Cellular Respiration: Electron Transport System

Electron Transport System (ETS)

• Final step in cellular respiration, involving protein complexes and mobile electron carriers.
• Electrons from NADH and FADH2 transfer through ETS, resembling a bucket brigade mechanism.
• Electrons transition from carriers with lower redox potential to those with higher potential.
• Four main types of electron carriers:
  • Cytochromes
  • Flavoproteins
  • Iron-sulfur proteins
  • Quinones

Aerobic Respiration

• Oxygen (O2) is the final electron acceptor, reducing to water (H2O).
• Cytochrome oxidase is the last electron carrier, differs among bacteria and aids in identification (e.g., Pseudomonas aeruginosa vs. Vibrio cholerae).

Conditions for Alternative Respiration

• Aerobic respiration may be limited by:
  • Absence of genes for specific cytochrome oxidases.
  • Lack of mechanisms to handle harmful oxygen radicals like hydrogen peroxide.
  • Insufficient oxygen availability.
• Alternative pathways involve anaerobic respiration using inorganic molecules as electron acceptors.
• Denitrifiers reduce nitrate (NO3-) and nitrite (NO2-) to nitrogen gas (N2).
• Some bacteria, such as E. coli, can switch to nitrate usage when oxygen is scarce.

Energy Dynamics in Electron Transfer

• Electrons lose energy during their transfer in the electron transport system (ETS).
• Some of the released energy is utilized to pump hydrogen ions (H+) across membranes.

Hydrogen Ion Pumping

• In prokaryotic cells, H+ is pumped outside the cytoplasmic membrane into the periplasmic space.
• In eukaryotic cells, H+ is pumped from the mitochondrial matrix to the intermembrane space.

Electrochemical Gradient

• An uneven distribution of H+ creates an electrochemical gradient.
• Higher H+ concentration leads to a proton motive force (PMF) and makes the side with more H+ more acidic.

Uses of Proton Motive Force (PMF)

• PMF facilitates ATP synthesis and powers processes like nutrient transport and flagella movement.

Chemiosmosis

• H+ diffuse across membranes through channels in ATP synthase, likened to water flowing through a dam.
• ATP synthase functions as a generator, turning as H+ flows through it.

ATP Synthase Function

• H+ movement leads to ATP regeneration from ADP and inorganic phosphate (Pi) via oxidative phosphorylation.
• In prokaryotes, H+ moves into the cytoplasm; in eukaryotes, H+ flows into the mitochondrial matrix.