Acetyl CoA formation occurs in the mitochondrial matrix through a process called beta-oxidation. Beta-oxidation is the breakdown of fatty acids to produce acetyl CoA.

3-phosphoglyceraldehyde and dihydroxyacetone phosphate.

RQ slightly more than unity is found when organic acids are broken down as respiratory substrates under aerobic conditions, e.g;
  \(2\underset { Oxalic\quad acid }{ (COOH)_{ 2 } } +{ O }_{ 2 }\rightarrow 4{ CO }_{ 2 }+2{ H }_{ 2 }O\)
\(\\ RQ\frac { { 4CO }_{ 2 } }{ { 1O }_{ 2 } } =4.0\)
\(\\ \underset { Malic\quad acid }{ { C }_{ 2 }{ H }_{ 6 }{ O }_{ 5 } } +3{ O }_{ 2 }\rightarrow { 4CO }_{ 2 }+{ 3H }_{ 2 }O\)
\(\\ RQ=\frac { { 4CO }_{ 2 } }{ { 3O }_{ 2 } } =1.3\)

the stepwise release of energy have advantage to the cell. Because if all the energy from the glucose would be released at once then most of it would be lost in the form of light and heat. This energy is trapped in ATP is utilised in various energy-requiring processes of the organisms and for the synthesis of the other biomolecules in the cell.

If a person is feeling dizzy, glucose or fruit juice is given immediately because, it is readily oxidised and oxidation of 1 g of glucose yields more energy than a cheese sandwich.

The aerobic respiration is a high energy yielding process.During the process of aerobic respiration as many as 36 molecules of ATP are produced for every molecule of glucose that is utilised.
This shows that aerobic respiration produces much more energy than anaerobic respiration, which produces only 2 ATPmolecules

The main steps in aerobic respiration are as follows
(i) Glycolytic breakdown of glucose into pyruvic acid.
(ii) Oxidative decarboxylation of pyruvic acid to acetyl Co-A(acetyl coenzyme-A)
(iii) Krebs' cycle.
(iv) Terminal oxidation and phosphorylation in respiratory chain.
It occurs inside the mitochondrial matrix.

It helps in maintaining proton gradient on the two sides of the membrane. It also helps in synthesis of ATP molecule.

Oxygen is the ultimate acceptor of electrons in ETS because it is highly reactive and combines with protons to form metabolic water.

3NADH₂ molecules are produced from one molecule of acetyl Co-A in TCA cycle.

(i) The anaerobic respiration occurs even in organisms that live in aerobic condition like human beings and angiosperms under conditions of oxygen scarcity that happens in the muscle tissue. Under intense use, muscles demand too much energy (ATP) and consume much more oxygen to produce that energy.
(ii) This high consumption leads to oxygen scarcity and the muscle cells begin to make lactic acid by anaerobic respiration trying to fulfill their energetic needs.
(iii) Similarly, yeast cells under deficient conditions carry out anaerobic respiration.

It is an alternative method of aerobic respiration which occurs in cytoplasm of mature cells. This pathway accerents 60% of total respiration occurring in lower cells.

Pyruvate synthesis takes place in cytoplasm by the process of glycolysis.
The chemical reaction,wherein pyruvic acid dehydrogenase acts as a catalyst is as follows
The reactions catalysed by pyruvic dehydrogenase require the  participation of several coenzymes, including NAD⁺ and Co-A
Pyruvic acid + Co-A + NAD+ \(\underrightarrow { { Mg }^{ 2+ } } \)      Acetyl Co-A + NADH + H⁺
                        Pyurvate dehydrogenase               

Red or dark muscles store myoglobin and hence, oxygen for meeting the gap between supply and consumption during continuous activity. Therefore, these muscles can continuously work and respire aerobically for long periods. High proportion of these muscles allow athletes to participitate in long duration events like cycling, swimming and distance running etc.

The following steps in the respiratory process are to release and utilize the energy stored in NADH+ H⁺and FADH₂. This is accomplished when they are oxidised through the electron transport system and the electrons are passed on to O₂ resulting in the formation of H₂O. The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS) and it is present in the inner mitochondrial membrane.
Electrons from NADH produced in the mitochondrial matrix during citric acid cycle are oxidised by an NADH dehydrogenase (complex I), and electrons are then transferred to ubiquinone located within the inner membrane. Ubiquinone also receives reducing equivalents via FADH₂ (complex II) that is generated during oxidation of succinate in the citric acid cycle.
The reduced ubiquinone (ubiquinol) is then oxidised with the transfer of electrons to cytochrome c via cytochrome bc l complex (complex III).
Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV. Complex IV refers to cytochrome c oxidase complex containing cytochromes a and a³, and two copper centres.
When the electrons pass from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase (complex V) for the production of ATP from ADP and inorganic phosphate. The number of ATP molecules synthesized depends on the nature of the electron donor.
Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while that of one molecule of FADH₂ produces 2 molecules of ATP. Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process. Yet, the presence of oxygen is vital, since it drives the whole process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor.
Unlike photophosphorylation where it is the light energy that is utilised for the production of proton gradient required for phosphorylation, in respiration it is the energy of oxidation-reduction utilised for the same process. It is for this reason that the process is called oxidative phosphorylation.
The energy released during the electron transport system is utilised in synthesising  ATP with the help of ATP synthase (complex V). This complex consists of two major components, F₁ and F₀. The F₁ head piece is a peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate. F₀ is an integral membrane protein complex that forms the channel through which protons cross the inner membrane. The passage of protons through the channel is coupled to the catalytic site of the F₁ component for the production of ATP. For each ATP produced, 2H⁺passes through F₀ from the intermembrane space to the matrix down the electrochemical proton gradient.

(a) Respiration and Combustion

(b) Glycolysis and Krebs’ cycle :
Glycolysis : Glycolysis is a metabolic pathway that is found in the cytoplasm of cells in all living organisms and is anaerobic, or doesn't require oxygen. The process converts one molecule of glucose into two molecules of pyruvate, and makes energy in the form of two net molecules of ATP.
Citric acid Cycle or Krebs Cycle : When oxygen is present, acetyl-CoA is produced from the pyruvate molecules created from glycolysis. Once acetyl-CoA is formed, two processes can occur, aerobic or anaerobic respiration.
When oxygen is present, the mitochondria will undergo aerobic respiration which leads to the Krebs cycle. However, if oxygen is not present, fermentation of the pyruvate molecule will occur. In the presence of oxygen, when acetyl-CoA is produced, the molecule then enters the citric acid cycle (Krebs cycle)inside the mitochondrial matrix, and gets oxidized to CO₂ while at the same time reducing NAD to NADH. NADH can be used by the electron transport chain to create further ATP as part of oxidative phosphorylation. To fully oxidize the equivalent of one glucose molecule, two acetyl-CoA must be metabolized by the Krebs cycle. Two waste products, H₂O and CO₂, are created during this cycle.
The citric acid cycle is an 8-step process involving 8 different enzymes. Throughout the entire cycle, acetyl-CoA changes into citrate, isocitrate, \(\alpha\) ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and finally, oxaloacetate. The net energy gain from one cycle is 3 NADH,1 FADH, and 1 ATP. Thus, the total amount of energy yield from one whole glucose molecule (2 pyruvate molecules) is 6 NADH, 2 FADH, and 2 ATP.
(c) Aerobic respiration and Fermentation :
Aerobic respiration :
Aerobic respiration is the main means by which both plants and animals utilize energy in the form of organic compounds that was previously created through photosynthesis. Respiration requires oxygen in order to generate energy (ATP). It is the preferred method of pyruvate breakdown from glycolysis and requires that pyruvate enter the mitochondrion in order to be fully oxidized by the Krebs cycle. The product of this process is energy in the form of ATP (Adenosine Triphosphate), by substrate-level phosphorylation, NADH and FADH₂.
Anaerobic Respiration or Fermentation :
Without oxygen, pyruvate is not metabolized by cellular respiration but undergoes a process of fermentation. The pyruvate is not transported into the mitochondrion, but remains in the cytoplasm, where it is converted to waste products that may be removed from the cell. This serves the purpose of oxidizing the hydrogen carriers so that they can perform glycolysis again and removing the excess pyruvate. This waste product varies depending on the organism. In skeletal muscles, the waste product is lactic acid. This type of fermentation is called lactic acid fermentation. In yeast, the waste products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic or ethanol fermentation. The ATP generated in this process is made by substrate phosphorylation, which is phosphorylation that does not involve oxygen.

Glycolysis : Glycolysis is a metabolic pathway that is found in the cytoplasm of cells in all living organisms and is anaerobic, or doesn't require oxygen. The process converts one molecule of glucose into two molecules of pyruvate, and makes energy in the form of two net molecules of ATP.
Citric acid Cycle or Krebs Cycle : When oxygen is present, acetyl-CoA is produced from the pyruvate molecules created from glycolysis. Once acetyl-CoA is formed, two processes can occur, aerobic or anaerobic respiration.
When oxygen is present, the mitochondria will undergo aerobic respiration which leads to the Krebs cycle. However, if oxygen is not present, fermentation of the pyruvate molecule will occur. In the presence of oxygen, when acetyl-CoA is produced, the molecule then enters the citric acid cycle (Krebs cycle)inside the mitochondrial matrix, and gets oxidized to CO₂ while at the same time reducing NAD to NADH. NADH can be used by the electron transport chain to create further ATP as part of oxidative phosphorylation. To fully oxidize the equivalent of one glucose molecule, two acetyl-CoA must be metabolized by the Krebs cycle. Two waste products, H₂O and CO₂, are created during this cycle.
The citric acid cycle is an 8-step process involving 8 different enzymes. Throughout the entire cycle, acetyl-CoA changes into citrate, isocitrate, \(\alpha\) ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and finally, oxaloacetate. The net energy gain from one cycle is 3 NADH,1 FADH, and 1 ATP. Thus, the total amount of energy yield from one whole glucose molecule (2 pyruvate molecules) is 6 NADH, 2 FADH, and 2 ATP.

Aerobic respiration :
Aerobic respiration is the main means by which both plants and animals utilize energy in the form of organic compounds that was previously created through photosynthesis. Respiration requires oxygen in order to generate energy (ATP). It is the preferred method of pyruvate breakdown from glycolysis and requires that pyruvate enter the mitochondrion in order to be fully oxidized by the Krebs cycle. The product of this process is energy in the form of ATP (Adenosine Triphosphate), by substrate-level phosphorylation, NADH and FADH₂.
Anaerobic Respiration or Fermentation :
Without oxygen, pyruvate is not metabolized by cellular respiration but undergoes a process of fermentation. The pyruvate is not transported into the mitochondrion, but remains in the cytoplasm, where it is converted to waste products that may be removed from the cell. This serves the purpose of oxidizing the hydrogen carriers so that they can perform glycolysis again and removing the excess pyruvate. This waste product varies depending on the organism. In skeletal muscles, the waste product is lactic acid. This type of fermentation is called lactic acid fermentation. In yeast, the waste products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic or ethanol fermentation. The ATP generated in this process is made by substrate phosphorylation, which is phosphorylation that does not involve oxygen.

The process of aerobic respiration is divided into four phases – glycolysis, TCA cycle, ETS, and oxidative phosphorylation. It is generally assumed that the process of respiration and production of ATP in each phase takes place in a step-wise manner. The product of one pathway forms the substrate of the other pathway. Various molecules produced during respiration are involved in other biochemical processes. The respiratory substrates enter and withdraw from pathway on necessity. ATP gets utilized wherever required and enzymatic rates are generally controlled. Thus, the step-wise release of energy makes the system more efficient in extracting and storing energy.