Electron Transport Chain (ETC) in Cellular Respiration: Definition, Location and Steps Simplified

Cellular respiration is a catabolic process which involves the intracellular oxidation of glucose or organic molecules  through series of enzymatic reaction producing energy in the form of ATP with the release of CO2 and H2O as byproducts.

This is the summarized video on Electron transport chain

3 stages of cellular respiration:

1. Glycolysis (Glyco=Glucose; lysis= splitting) is the oxidation of glucose (C 6) to 2 pyruvate (3 C) with the formation of ATP and NADH.

2.The Krebs cycle, Citric acid cycle or TCA cycle is an eight step cyclic reactions in which acetyl CoA is oxidized producing CO2, reduced coenzymes (NADH + H+ and FADH2), and ATP.

3. Electron Transport Chain: 

ETC is the step by step transfer of high energy electrons through a series of electron carriers located in multienzyme complexes, finally reducing molecular O2 to form water with the formation of ATP by chemiosmosis.

Electron Transport Chain Definition


  • Organelle: Mitochondrion
  • Site of Electron transport chain: Mitochondrial Inner membrane
  • Proton (H+) pumped into the intermembrane space creating proton gradient
  • ATP synthesis occurs towards the matrix region (see the above figure)
Electron Transport Chain ETC Location

Background info: At the beginning of electron transport chain we have NADH and FADH2 synthesized during Kerb’s cycle and glycolysis. Approximately only 4 ATP are synthesized directly (2 from glycolysis and 2 from Krebs cycle) from a glucose molecule. The rest ~32-34 ATP are synthesized during Electron transport chain (ETC) by chemiosmosis.

Now let as  move into the detail
1. Electron flow and Energy release:
NADH and FADH2 donates high energy electrons that pass through different protein complexes and electron carriers in the ETC.
As the electrons moves from high energy to low energy level, some amount of energy is released. The final electron acceptor is O2 which splits and takes up H+ to form water (H2O)
2. Proton movement and gradient formation
The energy released during electron flow is used to pump proton (H+ ions) from matrix side to the intermembrane space of mitochondrion. (see figure). This creates a proton gradient or (Electrochemical gradient or proton motive force) across the inner mitochondrial membrane (that is higher concentration of H+ ions in the intermembrane space compared to the matrix).
Electron transport chain (ETC) by chemiosmosis.

3. Proton motive force (PMF) driven ATP synthesis
The H+ ions should move to matrix to maintain equilibrium (to balance H+ ion concentration). As phopholipid bilayer of inner mitochondrial membrane is impermeable, the only way out is through the protein complex called ATP synthase which spans the inner mitochondrial membrane and has a proton channel.
The flow of H+ ions through ATP synthase provides energy for the addition of phosphate to ADP thus forming ATP. (just like turbine in hydroelectric power plant where water forces turbine movement, here flow of H+ ions drives ATP synthesis)
The proton gradient (Proton motive force) driven ATP synthesis is called Chemiosmosis.
Hope things are clear. Watch the video for better understanding. Thank you and enjoy learning Biology
10 steps of Glycolysis Video: https://youtu.be/XcdL9o3yidU
8 Steps of Krebs cycle:  https://youtu.be/W05eIbXeiMA
How Hans Krebs Discovered Krebs cycle: https://youtu.be/sLu7oGGy2cw

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