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Oxidative Phosphorylation: The major energy provider of the cell

There are a lot of different ways organisms acquire food. Just think about how sharks, bees, plants, and bacteria eat. Almost all aerobic organisms (organisms that require oxygen to live) use oxidative phosphorylation, in one way or another, to produce the basic energy currency of the cell needs to function: ATP (adenosine triphosphate). Oxidative phosphorylation is the fourth step of cellular respiration, and produces the most of the energy in cellular respiration.
Where does oxidative phosphorylation fit into cellular respiration?
  1. Glycolysis, where the simple sugar glucose is broken down, occurs in the cytosol.
  2. Pyruvate, the product from glycolysis, is transformed into acetyl CoA in the mitochondria for the next step.
  3. The citric acid cycle, where acetyl CoA is modified in the mitochondria to produce energy precursors in preparation for the next step.
  4. Oxidative phosphorylation, the process where electron transport from the energy precursors from the citric acid cycle (step 3) leads to the phosphorylation of ADP, producing ATP. This also occurs in the mitochondria.

What is oxidative phosphorylation?

Oxidative phosphorylation is the process where energy is harnessed through a series of protein complexes embedded in the inner-membrane of mitochondria (called the electron transport chain and ATP synthase) to create ATP. Oxidative phosphorylation can be broken down into two parts: 1) Oxidation of NADH and FADH2, and 2) Phosphorylation.

1. Oxidation of NADH and FADH2 - losing electrons via high energy molecules

Step 1
Oxidative phosphorylation starts with the arrival of 3 NADH and 1 FADH2 from the citric acid cycle, which shuttle high energy molecules to the electron transport chain. NADH transfers its high energy molecules to protein complex 1, while FADH2 transfers its high energy molecules to protein complex 2. Shuttling high energy molecules causes a loss of electrons from NADH and FADH2, called oxidation (other molecules are also capable of being oxidized).
The opposite of oxidation is reduction, where a molecule gains electrons (which is seen in the citric acid cycle). Here’s an easy way to remember which process gains or loses electrons:
LEO the lion says GER
Lose Electrons Oxidation (LEO)
Gain Electrons Reduction (GER)
Step 2 - Hitting the gym to pump some serious hydrogens
The process of NADH oxidation leads to the pumping of protons (single positively-charged hydrogen atoms denoted as H+) through protein complex 1 from the matrix to the intermembrane space. The electrons that were received by protein complex 1 are given to another membrane-bound electron carrier called ubiquinone or Q.
This process of transferring electrons drives the pumping of protons, which is seen as unfavorable. Electron transfer driving proton pumping is repeated in complexes 3 and 4 (which we will discuss in steps 2 - 5). As this action is repeated, protons will accumulate in the intermembrane space. This accumulation of protons is how the cell temporarily stores transformed energy.
Note - FADH2 has a slightly different route than NADH. After its arrival at protein complex 2, its high energy electrons are directly transferred to Q, to form reduced Q, or QH2. There is no hydrogen pumping for the exchange of the FADH2 electrons here.
Step 3
The rest of the steps are now the same for the high energy molecules from NADH and FADH2 in earlier steps. Inside the nonpolar region of the phospholipid bilayer, UQH2 (which is also a nonpolar compound) transports the electrons to protein complex 3. UQH2 also carries protons. When UQH2 delivers electrons to protein complex 3, it also donates its protons to be pumped.
Step 4
The electrons that arrived at protein complex 3 are picked up by cytochrome C (or “cyt C”), the last electron carrier. This action also causes protons to be pumped into the intermembrane space.
Step 5
Cytochrome C carries the electrons to the final protein complex, protein complex 4. Once again, energy released via electron shuttling allows for another proton to be pumped into the intermembrane space. The electrons are then drawn to oxygen, which is the final electron acceptor. Its important to note that oxygen must be present for oxidative phosphorylation to occur. Water is formed as oxygen receives the electrons from protein complex 4, and combines with protons on the inside of the cell.
In summary
  • +3 NADH
  • +1 FADH2
  • +3 Hydrogen protons (H+)
  • -2 Hydrogen protons (H+)
  • -½ O2
  • +1 H2O

2. Phosphorylation - the production of ATP

Step 6
As a result of part 1 (Oxidation of NADH and FADH2), an electrochemical gradient is created, meaning there is a difference in electrical charge between the two sides of the inner mitochondrial membrane. The outside, or exterior, of the mitochondrial membrane is positive because of the accumulation of the protons (H+), and the inside is negative due to the loss of the protons. A chemical concentration gradient has also developed on either side of the membrane. The electrochemical gradient is how the cell transfers the stored energy from the reduced NADH and FADH2.
Step 7
When there is a high concentration of protons on the outside of the mitochondrial membrane, protons are pushed through ATP synthase. This movement of protons causes ATP synthase to spin, and bind ADP and Pi, producing ATP. Finally, ATP is made!
In summary
  • -ADP
  • -Pi
  • +ATP

Consider the following:

In oxidative phosphorylation, oxygen must be present to receive electrons from the protein complexes. This allows for more electrons and high energy molecules to be passed along, and maintains the hydrogen pumping that produces ATP. What happens if we run out of oxygen? How do we break down our food to make energy? The body has a plan B for this situation called fermentation. It happens all the time in athletes, like runners, when they use all their oxygen and produce lactic acid. Fermentation starts after glycolysis, replacing the citric acid cycle and oxidative phosphorylation. During glycolysis, only two ATP molecules are produced. NADH is then oxidized to transform the pyruvates made in glycolysis into lactic acid.

Want to join the conversation?

  • duskpin ultimate style avatar for user oskargonzalez
    Are these protons, actually protons or just short hand for +H30?
    (12 votes)
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  • blobby green style avatar for user On MeiYan
    Are electron transport system same as the oxidative phosphorylation?
    (8 votes)
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  • blobby green style avatar for user Kellie508
    I understand that Oxygen must be present for the cycle to work because it accepts the electrons at the end of the chain of proteins. But, what exactly about accepting those electrons makes it necessary? Is it that a build up of electrons at that protein are harmful? Do they change the energy of the system so that more electrons wouldn't then flow towards that lower-energy protein? What, chemically, would happen if Oxygen didn't take them??
    (3 votes)
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    • piceratops ultimate style avatar for user swa256
      It would back up. The whole system is predicated upon rolling energetically downhill and passing the electrons to a more readily reduced molecule. If this molecule (oxygen) isn't there then complex 4 has nowhere to dump them (you can't just eject electrons from a molecule without some solid energy investments). As complex 4 already has electrons in this case, it loses the great ability to be reduced that made it a good place for cytochrome C (from complex 3) to dump it electrons from earlier in the chain. This causes complex 3 to back up etc., until the whole chain is effectively clogged with electrons until they can be unloaded onto an O2.
      (11 votes)
  • blobby green style avatar for user aminbboy69
    i just wanna know specificly how many H+ are pumped in the the space : for each NADH one or two h+ are pumped out ?
    (5 votes)
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    • starky ultimate style avatar for user ♪♫  Viola  ♫♪
      Charles is right. The most recent research points out that complex I pumps out 4 H+, Complex III pumps out 4 H+, and complex IV pumps out 2 H+. Therefore, NADH results in 10 H+ being pumped out, but FADH2 results in 6H+ being pumped out since it bypasses complex I.

      The current research estimates that ATP synthase makes 1 ATP for every 4 H+ that pass through it, thus, the 10 H+ coming from the energy of one NADH would lead to the creation of 10/4 or 2.5 ATP.
      (4 votes)
  • blobby green style avatar for user Daniel Smith
    If the inner membrane is impermeable to NADH, how does the NAD+ and FAD go from the citric acid cycle to the matrix in the electron transport chain?
    (2 votes)
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    • starky tree style avatar for user Sakura Nakada
      The citric acid cycle happens in the matrix of mitochondria, so NADH and FADH2 (from the reduction of NAD+ and FAD in the citric acid cycle) already have access to the ETC. Keep in mind that the citric acid cycle occurs in the mitochondrial matrix, and that the protons from NADH and FADH2 are pumped from the matrix of mitochondria across their inner membranes into the intermembrane space.
      (4 votes)
  • marcimus pink style avatar for user ratmkino
    It would be much better if we recognized NAD+ accepts what is essentially a hydride anion, and FAD accepts what is essentially a hydrogen diatom
    (3 votes)
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  • mr pants purple style avatar for user Anika Wadhera
    What is the difference between ATP synthase and ATP synthetase? I have seen different sources interchange these words.
    (1 vote)
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    • primosaur ultimate style avatar for user molly.sun13
      ATP synthetase is real, much like synthase, it's an enzyme that helps form covalent bonds however:
      synTHase = without much energy input, and
      synTHETase = with energy input
      So an example of this would be in the Krebs cycle, at the beginning, when forming citrate, it uses citrate synthase, which is pretty much without ATP usage. However, later on in the cycle, when forming succinate, the reaction actually takes GDP and produces GTP, so it uses energy - succinyl-CoA synthetase
      (4 votes)
  • duskpin tree style avatar for user Hae Young Ham
    I am a little confused by the summary of "1. Oxidation of NADH and FADH2". For example, the number of protons involved in the summary is wrong.... Can anyone explain?
    (2 votes)
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  • blobby green style avatar for user Anne Garava
    Where is phosphorylation undertaken?
    (1 vote)
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  • leafers sapling style avatar for user anagha pisal
    Is it so that the 3 NADH and 1 FADH2 as from citric acid cycle are directly used in Electron transport chain and 1 water molecule is produced as per 3 NADH and 1 FADH2 ?
    (1 vote)
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    • starky ultimate style avatar for user ♪♫  Viola  ♫♪
      One round of the citric acid cycle makes 3 NADH and 1 FADH2 (you are correct).
      But, one water is produced per electron carrier (as well as for every ATP made by ATP synthase...but that's another story). So, one round of the citric acid cycle leads to the production of 4 reduced electron carriers (three NADH and one FADH2) as the electrons reduce oxygen into water, leading to the production of 4 waters in the ETC.
      (3 votes)