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 cytochromes. 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): $$\begin{aligned} &\text { cytochrome } \mathrm{a}\left(\mathrm{Fe}^{3+}\right)+\mathrm{e}^{-} \longrightarrow \text { cytochrome } \mathrm{a}\left(\mathrm{Fe}^{2+}\right)\ &\mathscr{E}^{\circ}=0.385 \mathrm{V}\\\ &\text { cytochrome } \mathbf{b}\left(\mathrm{Fe}^{3+}\right)+\mathrm{e}^{-} \longrightarrow \text { cytochrome } \mathrm{b}\left(\mathrm{Fe}^{2+}\right)\ &\mathscr{E}^{\circ}=0.030 \mathrm{V}\\\ &\text { cytochrome } c\left(\mathrm{Fe}^{3+}\right)+\mathrm{e}^{-} \longrightarrow \text { cytochrome } \mathrm{c}\left(\mathrm{Fe}^{2+}\right)\ &\mathscr{E}^{\circ}=0.254 \mathrm{V} \end{aligned}$$ 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 \(\mathrm{O}_{2}\)

Step by step solution

01

Understanding Reduction Potentials

The reduction potential of a chemical species measures its ability to gain electrons i.e., its affinity for electrons. Higher the reduction potential, greater is the species' affinity for electrons. In certain reactions, molecules donate electrons, while in others, they gain electrons. Here, cytochromes are gaining an electron and converting iron from +3 to +2 state.

02

Identifying the Reduction Potentials

From the provided information, we have the reduction potentials of three cytochromes. - Cytochrome a: 0.385 V - Cytochrome b: 0.030 V - Cytochrome c: 0.254 V

03

Arranging Cytochromes in Increasing Order of Potential

For spontaneous electron transfer, electrons flow from a lesser potential cytochrome to a higher potential cytochrome. So we arrange the given cytochromes in increasing order of their reduction potential. 1. Cytochrome b 2. Cytochrome c 3. Cytochrome a

04

Conclusion

The order in which the cytochromes should be arranged for an electron to spontaneously move from one to another along the electron transfer chain and eventually be transferred to \( \mathrm{O}_2 \) is from cytochrome b to cytochrome c, and eventually to cytochrome a (b → c → a). This is because it sequentially increases in their affinity (reduction potential) for electrons. This series will ensure that each successive cytochrome has a higher affinity for electrons than the previous one, creating a flow of electrons through the chain.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Cytochromes

Cytochromes are integral proteins that play a crucial role in cellular respiration through the electron transfer chain, found in the mitochondria of eukaryotic cells and in the cell membranes of prokaryotes. These proteins are unique as they contain a heme group, a complex molecule with an iron atom at its center. This iron is where the magic happens; it can alternate between different oxidation states, typically between Fe²⁺ (ferrous) and Fe³⁺ (ferric), enabling the cytochrome to transfer electrons.

During respiration, as electrons pass through the chain of cytochromes, the iron within these heme groups gets reduced (gains an electron) and oxidized (loses an electron) in a cyclical process. This transition of iron ions between their oxidation states is pivotal, acting as a microscopic shuttle for electrons. Not only does the electron transfer drive the production of ATP – the cell's energy currency – but it also contributes to the generation of a gradient used for energy storage. As electrons move stepwise from cytochrome to cytochrome, they carry energy with them, which is harnessed to pump protons across the mitochondrial membrane, creating an electrochemical gradient known as the proton motive force.

Reduction Potentials

Reduction potentials, often represented in volts (V), are a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. When analyzing reduction potentials in biological systems, we gauge an entity's electron affinity. The higher the reduction potential, the greater the affinity for electrons, and the more likely the substance is to become reduced in a redox reaction.

Reduction potentials are fundamental in determining the direction of electron flow in the electron transfer chain. Electrons naturally move from species with lower reduction potential to those with higher potential. This concept explains how the electron transfer chain operates, with electrons spontaneously traveling from donors (like NADH or FADH₂) to acceptors (like oxygen) via a series of redox reactions. In the solved exercise, the student had to arrange the cytochromes according to their reduction potentials to ensure that the flow of electrons from one to another was energetically favorable, leading to the efficient production of ATP.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, or redox reactions, are processes where electrons are transferred between molecules, resulting in changes in their oxidation states. These reactions are foundations of energy transfer in biological systems, particularly respiration and photosynthesis.

In the context of the electron transfer chain, the cytochromes alternately undergo oxidation (loss of electrons) and reduction (gain of electrons) as the electrons move from one component to another. The key to understanding these reactions is to identify the flow of electrons: from a donor, which gets oxidized as it loses electrons, to an acceptor, which gets reduced as it gains electrons. Oxidation and reduction always occur together; when one substance is oxidized, another is reduced in a complementary fashion. This exchange of electrons is harnessed by the cell to generate ATP, illustrating the critical nature of redox reactions in cellular metabolism.