2.2 Part 2: Electron Transport Chain & ATP Synthase

The Electron Transport Chain (ETC) is a crucial metabolic pathway involved in cellular respiration.
Location: It consists of a series of protein complexes embedded within the inner mitochondrial membrane.
Primary Function: To transfer electrons, captured from donor molecules, through these protein complexes.
Coupled Process: This electron transfer is intrinsically coupled with the pumping of hydrogen ions (H^+).
Gradient Generation: The pumping of H^+ ions creates a proton gradient across the membrane.
ATP Synthesis: The potential energy stored in this proton gradient is then utilized by the ATP synthase complex to synthesize ATP (adenosine triphosphate), the primary energy currency of the cell.

NADH Dehydrogenase (Complex I): The initial complex where electrons from NADH enter the chain.
Cytochrome bc1 (Complex III): An intermediate complex involved in electron transfer.
Cytochrome Oxidase (Complex IV): The terminal complex where electrons are transferred to molecular oxygen.
ATP Synthase (Complex V): The enzyme complex responsible for synthesizing ATP using the proton gradient.

Ubiquinone (Coenzyme Q): A lipid-soluble electron carrier that moves electrons between NADH dehydrogenase and cytochrome bc1.
Cytochrome c: A small, water-soluble protein that carries electrons between cytochrome bc1 and cytochrome oxidase.

NADH: A high-energy electron donor molecule, providing 2 electrons at the start of the chain.
Electrons: The fundamental particles being transferred, carrying energy.
Hydrogen Ions (H^+ / Protons): Pumped across the membrane to create the electrochemical gradient.
Molecular Oxygen (O_2): The final electron acceptor in the chain.
Water (H_2O): A product formed at the end of the ETC.
ADP (Adenosine Diphosphate) and Pi (Inorganic Phosphate): The substrates used by ATP synthase to form ATP.

Electron Entry: 2 electrons are passed from NADH into the NADH dehydrogenase complex.
Proton Pumping: Coupled with this transfer, one hydrogen ion (H^+) is pumped for each electron passed. This means a total of 2 H^+ ions are pumped across the membrane at this stage.
Electron Transfer: The 2 electrons are then transferred from NADH dehydrogenase to Ubiquinone.

Mobility: Ubiquinone acts as a mobile transfer molecule, physically moving the 2 electrons from NADH dehydrogenase to the Cytochrome bc1 complex.

Electron Transfer: Each electron is then passed from the cytochrome b c one complex to Cytochrome c. This occurs one electron at a time.
Proton Pumping: As each electron is transferred through this complex, one hydrogen ion (H^+) is pumped across the membrane.

Cytochrome c accepts each electron one at a time and transfers it to the Cytochrome Oxidase complex.

Electron Requirement: This final major step requires a total of four electrons (4e^-).
Molecular Interaction: These four electrons interact with:
- One molecular oxygen molecule (O_2).
- Eight hydrogen ions (8H^+).
Water Formation: Four of the hydrogen ions (4H^+) combine with the four electrons (4e^-) and the molecular oxygen (O2) to form two water molecules (2H2O).
- Reaction: 4e^- + 4H^+ + O2 ightarrow 2H2O
Proton Pumping: The remaining four (4H^+) hydrogen ions (from the initial eight) are pumped across the inner mitochondrial membrane.

Gradient Formation: The continuous series of hydrogen ion pumping steps ( 2H^+ at Complex I, 2H^+ at Complex III, and 4H^+ at Complex IV, totaling 8H^+ for 4 electrons) creates a significant electrochemical gradient across the inner mitochondrial membrane.
Potential Energy: This gradient represents a store of potential energy.
ATP Synthase Action: The potential energy stored in this proton gradient is precisely what ATP synthase uses to drive the synthesis of ATP.
Mechanism: ATP synthase facilitates the binding of ADP and inorganic phosphate (Pi) to form ATP ( ADP + Pi
ightarrow ATP ).
Detailed Explanation: The specific steps involved in ATP synthesis by ATP synthase are discussed in greater detail in various resources, such as the referenced