Electron Transport Chain (ETC)

Electron Transport Chain Is a protein gradient coupled (where energy is transfer from one side to another) electron reaction between a electron donor (NADH and FADH2 which are donated from the krebs cycle) and the electron acceptor (O2) with the transfer of H+ across a membrane.

  • Electron transport chain is found in plants (radiant energy à chemical) and animals.
  • We will focus in animals, specifically the mitochondria (respiration).
  • Now as NADH and sometimes FADH2 go through a redox reaction and thus electrons pass through an enzymatic series of electron donors and acceptors.
  • Remember, each electron donor will pass electrons to a more electronegative acceptor.
  • This continues until the electrons reach the electron acceptor (O2).
  • Now, ATP is produce at the end of this reaction using a mechanism called ATP synthase.
  • ATP synthase uses the H+ in the intermembrane space (pumped from the mitochondria’s matrix) to pump back into the matrix and perform chemiosmosis.

 So what is the overall goal?

To create a trans membrane proton gradient pump from redox reactions to create energy in the form of ATP.  Yes, all of this at first may be over whelming but I will take it step by step.

 Here is a picture of the process called Electron Transport Chain

As you see above, the picture shows the complexes (labeled I II III and IV) and the flow of electrons,. Now lets dive in and figure out how this works!

Complex I

  • Called NADH Dehydrogenase.
  • When NADH is oxidize to NAD+, two electrons are removed and so is an H+.
  • The electrons are passed to Co-enzyme Q (ubiquinone Q). Co-enzyme is lipid soluble.
  • When electrons are passed to Co-enzyme Q, energy is released and complex I pumps H+ across the membrane into the intermembrane space, creating a proton gradient pump.

Complex II

  • Called succinate dehydrogenase
  • FADH is oxidize to FAD. Electrons and Hydrogens are released.
  • Electrons are passed to Co-enzyme Q.

Complex III

  • -Called cytochrome c reductase
  • Two electrons are transferred to complex III from Co-enzyme Q.
  • Once again, energy is release and the energy is use to drive H+ across the membrane.

Complex IV

  • Called Cytochrome c oxidase
  • Electrons are transferred from cytochrome c too complex IV. The electrons then connect with O2 to make H2O.
  • At the same time, protons are pumped across the membrane.

Now you have a electrochemical gradient or just simply called proton gradient created by the transfers of electrons and the pumping action of proteins (complex I II IV). Now there is a charge difference, more positive in the intermembrane and more negative in the matrix of the mitochondria. The protons will now be use to create energy!


ATP Synthase

  • Since there is a high concentration of H+ in the intermembrane and a low concentration of H+ in the mitochondrial matrix; the protons will have a tendency to flow down their concentration gradient.
  • They flow through a channel called ATP Synthase.
  • When a hydrogen ion enters the ATP synthase from the intermembrane space, another hydrogen ion leaves creating energy.
  • This energy helps attach an inorganic Pi to a ADP molecule producing ATP.

So the reaction is called substrate phosphorylation (ADP–>ATP).

  • The overall reaction for the working of ATP Synthase can be called Chemiosmosis.


The link below is a great video of how ATP Synthase works… Check it out!



- So as glucose is broken down, CO2 is released and hydrogen atoms are donated to NAD+ and FAD to form NADH and FADH2 (in glycolysis and Krebs). These energy carriers are recycled to the Electron transport Chain. They then donate their Hydrogen atoms and the energy carriers are converted back to NAD+ and FAD. Next, in a series of redox reactions, electrons move from one energy level to another releasing energy which drives the hydrogen ions to be pumped across the membrane (into the intermembrane space). Now after Complex I III and IV have pumped hydrogen ions, the intramembrane space is now at a high concentration while the matrix is at a low concentration of hydrogen ions. That concentration drives the Hydrogen ions into ATP synthase which creates energy to make ATP from ADP.

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