It is characterised as an electron transfer chain driven by substrate oxidation that is connected to the synthesis of ATP through the use of an electrochemical transmembrane gradient (OXPHOS). As previously stated (Saraste, 1999), because they can be isolated in large quantities, bovine heart mitochondria have traditionally been the system of choice for the structural characterization of eukaryotic OXPHOS complexes. As a result of its accessibility to a wide range of molecular genetic tools, the yeast Saccharomyces cerevisiae is increasingly being used as a model organism for studies of mitochondrial complex formation and the effect of mutations on OXPHOS components. However, mitochondria of photosynthetic species have received little attention from a biochemical standpoint, owing in part to the difficulty in obtaining preparations that are free of chloroplast contamination. Despite this, great progress has been made in the characterisation of Arabidopsis mitochondrial components using proteomic techniques.

Oxidative Phosphorylation Steps

The following are the major phases in the process of oxidative phosphorylation in mitochondria:

• NADH and FADH2 are responsible for the delivery of electrons. 

Because of their reduced state, NADH and FADH2 can transmit electrons to molecules at the beginning of the transport chain. They are oxidised to form NAD+ and FAD, which are then used in the following phases of cellular respiration to complete the cycle.

• Electron Transport and Proton Pumping

The electrons go from a higher energy level to a lower energy level, releasing energy in the process. To transfer electrons from the matrix into intermembrane space, a portion of the energy must be utilised. This results in the establishment of an electrochemical gradient.

• Water is formed as a result of the splitting of oxygen.

When the electrons have been delivered to the oxygen molecule, it splits in half and absorbs H+, resulting in the formation of water.

• ATP Synthesis

While flowing back into the matrix, the H+ ions pass via an enzyme known as ATP synthase, which produces energy. This regulates the flow of protons into the cell to produce ATP.

• Chemiosmosis

The proton pumps found in complexes I, III, and IV of the electron transport chain are known as electron pumps. During the energetically downhill movement of electrons, the complexes catch the released energy and use it to pump H+. A voltage gradient across the inner mitochondrial membrane is created as a result of this pumping action. The gradient is also referred to as the proton-motive force, and it can be thought of as a sort of stored energy, similar to that of a battery.

Protons, like many other ions, are unable to flow straight through the phospholipid bilayer of the membrane because the core of the membrane is too hydrophobic. A channel protein that forms hydrophilic tunnels across the membrane is the only way for H+ ions to flow along their concentration gradient, rather than by themselves.

A membrane-spanning protein known as ATP synthase is responsible for transporting H+  ions through the inner mitochondrial membrane. ATP synthase is conceptually similar to a turbine in a hydroelectric power plant in that it produces energy. The flow of H+  ions travelling down their electrochemical gradient, rather than the flow of water, turn the rotor instead of the water turning the rotor. With each rotation of the enzyme, it catalyses the addition of a phosphate to ADP, collecting energy from the proton gradient and converting it into ATP.

Electron Transport Chain

• The coenzyme NADH is produced by the majority of metabolic catabolic processes, including the citric acid cycle, glycolysis, beta-oxidation, and others. It is composed of electrons with a high potential for the transmission of energy.
• The oxidation of these processes results in the release of a significant amount of energy. This type of reaction is also referred to as an uncontrollable reaction since the energy stored within the cells is not released all at once.
• During the process of separating the electrons from the NADH and passing them on to the oxygen, a little quantity of energy is released by a sequence of enzymes. 
• The electron transport chain is a collection of enzymes that have formed complexes with one another.
• The inner layer or membrane of mitochondria contains a chain that can be observed. This electron chain transport pathway is also responsible for the oxidation of succinic acid salts.
• It is in the case of eukaryotes that the energy released in the electron transport system from NADH oxidation is used by the enzymes to pump protons across the inner membrane of the mitochondria. 
• Consequently, an electrochemical gradient across the membrane is generated. This can be considered to be one of the most effective illustrations of the concept of oxidative phosphorylation available.

ATP yield

In cellular respiration, how many ATP molecules are produced per gramme of glucose? 

• There is a good chance that if you check in other books or question different teachers, you will get somewhat different responses. 
• In contrast, the majority of contemporary sources indicate that the highest possible ATP yield for one molecule of glucose is approximately 30 to 32 ATP 2,3,4. 
• This range is lower than prior estimates because it takes into consideration the essential transport of ADP into and ATP out of the mitochondrion, which is necessary for energy production.

Who or what is responsible for the figure of 30-32 ATP being used?

• Another two net ATP are generated by the glycolysis reaction and another two ATP (or energetically equivalent GTP) are generated by the citric acid cycle. 
• Aside from those four, all of the remaining ATP is produced through oxidative phosphorylation. To an extensive body of experimental evidence, it appears that four H+  ions must return to the matrix through ATP synthase to power the production of only one ATP. 
• Each NADH produces around 2.5 ATP when electrons from NADH move through the transport chain. Approximately 10 H+  ions are pushed from the matrix to the intermembrane space during the electron transfer process. 
• FADH2  electrons, which enter the chain later in the process, cause pumping of just 6 H+resulting in the creation of approximately 1.5 ATP.

Conclusion

When considering oxidative phosphorylation, it is vital to remember that oxygen is required for it to occur. Water is generated when oxygen absorbs electrons from protein complex 4 and interacts with protons on the inside of the cell, resulting in the formation of water.

Oxidative phosphorylation is a very effective method of making vast amounts of ATP, the energy-conserving molecule that is the building block of all metabolic reactions. This process involves the transfer of electrons between molecules, which results in the formation of a chemical gradient that allows for the generation of ATP.