and the other one is reduced. The species that gain hydrogen or electron is known as reduced form and the species that loss hydrogen or electron is known as oxidized form. The general representation of the redox reaction is,

  $\text{A}+{\text{H}}^{+}+{\text{e}}^{-}\to \text{AH}$

Here A is oxidized form and AH is reduced form.

Answer to Problem 23.89EP

The change $\text{cyt}\text{c}\left({\text{Fe}}^{2+}\right)\to \text{cyt}\text{c}\left({\text{Fe}}^{3+}\right)$ represents oxidation as there is a loss of an electron in the reaction.

Explanation of Solution

$\text{cyt c}$ is used to transfer the electrons from complex III to complex IV and consists of iron that changes its oxidation state. Iron in $\text{cyt}\text{c}\left({\text{Fe}}^{2+}\right)$ undergoes oxidation and leads to the formation of $\text{cyt}\text{c}\left({\text{Fe}}^{3+}\right)$. The reaction to this change is:

  $\text{cyt}\text{c}\left({\text{Fe}}^{2+}\right)\to \text{cyt}\text{c}\left({\text{Fe}}^{3+}\right)+{\text{e}}^{-}$

Interpretation Introduction

Interpretation:

Whether the change $\text{cyt}\text{b}\left({\text{Fe}}^{3+}\right)\to \text{cyt}\text{b}\left({\text{Fe}}^{2+}\right)$ represents oxidation or reduction has to be indicated.

Concept Introduction:

Electron transport chain is a sequence of biochemical reactions in which electrons and hydrogen atoms from the citric acid cycle are transferred to various intermediate carriers and finally reacts with molecular oxygen to form a water molecule.

There are four complexes associated with the electron transport chain that is present in the inner mitochondrial membrane. The four complexes that help in the electron transfer in the electron transport chain are:

Complex I: $\text{NADH-coenzyme}\text{Q reductase}$

Complex II: $\text{Succinate-coenzyme}\text{Q reductase}$

Complex III: $\text{Coenzyme}\text{Q-cytochrome c reductase}$

Complex IV: $\text{Cytochrome c oxidase}$

An overview of the electron transport chain is as follows:

Redox reactions involve oxidation and reduction reaction occurring simultaneously so that one species is oxidized and the other one is reduced. The species that gain hydrogen or electron is known as reduced form and the species that loss hydrogen or electron is known as oxidized form. The general representation of the redox reaction is,

  $\text{A}+{\text{H}}^{+}+{\text{e}}^{-}\to \text{AH}$

Here A is oxidized form and AH is reduced form.

Answer to Problem 23.89EP

The change $\text{cyt}\text{b}\left({\text{Fe}}^{3+}\right)\to \text{cyt}\text{b}\left({\text{Fe}}^{2+}\right)$ represents reduction as there is a gain of an electron in the reaction.

Explanation of Solution

$\text{Cyt}\text{b}\left({\text{Fe}}^{3+}\right)$ is a structural component of the complex III and consists of iron that changes its oxidation state from $+3$ to $+2$ in the complex.  Iron in $\text{cyt}\text{b}\left({\text{Fe}}^{3+}\right)$ undergoes reduction and leads to the formation of $\text{cyt}\text{b}\left({\text{Fe}}^{2+}\right)$. The reaction to this change is:

  $\text{cyt}\text{b}\left({\text{Fe}}^{3+}\right)+{\text{e}}^{-}\stackrel{\text{Reduction}}{\to }\text{cyt}\text{b}\left({\text{Fe}}^{2+}\right)$