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The Basics of Oxidative Phosphorylation

ATP Synthase:


Oxidative Phosphorylation (OP) is an ATP producing part of cellular respiration. "Oxidative" means that OP is an aerobic process, meaning it only occurs in the presence of oxygen (O2).


Oxidative phosphorylation uses the proton gradient established by the electron transport chain in mitochondria to power the synthesis of adenosine triphosphate (ATP) from adenoside diphosphate (ADP) and phosphate (Pi). OP produces much more ATP than glycolysis - about 28 molecules. This ATP can then be hydrolized by water to release free energy. OP is the main form of ATP production in aerobically respiring organisms.

Where it Takes Place:

Oxidative phosphorylation takes place in the mitochondria of eukaryotic cells, specifically in the inner membrane, matrix, and intermembrane space. In prokaryotic cells, it occurs in the cytosol.


Oxidative Phosphorylation is essentially an extension of the electron transport chain (ETC) of the mitochondria, occurring in a new protein complex, complex V. If you would like to review the electron transport chain before continuing this article, click the link above.

A quick review of the ETC: This is the "oxidation" part of oxidative phosphorylation. It involves the passage of electrons through four different protein complexes within the inner mitochondrial membrane, which simultaneously pumps protons into the intermembrane space between the inner and outer membranes. This creates a proton gradient, which is then used to power ATP synthesis. Now, on to the good stuff.

Chemiosmosis: The actual synthesis of ATP using the proton gradient constitutes the "Phosphorylation" aspect of oxidative phosphorylation. Due to the ETC, a high concentration of protons are outside the inner membrane, producing a positive charge, and a high concentration of electrons are inside the inner membrane, producing a negative charge. This creates a large difference in electrical charges, which is called a proton-motive force. This force just means that the protons on the outside are attracted to the electrons on the inside, so much so that they want to diffuse (move) through the inner membrane. The motive force pumps protons back into the mitochondrial matrix through the fifth complex in the inner membrane, known as ATP synthase.

Hint: Before continuing, it's important to understand the difference between exergonic reactions and endergonic reactions. Exergonic chemical reactions occur on their own, without the need of free energy within the cell, and usually release free energy. Endergonic chemical reactions, however, will not occur without the addition of some form of free energy that pushes the reaction along.

The synthesis of ATP from ADP and a phosphate is endergonic, meaning ATP will not be synthesized without energy powering the reaction - kind of like how electronics won't turn on unless you plug them in. This is where ATP synthase comes in. As protons flow through the inner membrane, ATP synthase couples the energy released from the proton-motive force with the reaction between ADP and phosphate, pushing the two compounds together to create ATP. This reaction also creates a molecule of water, but ATP is the real payout.

Oxidative Phosphorylation Steps:

ATP Synthesis Reaction:

The reaction that produces ATP is written as;

ADP + Pi + free energy ------> ATP + H2O

This reaction is freely reversible, which means that water can hydrolize, or break down, ATP into ADP, phosphate, and energy in the following reaction;

ATP + H2O ------> ADP +Pi + free energy

Since we have learned that the first reaction requires energy and is therefore endergonic, the reverse reaction releases energy and is therefore exergonic.

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Because of this reversibility, ADP can create ATP, and vice versa.


ATP: about 28 molecules of ATP are produced, which can be hydrolyzed to release free energy for use in other cell functions, such as glycolysis. Add these to the 2 ATP's produced from glycolysis and the citric acid cycle to get approximately 32 ATP molecules. 32 is the maximum, though, you'll most likely get around 30 most of the time.

Water: water produced is used to hydrolyze ATP.

OP Steps Video:

Terms to Know:

  • ADP: a molecule consisting of a 5-carbon pentose sugar, an adenine molecule, and two phosphate groups used to synthesise ATP and created as a result of ATP hydrolysis.
  • ATP: a molecule consisting of a 5-carbon pentose sugar, an adenine molecule, and three phosphate groups hydrolyzed to produce energy. Note that ATP consists of one more phosphate group than ADP
  • Electron: a basic particle of an atom (subatomic) consisting of a positive electrical charge
  • Inner membrane: The mitochondria have two cell membranes, this is the membrane that surrounds the matrix but is surrounded by the outer membrane.
  • Intermembrane Space: the thick, viscous liquid between the inner and outer membranes of the mitochondria; basically the cytosol of the mitochondria.
  • Mitochondria: An energy producing organelle within eukaryotic cells and the site of the ETC; contains two cell membranes.
  • Matrix: the thick, viscous liquid surrounded by the inner membrane of the mitochondria; basically the cytosol of the mitochondria.
  • Outer membrane: The mitochondria have two cell membranes, this is the membrane that surrounds the entire cell.
  • Oxidation: the loss of an electron or gain of a proton/hydrogen atom by a molecule.
  • Protein Complex: A site of electron transport embedded in the mitochondrial inner membrane
  • Proton: a basic particle of an atom (subatomic) consisting of a positive electrical charge.
  • Proton Gradient: a source of energy resulting from a higher concentration of protons in the intermembrane space of a mitochondrial inner membrane that in the mitochondrial matrix (more protons outside than in).
  • Redox Reaction: a reaction in which one reactant is oxidized, and one is reduced.
  • Reduction: the gain of an electron or loss of a proton/hydrogen atom by a molecule.


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