Aerobic respiration takes place within the mitochondria. Following are the main steps in aerobic respiration:
Step 1: Stepwise removal of all the hydrogen atoms leads to complete oxidation of pyruvate. This leaves three molecules of CO2. This step takes place in the matrix of mitochondria.
Step 2: Electrons removed from hydrogen atoms are passed on to molecular O2. This happens with simultaneous synthesis of ATP. This step takes place in the inner membrane of mitochondria.
Step 3: Pyruvate enters the mitochondria matrix and undergoes oxidative decarboxylation. This involves a complex set of reactions which are catalysed by pyruvic dehydrogenase.
Pyruvic acid + CoA + NAD+ (in presence pyruvic dehydrogenase) → Acetyl CoA + NADH + H+
During this process, two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid (produced from one glucose molecule during glycolysis).
After this, acetyl CoA enters a cyclic pathway. This pathway is called tricarboxylic acid cycle or Citric Acid Cycle or Krebs’ Cycle. This was first explained by Hans Krebs.
- The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid. This reaction is catalysed by the enzyme citrate synthase and a molecule of CoA is released. Citrate is then isomerised to isocitrate.
- It is followed by two successive steps of decarboxylation. These steps of decarboxylation lead to the formation of α-ketoglutaric acid and then formation of succinyl-CoA.
- After that, succinyl-CoA is oxidised to OAA allowing the cycle to continue. During this step, a molecule of GTP is synthesised. This is a substrate level phosphorylation.
- In a coupled reaction GTP is converted to GDP with the simultaneous synthesis of ATP from ADP. Moreover, there are three points in the cycle where NAD+ is reduced to NADH+H+ and one point where FAD+ is reduced to FADH2.
- The continued oxidation of acetic acid via the TCA cycle requires the continued replenishment of oxaloacetic acid. It also requires regeneration of NAD+ and FAD+ from NADH and FADH2 respectively.
Electron Transport System (ETS) and Oxidative Phosphorylation
The next steps 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). This pathway is present in the inner mitochondrial membrane.
- Electrons from NADH (produced in the mitochondria matrix) are oxidized by an NADH dehydrogenase (Complex I). After that, electrons are transferred to ubiquinone which is located within the inner membrane.
- Ubiquinone also receives reducing equivalents via FADH2 (Complex II). FADH2 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 bc1 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). This coupling is necessary for the production of ATP from ADP and inorganic phosphate. The nature of the electron donor decides the number of ATP molecules synthesized.
- Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while oxidation 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. But since oxygen drives the whole process by removing hydrogen from the system, the presence of oxygen is vital.
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.
During photophosphorylation, light energy is utilised for the production of proton gradient. But in respiration, the energy of oxidation-reduction is utilised for the production of proton gradient. Hence, this process is called oxidative phosphorylation.
The energy released during the electron transport system is utilised in synthesizing ATP with the help of ATP synthase (Complex V). This complex is composed of two major components, viz. F1 and F0. The F1 headpiece is a peripheral membrane protein complex. It contains the site for synthesis of ATP. F0 is an integral membrane protein complex which forms the channel through which protons cross the inner membrane. The passage of protons through the channel is accompanied by catalytic site of the F1 component for the production of ATP. For each ATP produced, 2H+passed through F0 down the electrochemical proton gradient.
The Respiratory Balance Sheet
The respiratory balance sheet gives theoretical value about net gain of ATP for every glucose molecule oxidized. The calculations for respiratory balance sheet are based on some assumptions which are as follows:
- There is a sequential and orderly pathway in which one substrate makes the next substrate. Glycolysis, TCA cycle and ETS pathway follow one after another.
- NADH is synthesized in glycolysis and is transferred into the mitochondria. The NADH undergoes oxidative phosphorylation within the mitochondria.
- None of the intermediates in the pathway are utilised to synthesise any other compound.
- Glucose is the only substrate undergoing respiration. No other alternative substrates are entering in the pathway at any stage.
- But these assumptions may not be valid in a living system because all pathways work simultaneously. There can be a net gain of 36 ATP molecules during aerobic respiration of one molecule of glucose.
Glucose is the most favoured substrate for respiration. Other substrates can also be respired but they do not enter the respiratory pathway at the first step. Respiratory process involves both catabolism and anabolism; because breakdown and synthesis of substrates are involved. Hence, respiratory pathway is considered as an amphibolic pathway rather than a catabolic one.
The ratio of the volume of CO2 evolved to the volume of O2 consumed during respiration is called the respiratory quotient (RQ) or respiratory ratio. The RQ for carbohydrates is 1. The RQ for fat and protein is less than 1.
Respiratory Quotient = Volume of CO2 ÷ Volume of O2 consumed
Reaction for respiration of fat:
2(C51H98O6 + 145O2 → 102CO2 + 98H2O
RQ of Fat = 102CO2 ÷ 145O2 = 0.7