Question

A molecule of ATP is formed when electron passes from (a) Cyt c to Cyt a (b) Cyt a to Cyt c (c) Cyt b to Cyt c \(_{1}\) (d) Cyt c to Cyt b

Short answer

The correct answer is (c) Cyt b to Cyt c1, as this electron transfer is a part of the electron transport chain that leads to ATP synthesis.

Step-by-step solution

Understanding electron transfer

In the electron transport chain, electrons move from donor to acceptor molecules. Cytochromes serve as electron carriers, helping to transfer electrons from one complex to another. The flow of electrons leads to the pumping of protons across the inner mitochondrial membrane, setting up a gradient that drives ATP synthesis.

Identifying the correct electron transfer

Given the options, the correct electron transfer that leads to ATP synthesis would involve an initial electron donor (from where the electron is coming from) and an electron acceptor (where the electron is going to). The electron flow in the electron transport chain typically proceeds from Cyt b to Cyt c1 (a part of the Cyt bc1 complex), then to Cyt c. This order follows the increasing reduction potential of the cytochromes.

Answering the Exercise

From the steps above, it is evident that the correct answer is (c) Cyt b to Cyt c1. The electron flow from Cyt b to Cyt c1 and subsequently to Cyt c is a crucial part of the electron transport chain that leads to ATP synthesis.

Key concepts

Electron Transport Chain

The electron transport chain (ETC) is a series of complexes located in the inner mitochondrial membrane. Its primary role is to facilitate the transfer of electrons from electron donors to electron acceptors, ultimately leading to the synthesis of ATP, the energy currency of the cell.
Electrons are transported through a sequence of proteins and molecules, which allows for the gradual release of energy. This energy is used to pump protons (H⁺ ions) across the membrane, creating a proton gradient that is essential for ATP production.

• The electron transport chain begins with the electron donors, such as NADH and FADH₂, which deposit electrons into the chain.
• The electrons move through a series of complexes (I, II, III, and IV), each playing a specific role in electron transport and proton pumping.
• This systematic movement of electrons ensures the efficient production of a proton gradient used by ATP synthase to generate ATP.

The chain ensures oxidative phosphorylation, coupling electron transport to ATP synthesis, an elegant solution for cellular energy management.

Cytochromes

Cytochromes are heme-containing proteins that play a crucial role in the electron transport chain by acting as electron carriers. These specialized proteins facilitate the movement of electrons through the chain, contributing to the formation of the proton gradient.
Each cytochrome has a heme group that can undergo oxidation and reduction, enabling it to accept and donate electrons efficiently. There are several types of cytochromes, each with unique properties and roles in the ETC.

• Cytochrome b: Part of the cytochrome bc1 complex, it plays a part in transferring electrons to cytochrome c₁.
• Cytochrome c₁: Receives electrons from cytochrome b and passes them to cytochrome c.
• Cytochrome c: A small, soluble electron carrier that transfers electrons between complex III and complex IV.

These proteins help maintain the flow of electrons, ensuring the continuation of the electron transport chain and effective ATP synthesis.

Proton Gradient

A proton gradient is a vital component for ATP synthesis, created by the electron transport chain in the inner mitochondrial membrane. This gradient is formed when energy released from electron transport is used to pump protons from the mitochondrial matrix to the intermembrane space.
The proton gradient establishes an electrochemical potential difference across the membrane, sometimes called "proton motive force." This force drives protons back into the matrix via ATP synthase, an enzyme responsible for synthesizing ATP.

• The movement of protons through ATP synthase causes it to rotate mechanically, which catalyzes the conversion of ADP and inorganic phosphate (Pi) into ATP.
• This process is known as chemiosmosis, where chemical energy stored in the proton gradient is harnessed to produce ATP.
• An intact membrane is critical for maintaining the gradient, as any leakage can reduce efficiency and limit ATP production.

The proton gradient is a key mechanism connecting electron transport to ATP production, showcasing the intricate design of cellular energy transformation.

Electron Carriers

Electron carriers are integral to the electron transport chain, allowing electrons to move efficiently between complexes within the chain. These carriers include both protein-bound and small molecule carriers, each facilitating the transfer of high-energy electrons.
Two primary types of electron carriers used in the ETC are NAD⁺/NADH and FAD/FADH₂.

• NADH: Donates electrons to the first complex, NADH dehydrogenase (Complex I), where electrons are released for further transport.
• FADH₂: Donates electrons to Complex II, the succinate dehydrogenase complex, also contributing to electron flow along the chain.
• These carrier molecules are regenerated and reused, allowing the cycle of electron donation and acceptance to continue.

Electron carriers, by accepting and donating electrons, set the stage for cellular respiration and energy production, underscoring their significance in sustaining cellular life.