(1) Electron Transport Chain - ATP Synthase, Chemiosmosis, & Oxidative Phosphorylation

Electron Transport Chain Overview

• Stage four of cellular respiration

• Involves the transfer of electrons to ultimately produce ATP

NADH and Its Role

• NADH generated in Krebs cycle and glycolysis donates electrons at Complex I (NADH dehydrogenase)

  • Removes hydrogen, loses electrons

  • Electrons travel to ubiquinone (Q), a mobile electron carrier

Ubiquinone's Function

• Transports electrons from Complex I to Complex III

• Essential because electrons cannot move through the phospholipid membrane alone due to their charge

Complex III (Cytochrome Reductase)

• Accepts electrons from ubiquinone

• Reduces cytochrome c by transferring electrons

• Integral protein embedded in the membrane

Complex IV (Cytochrome Oxidase)

• Receives electrons from cytochrome c

• Oxidizes cytochrome c by removing its electrons

• Transfers electrons to molecular oxygen, resulting in water formation (final product of cellular respiration)

FADH2 and Its Function

• Produced in Krebs cycle when succinate converts to fumarate via succinate dehydrogenase (Complex II)

• FADH2 gives up hydrogens and electrons, converting back to FAD

• Electrons travel through the same pathway as NADH, but FADH2 only activates Complexes II and III

Proton Gradient and Chemiosmosis

• As electrons are transferred, protons (H+) are pumped from mitochondrial matrix into intermembrane space

• Builds up positive charge in the intermembrane space, creating a concentration gradient

• Protons are drawn back into the matrix through ATP synthase (a membrane protein)

  • Mechanically generates ATP by phosphorylating ADP

ATP Synthase Functionality

• Compares to a turbine powered by flowing protons

• ATP production via diffusion of protons is called chemiosmosis

• Combines with the electron transport chain to create oxidative phosphorylation

Mechanism of Oxidative Phosphorylation

• NADH oxidized to NAD+ during the electron transport chain process

• The oxidation state of NADH changes as it loses electrons

• Results in the phosphorylation of ADP to ATP via ATP synthase

Electron Acceptors in Cellular Respiration

• Various electron acceptors such as NAD+ in glycolysis and FAD during Krebs cycle

• Oxygen is the primary electron acceptor in the electron transport chain

• Its electronegativity pulls electrons through the chain, facilitating energy release

Comparison of Electronegativities

• Electrons flow from less electronegative to more electronegative substances (e.g., from carbon to oxygen)

• Illustrative comparison to a battery where charge flow produces energy (e.g., lighting a bulb)

Efficiency of Electron Carriers

• Complex III has more electron affinity than Complex I (order of electron flow dictates this)

• Ubiquinone has greater electronegativity than Complex II

ATP Yield from NADH and FADH2

• One molecule of NADH yields three ATP (activates three complexes)

• One molecule of FADH2 yields two ATP (activates two complexes)

• Relevance when calculating glucose's total ATP yield during cellular respiration.