7.3 Biological Electron Transport Chains

Electron carriers and the mitochondrial electron transport chain are crucial for cellular energy production. These molecules and protein complexes work together to transfer electrons, creating a proton gradient that powers ATP synthesis.

ATP synthesis, driven by the electron transport chain, is the primary source of cellular energy. This process is not only vital in mitochondria but also plays a role in other cellular processes like photosynthesis, bacterial energy production, and various metabolic pathways.

Electron Carriers and Mitochondrial Electron Transport Chain

Structure and function of electron carriers

Top images from around the web for Structure and function of electron carriers

• 

  Reading: Electron Transport Chain | Biology I View original

  Is this image relevant?

• 

  File:2508 The Electron Transport Chain.jpg - Wikimedia Commons View original

  Is this image relevant?

• 

  4.3 Citric Acid Cycle and Oxidative Phosphorylation – Concepts of Biology – H5P View original

  Is this image relevant?

• 

  Reading: Electron Transport Chain | Biology I View original

  Is this image relevant?

• 

  File:2508 The Electron Transport Chain.jpg - Wikimedia Commons View original

  Is this image relevant?

1 of 3

Top images from around the web for Structure and function of electron carriers

• 

  Reading: Electron Transport Chain | Biology I View original

  Is this image relevant?

• 

  File:2508 The Electron Transport Chain.jpg - Wikimedia Commons View original

  Is this image relevant?

• 

  4.3 Citric Acid Cycle and Oxidative Phosphorylation – Concepts of Biology – H5P View original

  Is this image relevant?

• 

  Reading: Electron Transport Chain | Biology I View original

  Is this image relevant?

• 

  File:2508 The Electron Transport Chain.jpg - Wikimedia Commons View original

  Is this image relevant?

1 of 3

• Electron carriers accept and donate electrons in redox reactions transferring electrons between components of electron transport chains
• Nicotinamide adenine dinucleotide (NAD+/NADH) consists of two nucleotides joined by phosphate groups carrying two electrons and one proton
• Flavin adenine dinucleotide (FAD/FADH2) contains riboflavin attached to an ADP molecule carrying two electrons and two protons
• Coenzyme Q (ubiquinone) lipid-soluble benzoquinone with long isoprenoid tail shuttles electrons between complexes in mitochondrial membrane
• Cytochromes heme-containing proteins transfer single electrons through changes in iron oxidation state

Organization of mitochondrial electron transport

• Inner mitochondrial membrane houses electron transport chain components
• Complex I (NADH dehydrogenase) accepts electrons from NADH transferring to coenzyme Q
• Complex II (Succinate dehydrogenase) accepts electrons from FADH2 transferring to coenzyme Q
• Complex III (Cytochrome bc1 complex) accepts electrons from reduced coenzyme Q transferring to cytochrome c
• Complex IV (Cytochrome c oxidase) accepts electrons from cytochrome c transferring to molecular oxygen forming water
• Electrons move from complexes with lower to higher reduction potentials releasing energy used to pump protons into intermembrane space

ATP Synthesis and Other Cellular Processes

Coupling of electron transport to ATP synthesis

• Chemiosmosis drives ATP synthesis using electrochemical gradient
• Proton gradient forms as Complexes I, III, and IV pump protons into intermembrane space creating electrochemical gradient across inner mitochondrial membrane
• ATP synthase (Complex V) consists of F0 and F1 subunits catalyzing ATP synthesis using energy from proton flow
• ATP synthesis mechanism:

1. Protons flow through ATP synthase from intermembrane space to matrix
2. Proton flow energy drives F0 subunit rotation
3. F0 rotation causes F1 conformational changes leading to ATP synthesis

• ATP yield approximately 2.5 ATP molecules per NADH and 1.5 ATP molecules per FADH2

Electron transport chains in cellular processes

• Photosynthesis light-dependent reactions involve electron transport chains in thylakoid membranes generating proton gradient for ATP synthesis (chloroplasts)
• Bacteria and archaea use diverse electron transport chains for various energy sources with alternative terminal electron acceptors (nitrate, sulfate)
• Plasma membrane electron transport present in various cell types functions in redox homeostasis and cell signaling (red blood cells)
• Endoplasmic reticulum electron transport involved in protein folding and lipid metabolism including cytochrome P450 systems for detoxification and hormone synthesis (liver cells)
• Nitrogen fixation electron transport chains in nitrogen-fixing bacteria provide energy for N2 reduction (Rhizobium)