The electron transport chain

• Most of the ATP in a cell is synthesized through a series of reactions called the electron transport chain
• In the electron transport chain:

  1. A molecule in a reduced form is oxidized

  2. The hydrogen atom passes through a proton pump, where an electron is removed, and the remaining proton (H+) is pumped to the other side of the membrane

  3. Electrons are passed from one electron carrier protein to another, releasing energy used by the proton pumps to pump more protons across the membrane

  4. A higher concentration of protons builds up on one side of the membrane, creating a difference in pH and charge across the membrane

  5. Embedded in the membrane are stalked particles containing the enzyme ATP synthetase

  6. Protons pass down a concentration gradient through a channel in the enzyme, providing high energy used by ATP synthetase to combine ADP with inorganic phosphate (Pi) to form ATP

  7. Some protons are pumped back across the membrane to maintain the concentration gradient
• 8. Electrons passed through the electron carrier proteins are picked up by a final electron acceptor, which also picks up some protons and is reduced in the process
• Electron transport chains are used in both respiration and photosynthesis in eukaryotes and prokaryotes to synthesize ATP
• All electron transport chains operate by creating a region of high H+ concentration, using the movement of H+ down a concentration gradient, providing energy to ATP synthetase, and combining ADP and Pi to make ATP
• The presence of electron transport chains, proton pumps, and the synthesis of ATP by ATP synthetase in all cells supports the endosymbiotic theory
• NADH and FADH2 donate their electrons to the first protein in the ETC, NADH passing its electrons to complex I while FADH2 passes them to complex II.
• Electron carriers include cytochromes (proteins with iron atoms) and flavoproteins (containing FAD).
• The electron transport chain is the final step in cellular respiration, where electrons are passed from one carrier to another.
• Oxygen acts as the terminal electron acceptor, accepting electrons from cytochrome c at complex IV.
• Protons are pumped across the membrane during electron transfer, resulting in an electrochemical gradient that drives ATP production.
• As electrons move along the ETC, they lose potential energy, causing a drop in redox potential.
• ATP synthase uses this gradient to produce ATP from ADP and phosphate.
• Protons are pumped across the inner mitochondrial membrane during this process, generating an electrochemical gradient that drives the production of ATP through oxidative phosphorylation.
• ATP synthase uses the energy stored in the electrochemical gradient to drive the formation of ATP molecules from ADP and Pi.
• ATP synthase uses this energy to produce ATP through chemiosmosis.
• In eukaryotes, the mitochondrial inner membrane contains proteins involved in oxidative phosphorylation, including complexes I-V and ATP synthase.
• In eukaryotes, the electron transport chain takes place within the inner mitochondrial membrane, while in bacteria it occurs on the plasma membrane or in specialized structures called thylakoids.
• FADH2 enters the ETC at Complex II and releases two electrons, which pass through three different proteins and release protons into the intermembrane space.
• Flavin adenine dinucleotide (FAD) is a coenzyme found in some enzymes involved in the ETC.