Electron Transport Chain and Energy Production: An Explanation

Electron Transport Chain
Electron Transport Chain and Oxidative Phosphorylation. OpenStax College/Wikimedia Commons/CC BY-SA 3.0

The electron transport chain is the third step of aerobic cellular respiration. Cellular respiration is the process by which energy is obtained from the foods we eat. The electron transport chain is where most of the energy cells need to operate is generated. This chain is actually a series of protein complexes and electron carrier molecules within the inner membrane of eukaryotic cell mitochondria.

Electrons are passed along the chain creating an electrochemical gradient that facilitates the production of adenosine triphosphate (ATP). ATP is the main source of energy for many cellular processes including muscle contraction and cell division. Energy is released during cell metabolism when ATP is hydrolized (chemically decomposed by reacting with water) to adenosine diphosphate (ADP). ADP is in turn used to synthesize ATP in cellular respiration.

Cellular Respiration

The first step of cellular respiration is glycolysis. Glycolysis occurs in the cytoplasm and involves the splitting of glucose into two pyruvate molecules. Two molecules of ATP and two molecules of NADH (high energy, electron carrying molecule) are also generated. Pyruvate is transported across the outer and inner mitochondrial membranes into the mitochondrial matrix. This is where the second step of cellular respiration, the citric acid cycle or Krebs cycle, occurs.

Pyruvate is further oxidized in the Krebs cycle producing two more molecules of ATP, as well as NADH and FADH2 molecules. Electrons from NADH and FADH2 are transfered to the third step of cellular respiration, the electron transport chain.

Electron Transport Chain Protein Complexes

There are four protein complexes that are part of the electron transport chain.

A fifth protein complex serves to transport hydrogen ions back into the matrix. These complexes are embedded within the inner mitochondrial membrane and electron transport chain proteins function to pass electrons down the chain. Oxygen is required for aerobic respiration as the chain terminates with the donation of electrons to oxygen. As electrons are passed from protein complex to protein complex, energy is released and hydrogen ions (H+) are pumped out of the matrix (compartment within the inner membrane) and into the intermembrane space (compartment between the inner and outer membranes). This creates both a chemical gradient (difference in solute concentration) and an electrical gradient (difference in charge) across the inner membrane. As more H+ ions are pumped into the intermembrane space, the higher concentration of ions creates a gradient whereby the ions "want to" flow down their concentration gradient and return to the matrix. Since they are not able to passively flow across the phospholipid bilayer of the mitochondrial inner membrane, the ions must be helped across by the intermembrane protein ATP synthase. ATP synthase uses the energy generated from the movement of H+ ions into the matrix for the conversion of ADP to ATP.

This entire metabolic pathway of oxidizing molecules to generate energy for the production of ATP is called oxidative phosphorylation.

Electron Transport

  • Complex I: NADH transfers two electrons to Complex I resulting in four H+ ions being pumped across the inner membrane. NADH is oxidized to NAD+, which is recycled back into the Krebs cycle. Electrons are transfered from Complex I to a carrier molecule ubiquinone (Q), which is reduced to ubiquinol (QH2). Ubiquinol carries the electrons to Complex III.
  • Complex II: FADH2 transfers electrons to Complex II and the electrons are passed along to ubiquinone (Q). Q is reduced to ubiquinol (QH2), which carries the electrons to Complex III. No H+ ions are transported to the intermembrane space in this process.
  • Complex III: The passage of electrons to Complex III drives the transport of four more H+ ions across the inner membrane. QH2 is oxidized and electrons are passed to another electron carrier protein cytochrome C.
  • Complex IV: Cytochrome C passes electrons to the final protein complex in the chain, Complex IV. Two H+ ions are pumped across the inner membrane. The electrons are then passed from Complex IV to an oxygen (O2) molecule, causing the molecule to split. The resulting oxygen atoms quickly grab H+ ions to form two molecules of water.
  • ATP synthase: ATP synthase moves H+ ions that were pumped out of the matrix by the electron transport chain back into the matrix. The energy from the influx of protons into the matrix is used to generate ATP by the phosphorylation (addition of a phosphate) of ADP. The movement of ions across the selectively permeable mitochondrial membrane and down their electrochemical gradient is called chemiosmosis.

In summary, electrons are donated to the electron transport chain by NADH and FADH2. Electrons are passed along the chain from protein complex to protein complex until they are donated to oxygen forming water. During the passage of electrons, protons are pumped out of the mitochondrial matrix across the inner membrane and into the intermembrane space. The accumulation of protons in the intermembrane space creates an electrochemical gradient that causes protons to flow down the gradient and back into the matrix through ATP synthase. This movement of protons provides the energy for the production of ATP. NADH generates more ATP than FADH2. For every NADH molecule that is oxidized, 10 H+ ions are pumped into the intermembrane space. This yields about 3 ATP molecules. Because FADH2 enters the chain at a later stage (Complex II), only 6 H+ ions are transfered to the intermembrane space. This accounts for about 2 ATP molecules. A total of 32 ATP molecules are generated in electron transport and oxidative phosphorylation.