Oxidative Phosphorylation

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Summary

  • Oxidative phosphorylation is comprised of the electron transport chain and chemiosmosis.
  • It is the most efficient producer of ATP in the process of aerobic respiration
  • Electrons carried from previous steps of respiration enter the electron transport chain, and are sequentially passed through membrane bound proteins
  • The final member of the chain is oxygen, which forms water upon accepting the electron
  • The electron transport chain generates a protein gradient
  • The protein gradient drives ATP synthase activity, which generates ATP

 Oxidative Phosphorylation is the fourth and final step in cellular respiration, and is the main producer of ATP in the process. Oxidative phosphorylation is an aerobic process, meaning it only occurs in the presence of oxygen.

Oxidative phosphorylation consists of two elements: the electron transport chain and chemiosmosis. In the electron transport chain, electrons are passed from one carrier to another, forming an electrochemical gradient that can be used to power oxidative phosphorylation, Chemiosmosis describes the formation of ATP using this gradient. The process of oxidative phosphorylation produces much more ATP than glycolysis – about 28 molecules.

The electron transport chain

The electron transport chain comprises the part of the final stages of aerobic respiration. The electron transport chain consists of a series of redox reactions where electrons are passed between membrane-spanning proteins. The final link in the chain is oxygen, which is the last acceptor of the electrons. Oxygen is reduced by the electrons, forming water. If oxygen isn’t present to accept electrons, the electron transport chain will stop running, and ATP will no longer be produced by chemiosmosis. Without enough ATP, cells can’t carry out the reactions they need to function.

The components of the electron transport chain are organized into four large complexes labelled I to IV. All of the electrons that enter the transport chain come from NADH and FADH2 molecules produced during earlier stages of cellular respiration. Energy is released when these electrons transfer across the electrochemical gradient, and several of the protein complexes use the released energy to pump protons from the mitochondrial matrix to the intermembrane space, forming a proton gradient.

The electron transport chain

Firstly, electrons enter the transport chain through delivery by electron carriers NADH and FADH.  These reduced electron carriers from the previous steps of cellular respiration transfer their electrons to molecules near the beginning of the transport chain. Following loss of their electrons, they are oxidised into NAD+ and FADH when can then be recycled to other steps of respiration.

More specifically, NADH starts the process by depositing its electrons at Complex I, turning into NAD+ and releasing a proton into the matrix. FADH2 is not as good at donating electrons as NADH (its electrons are at a lower energy level), so it cannot transfer its electrons to Complex I. Instead, FADH2 deposits electrons at Complex II. It is then reduced to FAD and releases 2 hydrogen atoms.

The electrons from Complexes I and II are then passed to another carrier, ubiquinone (Q). Q (now in the reduced form QH2) is a mobile electron carrier, free to travel through the membrane. Q transports the electrons to Complex III. As electrons pass through Complex III, more hydrogen ions are pumped across the membrane, and the electrons are passed to cytochrome C, which is also mobile and free to pass through the membrane.

Cytochrome C passes the electrons to Complex IV. Complex IV passes the electrons to oxygen, the terminal electron acceptor. Oxygen is split into two oxygen atoms, and accepts H+ from the matrix to form water. It takes two electrons, one oxygen (1/2 O2 molecule), and 2 H+ ions to form one water molecule. Complexes I, III, and IV use the energy released from electrons moving from higher to lower energy levels to move protons out of the matrix and into the intermembrane space. This generates a proton gradient.

Chemiosmosis and oxidative phosphorylation

The movement of electrons through the electron transport chain causes a proton gradient, owing to the positively charged hydrogen ions accumulating at one side of the membrane. This proton is then used to power ATP synthesis.

The hydrogen ions in the mitochondrial matrix can only pass through the phospholipid bilayer of the inner membrane with the help of a membrane-embedded protein called ATP synthase.  ATP synthase acts a lot like a water wheel; it is turned by the flow of the hydrogen ions moving through it, down their electrochemical gradient. As ATP synthase turns, it catalyses the addition of a phosphate to ADP, generating ATP. This process, in which energy from a proton gradient is used to make ATP, is called chemiosmosis.

ATP production

In a eukaryotic cell, cellular respiration can generate 30 to 32 ATP molecules per molecule of glucose. The process of glycolysis only produces 2 ATP; the rest are produced using the electron transport chain. The electron transport chain is hugely efficient at generating energy in the cell, but relies on an abundance of oxygen to be carried out.

 

Further reading and references:

[1]. https://slideplayer.com/slide/8184099/ (Image)

[2]. https://www.ncbi.nlm.nih.gov/books/NBK9885/

[3]. https://www.nature.com/scitable/topicpage/mitochondria-14053590