Question

The ultimate electron acceptor in the respiration process is molecular oxygen. Electron transfer through the respiratory chain takes place through a complex series of oxidationreduction reactions. Some of the electron transport steps use iron-containing proteins called \(c y\) tochromes. All cytochromes transport electrons by converting the iron in the cytochromes from the +3 to the +2 oxidation state. Consider the following reduction potentials for three different cytochromes used in the transfer process of electrons to oxygen (the potentials have been corrected for \(\mathrm{pH}\) and for temperature): cytochrome \(\mathrm{a}\left(\mathrm{Fe}^{3+}\right)+\mathrm{e}^{-} \longrightarrow\) cytochrome \(\mathrm{a}\left(\mathrm{Fe}^{2+}\right)\) $$ \begin{array}{c}\mathscr{C}=0.385 \mathrm{~V}\end{array} $$ cytochrome \(\mathrm{b}\left(\mathrm{Fe}^{3+}\right)+\mathrm{e}^{-} \longrightarrow\) cytochrome \(\mathrm{b}\left(\mathrm{Fe}^{2+}\right)\) $$ \begin{array}{l}\mathscr{E}=0.030 \mathrm{~V}\end{array} $$ cytochrome \(\mathrm{c}\left(\mathrm{Fe}^{3+}\right)+\mathrm{e}^{-} \longrightarrow \mathrm{cytochrome} \mathrm{c}\left(\mathrm{Fe}^{2+}\right)\) $$ \begin{array}{c}\mathscr{C}=0.254 \mathrm{~V}\end{array} $$ In the electron transfer series, electrons are transferred from one cytochrome to another. Using this information, determine the cytochrome order necessary for spontaneous transport of electrons from one cytochrome to another, which eventually will lead to electron transfer to \(\mathrm{O}_{2}\).

Short answer

The correct order for spontaneous electron transfer in the cytochromes is: Cytochrome b \(\rightarrow\) Cytochrome c \(\rightarrow\) Cytochrome a. This order ensures that the transfer process occurs spontaneously and ultimately leads to electron transfer to molecular oxygen.

Step-by-step solution

List down the reduction potentials

We are given three reduction potentials for cytochromes: Cytochrome a: \(E_{a}=0.385 \mathrm{~V}\) Cytochrome b: \(E_{b}=0.030 \mathrm{~V}\) Cytochrome c: \(E_{c}=0.254 \mathrm{~V}\)

Arrange the cytochromes in order of increasing reduction potential

First, we need to arrange the three given cytochromes in order of increasing reduction potential: Cytochrome b: \(E_{b}=0.030 \mathrm{~V}\) < Cytochrome c: \(E_{c}=0.254 \mathrm{~V}\) < Cytochrome a: \(E_{a}=0.385 \mathrm{~V}\)

Determine the correct order for spontaneous electron transfer

For the electron transfer process to occur spontaneously, the final acceptor must have a more positive reduction potential than the initial donor. Therefore, the correct order for spontaneous electron transfer is Cytochrome b \(\rightarrow\) Cytochrome c \(\rightarrow\) Cytochrome a. This means that electrons are first transferred from Cytochrome b (with the lowest reduction potential) to Cytochrome c, and then from Cytochrome c to Cytochrome a (with the highest reduction potential). This order ensures that the electron transfer process occurs spontaneously and ultimately leads to the electron transfer to molecular oxygen.

Key concepts

Electron Transport Chain

The electron transport chain is a crucial part of cellular respiration that takes place in the mitochondria. Its primary purpose is to transfer electrons from electron donors to electron acceptors via a series of reactions. This process helps in generating a proton gradient across the mitochondrial membrane, which is then used to produce ATP.

The electron transport chain comprises various protein complexes and small organic molecules. As electrons pass through these complexes, protons are pumped from the mitochondrial matrix into the intermembrane space, creating a proton gradient. This gradient provides the energy needed for ATP synthesis through an enzyme called ATP synthase.

• Electrons are transferred by protein complexes like Complex I, II, III, and IV.
• The final electron acceptor in this chain is oxygen, which combines with electrons and protons to form water.

Ensuring a correct flow of electrons through the chain is essential for the efficient production of energy.

Cytochromes

Cytochromes are a group of iron-containing proteins that play an integral role in the electron transport chain. Each cytochrome acts as an electron carrier during the transfer of electrons through the chain.

Here's how cytochromes work:

• Cytochromes hold iron atoms, which can oscillate between two oxidation states: +2 (reduced) and +3 (oxidized).
• During electron transport, cytochromes pick up electrons, reducing the iron from +3 to +2.
• When passing electrons to the next carrier, the iron gets oxidized back to +3.

These redox transformations help in maintaining the essential flow of electrons. Different cytochromes, such as cytochrome a, b, and c, have varying reduction potentials which dictate the order of electron transfer in the chain.

Reduction Potential

Reduction potential, also known as redox potential, is a measure of a substance's ability to gain electrons and be reduced. It is expressed in volts (V) and is crucial in determining the direction of electron flow.

Within the electron transport chain:

• Substances with higher reduction potentials will accept electrons more easily than those with lower potentials.
• The order of the cytochromes' reduction potentials is pivotal in ensuring the spontaneous flow of electrons.

For instance, in the given problem, Cytochrome b (0.030 V) transfers electrons to Cytochrome c (0.254 V), which then transfer to Cytochrome a (0.385 V), and finally to oxygen, the ultimate electron acceptor. This order enables efficient and spontaneous electron transfer.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, or redox reactions, are fundamental chemical reactions that involve the transfer of electrons between substances. In one part of the reaction, a substance loses electrons (oxidation) while another gains electrons (reduction).

In the context of the electron transport chain:

• Cytochromes undergo redox changes as they shuttle electrons from one complex to another.
• Every transfer involves a cytochrome accepting electrons (reduction) and then passing them on (oxidation).
• This series of redox reactions is vital for maintaining the energy flow in cells.

The precision of these reactions ensures that energy is efficiently captured and utilized to form ATP, the cell's energy currency. Understanding these transformations is essential for grasping how cells generate energy through respiration.