Cellular Respiration: Electron Transport System

Study Notes

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

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).

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.

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.

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.

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.

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

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.

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.