Aerobic and Anaerobic Respiration: Definition, Mechanism

Cellular respiration is divided into two categories depending upon the availability of atmospheric oxygen. Which is -

1. Aerobic Respiration
2. Anaerobic Respiration

1. Aerobic respiration - The oxidative breakdown of respiratory substrates with the help of atmospheric O2 is known as aerobic respiration. It involves the complete breakdown of substrates into end product CO2 and water, releasing energy. This is a common type of respiration in all animals.

          C6H12O6 + 6O2 → 6CO2 + H2O + 674 kcal

2. Anaerobic respiration (fermentation) -  Oxidation of respiratory substrates in absence of atmospheric O2 is termed as anaerobic respiration. It involves incomplete break down of respiratory substrates in which the end products such as ethanol or lactic acid produced and CO2 is released. Anaerobic respiration is common among certain microorganisms. The following equation is usually given for anaerobic respiration and fermentation-

C6H12O6 → 2C2H5OH + 2CO₂ + 56 K Cal

Anaerobic respiration carried out by some fungi (e.g. yeast) and bacteria is sometimes termed as fermentation.

Mechanism of Respiration

The respiration starts with glucose (usually). In aerobic and anaerobic respiration initial reactions are common as a result of pyruvic acid is formed by breakdown of glucose. The process is called Glycolysis or EMP pathway (Embden Meyerhof Parnas pathway).

After this stage the fate of pyruvic acid is different depending upon the absence or presence of oxygen. If oxygen is present there is complete oxidation of pyruvic acid into H2O and CO2 and chemical reactions through which this occurs is called Tri-Carboxylic Acid Cycle (TCA Cycle) or on the basis of the name of discoverer Krebs cycle. This cycle occurs in mitochondria. If oxygen is absent, pyruvic acid is form ethyl alcohol (C2H5OH) and CO2 without the help of any cell organelle. This process is called anaerobic respiration (Intramolecular respiration).

Mechanism of Aerobic Respiration

Stepwise breakdown of respiratory substrates to CO₂ and H₂O in presence of oxygen is referred to as aerobic respiration. It involves at least 3 major stages. These stages are as follows:

(i) Decarboxylation & dehydration
(ii) Krebs cycle
(iii) Electron Transport System (E.T.S.)

(i) Decarboxylation and dehydration - Pyruvic acid produced in cytosol enters mitochondria. First of all there is activation of pyruvic acid in which NAD and Coenzyme A (CoA.SH) react with pyruvic acid and oxidative decarboxylation takes place in the presence of Pyruvic dehydrogenase. In this reaction a molecule of CO₂ is released and NAD is reduced to NAD.2H, leading to the formation of Acetyl CoA (Activated acetic acid = activated acetate). This reaction is not part of Krebs cycle.


CH3CO.COOH + CoASH + NAD   →    CH3.CO.S.CoA  +  NADH+ + CO2



(ii) Krebs cycle (Tricarboxylic Acid Cycle = TCA Cycle) - The steps of this cycle were given by Hans Krebs in 1931. Acetyl-CoA enters into the Krebs cycle. Various steps of the cycle take place in mitochondria. As a result of different reactions taking place in a cyclic manner, complete oxidation of acetyl-CoA takes place. Several Krebs cycle intermediates are used in various anabolic pathways.

(iii) Electron Transport System (ETS) or terminal oxidation - In the process of oxidation taking place in the Krebs cycle, dehydrogenase enzymes release pairs of Hydrogen and electrons from different substrates. Hydrogen and electrons pass through different electron carriers and ultimately react with oxygen (at the end of ETS) and form water molecules. During the transfer of hydrogen atoms from one enzyme carrier to another in ETS, a large amount of energy is released, which is stored in the pyrophosphate bond of ATP, produced from ADP and inorganic phosphate (iP). The enzymes which participate in this process are called Respiratory Chain Enzymes or Electron Transport System Enzymes or terminal oxidation. All these enzymes are found in F1 particles of mitochondria.

Factors Affecting Respiration

(A) External factors

1. Temperature - An increase in temperature from 10°C to 35°C bring an increased rate of respiration. The Q10 for respiration being 2 to 3. At low temperatures the respiratory enzyme become inactive, so the rate of respiration falls.
2. Light - Its effect is indirect. Under suitable light the rate of photosynthesis is optimum which supplies respiratory substrates at a moderate rate. So rate of respiration increased.
3. Oxygen - The minimum oxygen concentration at which aerobic respiration occurs is known as extinction point (3 to 10% of O2). Reduction in the amount of breakdown of respiratory substrate and evolution of CO₂ , when an anaerobically respiring material is brought in oxygen containing environment is called Pasteur effect.
4. Carbon dioxide - Higher concentration of CO₂ in the atmosphere especially in the poorly aerated soil has inhibitory effect on the rate of respiration.
5. Inorganic salts - If the plants are placed in adequate salt solution, their rate of respiration is increased. The process is called as salt respiration.
6. Water -Proper hydration of the respiratory cells is essential for respiration. Dehydration slow down the enzymic activity so respiration also. In dry seeds the respiration is at its minimum because in absence of medium (H2O) the respiratory enzymes become inactive.
7. Injury - Injury or wounds cause increase in the rate of respiration because the cells at the injured portion become meristematic to form new cells to heal up the wound hence they require more energy which is supplied by increased rate of respiration.

(B) Internal factors

1. Protoplasmic factors - Actively dividing meristematic cells which contain dense protoplasm, show higher rate of respiration. The old and mature cells have lower rate of respiration. But in some fruits like banana rate of respiration increased during maturation or ripening due to ethylene production it is called as respiratory climacteric.
2. Concentration of respiratory substrate - Increased concentration of respirable food material brings about an increase in the rate of respiration.

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