Cellular Respiration: ETC and Chemiosmosis Notes

Cellular Respiration: ETC and Chemiosmosis

Overview

  • Majority of ATP production occurs via the electron transport chain (ETC).
  • ETC takes place across the inner mitochondrial membrane.
  • Involves the transfer of high-energy electrons (from NADH & FADH2) through a series of carrier proteins.

Electron Transport Chain (ETC) Steps

  1. Electron Transfer and Energy Release:

    • As electrons move down the chain, energy is released.
    • This energy fuels the pumping of hydrogen ions (H^+) across the inner mitochondrial membrane.
    • H^+ is moved from the matrix to the intermembrane space, establishing a concentration gradient.
  2. Chemiosmosis and ATP Synthesis:

    • The resulting concentration gradient (high H^+ in the intermembrane space, low H^+ in the matrix) drives H^+ through ATP synthase.
    • ATP synthase is a membrane-embedded protein complex.
    • The flow of H^+ through ATP synthase powers the conversion of ADP to ATP.
  3. Final Electron Acceptor:

    • Oxygen acts as the final electron acceptor in the ETC.
    • Oxygen accepts electrons and hydrogen ions to form water (H_2O).
    • The water molecule is released as a byproduct.

Role of NADH and FADH2

  • NADH and FADH2 deliver electrons to the electron transport chain.
  • They are produced during glycolysis and the Krebs cycle.
  • After donating electrons, they are recycled to NAD+ and FAD, respectively.
  • NAD+ and FAD can then be reused in glycolysis and the Krebs cycle.

Energy Release and H+ Gradient

  • As electrons move from one protein to another in the ETC, a small amount of energy is released.
  • Released energy is used to pump H^+ into the intermembrane space, creating a concentration gradient.

Oxygen as the Final Electron Acceptor

  • Oxygen is crucial for the ETC to function.
  • The reduction of oxygen to form water is represented by the equation: 2H^+ + \frac{1}{2}O2 + 2e^- \rightarrow H2O

Consequences of Oxygen Deprivation

  • If oxygen is absent, the ETC backs up.
  • NADH and FADH2 cannot be recycled back to NAD+ and FAD.
  • This backup affects the entire process, including glycolysis.
  • Lack of oxygen leads to no ATP production leading to cell death which in turn leads to organism death.

Chemiosmosis Details

  • Chemiosmosis is the process responsible for producing the majority of ATP in cellular respiration i.e., ~$32 ATP per glucose molecule.
  • Chemiosmosis necessitates:
    • An ATP synthase channel located in the inner mitochondrial membrane.
    • The buildup of H^+ in the intermembrane space due to the electron transport chain.

Mechanism of Chemiosmosis

  • The inner mitochondrial membrane is generally impermeable to H^+ ions.
  • The ATP synthase channel provides the only route for H^+ to diffuse back into the matrix.
  • As H^+ flows down its concentration gradient through ATP synthase, energy is released.
  • ATP synthase harnesses this energy to synthesize ATP.
  • The yield is ~32 ATP per glucose molecule.

Summary of Aerobic Cellular Respiration

  • Four main stages: glycolysis, Krebs cycle preparation, Krebs cycle, and electron transport system.
    • Glycolysis
      • Occurs in the cytoplasm.
      • Glucose is split into two three-carbon molecules called pyruvate.
      • A small amount of ATP is produced.
      • Proceeds without oxygen.
    • Krebs Cycle Preparation
      • Occurs in the matrix of the mitochondrion.
      • Pyruvate is used to make a molecule called acetyl CoA.
      • Carbon dioxide is released.
    • Krebs Cycle
      • Processes acetyl CoA through a series of reactions that extract electrons and hydrogen ions.
      • A small amount of ATP is produced.
      • Electrons and hydrogen ions are carried to an electron transport system.
      • Carbon dioxide is released.
    • Electron Transport System
      • Occurs inside the mitochondrion.
      • Electrons are transferred through a series of molecules that accept and then pass on the electrons.
      • A large amount of ATP is produced.
      • Oxygen is the final acceptor of electrons and combines with hydrogen ions to form water.

ATP Production Numbers

  • Glycolysis: 2 ATP
  • Krebs Cycle: 2 ATP
  • Electron Transport System: Up to 32 ATP
  • Total: ~36 ATP per glucose molecule