Electron Transport Chain and Oxidative Phosphorylation

Electron Transport Chain and Oxidative Phosphorylation

Only four ATP molecules is directly synthesized when one glucose is oxidized to six CO₂ molecules by way of glycolysis and the TCA cycle. Most ATP generated comes from the oxidation of NADH and FADH2 in the electron transport chain. The Electron Transport Chain (ETC) and Oxidative phosphorylation are the final and most energy-yielding stages of aerobic cellular respiration. This process occurs in the inner mitochondrial membrane in eukaryotes and the plasma membrane in prokaryotes.

ETC or the respiratory chain contain four major protein complexes (I to IV) and two mobile electron carriers embedded in the inner mitochondrial membrane.  These complexes and carriers transfer electrons from electron donors such as NADH and FADH2 to a final electron acceptor. The energy released during this stepwise transfer is used to pump protons (H+), which create an electrochemical gradient and results in ATP synthesis.  The process of using the energy from electron transport to make ATP is called oxidative phosphorylation or Electron Transport Phosphorylation or Chemiosmosis.

A pair of electrons from NADH can produce up to 2.5 to 3 ATP molecules, while those from FADH2 produce up to 1.5 to 2 ATP molecules, because FADH2 enters the chain later.

(Prescott−Harley−Klein:Microbiology, Fifth Edition)

Component	Name	Function	Proton Pumping
Complex I	NADH-Ubiquinone Oxidoreductase	Oxidizes NADH to NAD+ and transfers electrons to Ubiquinone (Q).	Pumps 4 H+ from matrix to intermembrane space.
Complex II	Succinate Dehydrogenase	Oxidizes FADH2​ and transfers electrons to Q.
Ubiquinone (Q)	Coenzyme Q	A small, lipid-soluble electron and proton carrier. Shuttles electrons from Complexes I and II to Complex III.
Complex III	Ubiquinol-Cytochrome c Oxidoreductase	Transfers electrons from Ubiquinol (QH2​) to Cytochrome c via the Q cycle.	Pumps 4 H+ from matrix to intermembrane space.
Cytochrome c	Cyt c	A small, water-soluble protein. Shuttles electrons from Complex III to Complex IV.
Complex IV	Cytochrome c Oxidase	Transfers electrons from Cyt c to molecular oxygen, reducing it​ to water.	Pumps 2 H+ from matrix to intermembrane space.

 

The H+ pumping by Complexes I, III, and IV moves protons from the mitochondrial matrix into the intermembrane space. This creates an electrochemical gradient, known as the Proton-Motive Force (PMF), with a higher concentration of H+ and a more positive electrical charge in the intermembrane space compared to the matrix.

The ATP synthase (sometimes called Complex V) is a large enzyme complex embedded in the inner membrane.  It acts as a channel that allows protons to flow back down their concentration gradient (from the intermembrane space back into the matrix).  The movement of protons causes the F0​ rotor subunit of ATP synthase to spin.  This mechanical rotation energy is transferred to the F1​ head subunit, which catalyzes the phosphorylation of ADP to form ATP.

(Prescott−Harley−Klein:Microbiology, Fifth Edition)

 

Electron Transport and Oxidative Phosphorylation - https://www.youtube.com/watch?v=zJNx1DDqIVo

 

 

Comparison of Mitochondrial and Bacterial ETC

While prokaryotic (bacterial) ETCs operate on the same fundamental principles, they often differ significantly from the eukaryotic mitochondrial chain. Structural and functional variations exist between eukaryotic (mitochondrial) and prokaryotic (bacterial) ETC.

Feature	Mitochondrial ETC (Eukaryotes)	Bacterial ETC (Prokaryotes)
Location	Inner mitochondrial membrane	Plasma membrane
Complexity/Organization	Usually four main complexes (I to IV) and ATP synthase (Complex V), often organized into supercomplexes.	Shorter and simpler; may contain fewer complexes or complexes with different subunits. Often branched pathways with alternative components.
Electron Carriers	Fixed carriers: NADH, FADH2​ Mobile carriers: Ubiquinone (Q), Cytochrome c.	Can use various electron donors and terminal oxidases/reductases Multiple quinones and cytochromes possible.
Terminal Electron Acceptor	Typically Oxygen.	Typically Oxygen (O2​) under aerobic conditions Can use alternative inorganic/organic acceptors (e.g., nitrate, sulfate, fumarate) under anaerobic conditions.
Efficiency (P/O ratio)	Higher (≈2.5 ATP per NADH; ≈1.5 ATP per FADH2​)	Lower, as the number of proton pumps can vary.