Cellular Respiration: Electron Transport Chain and Chemiosmosis

Cellular Respiration Step 4: The Electron Transport Chain and Chemiosmosis

Electron Transport Chain (ETC)

• Definition: A series of protein complexes located on the inner mitochondrial membrane that facilitate electron transport.
• Function: Receives electrons from NADH and FADH2, creating an energy cascade that generates energy.
• Arrangement:
  • Proteins are arranged in order of electron attraction strength (weakest to strongest).
  • Electrons flow through these proteins, culminating in oxygen, which is the terminal electron acceptor.
• Key Mechanism:
  • As electrons move, they release energy used to pump protons (H+) from the mitochondrial matrix into the intermembrane space via proton pumps.
  • The resultant electrochemical gradient is critical for ATP synthesis.
  • Oxygen combines with the electrons and protons to form water (H2O).

Key Components of the Electron Transport Chain

• NADH and FADH2:
  • Both are carriers of electrons generated during earlier stages (glycolysis and Krebs cycle).
  • NADH can produce 3 ATP; FADH2 can produce 2 ATP.
  • NADH is typically derived from glycolysis and the conversion of pyruvate to acetyl-CoA.
• Proton Pumps: Move H+ into the intermembrane space, contributing to the proton gradient.
  • Key Complexes:
  • NADH Dehydrogenase: First complex that accepts electrons from NADH.
  • Cytochrome b-c1 Complex: Intermediary transfer of electrons.
  • Cytochrome Oxidase Complex: Final complex that transfers electrons to oxygen.
• Ubiquinone and Cytochrome C: Serve as mobile carriers of electrons within the chain.

Chemiosmosis

• Definition: The process by which the stored energy in the proton gradient is converted into ATP.
• Mechanism:
  • Protons in the intermembrane space flow down their gradient through ATP synthase, a protein channel that synthesizes ATP from ADP and inorganic phosphate (Pi).
• Proton-Motive Force: The energy generated from protons moving through the ATP synthase creates a powerful driving force for ATP generation.

ATP Yield

• Each NADH contributes to the production of 3 ATP.
• Each FADH2 contributes to the production of 2 ATP.
• Maximal Theoretical Yield:
  • Glycolysis produces 4 ATP and 2 NADH.
  • One cycle of the Krebs cycle generates 6 NADH and 2 FADH2, contributing a total of 18 ATP.
• Total Theoretical ATP Yield from Glucose Oxidation: 36 ATP.

Control of Aerobic Respiration

• Regulation: Feedback inhibition and product activation loops controlled by various metabolites.
  • Positive Regulators:
  • ATP and ADP levels affect the activity of key enzymes like phosphofructokinase.
  • Negative Regulators:
  • Fructose 1,6-bisphosphate inhibits pyruvate decarboxylase.
  • Acetyl-CoA and citrate can inhibit pathways in Krebs cycle.

Summary of Pathways Involved

• Glycolysis: Breaks down glucose to pyruvate, generating NADH and direct ATP.
• Krebs Cycle: Processes acetyl-CoA to produce NADH and FADH2 for the ETC.
• ETC: Transports electrons, pumps protons, establishes a gradient for ATP synthesis via oxidation phosphorylation (chemiosmosis).