Anaerobic & Aerobic Respiration

1.0Anaerobic Respiration or Fermentation

Fermentation takes place under anaerobic conditions in many prokaryotes, unicellular eukaryotes and in germinating seeds. By incomplete oxidation of glucose, enzymes like pyruvic acid decarboxylase and alcohol dehydrogenase catalyse these reactions. 

Lactic acid anaerobic respiration 

In human muscles (during exercise when oxygen is inadequate), pyruvic acid is reduced to lactic acid by lactate dehydrogenase. The reducing agent is NADH + H⁺ which is reoxidised to NAD⁺. 

Lactic acid fermentation is also performed by bacteria Lactobacillus. It is used in curd and other dairy products formation. Curd become sour due to excess lactic acid fermentation. In aerobic respiration, there is an external final electron acceptor i.e. O₂ while in fermentation, there is no external electron acceptor. The final electron acceptor is organic intermediate of the process.

Alcoholic fermentation (In yeast)

Alcoholic fermentation is used in the formation of beverages (alcoholic drinks) and bread. Bread become puffed or spongy due to release of CO₂ during the process. 

In both lactic acid and alcohol fermentation, not much of the energy is released. Less than seven percent of the energy in glucose is released and not all of it is trapped as high energy bonds of ATP. Also, the processes are hazardous – either acid or alcohol is produced. Yeasts poison themselves to death when the concentration of alcohol reaches about 13 percent.

In Anaerobic respiration or fermentation, the net gain of ATP is 2 ATP because during the processes of glycolysis, 4 ATP are synthesised by the substrate level phosphorylation and 2 ATP are consumed, so net gain = 4 – 2 = 2 ATP.

(The 2NADH + H⁺ produced during glycolysis do not enter into ETS, instead they are utilised to form alcohol or lactic acid).

2.0Aerobic Respiration

For aerobic respiration to take place within the mitochondria, the final product of glycolysis, pyruvate is transported from the cytoplasm into the mitochondria. The crucial events in aerobic respiration are: 

• The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of CO₂. (Link reaction and Krebs cycle)
• The passing on of the electrons removed as part of the hydrogen atoms to molecular O₂ with simultaneous synthesis of ATP. (ETS and Oxidative phosphorylation)

What is interesting to note is that the first process takes place in the matrix of the mitochondria while the second process is located on the inner membrane of the mitochondria.

3.0Link/Gateway Reaction (Formation of Acetyl-CoA)

i. This process connects Glycolysis and Krebs cycle so it is called Link reaction or Gateway reaction. During this process, first time CO₂ is evolved during respiration.

ii. Acetyl CoA is a connecting link between glycolysis & Krebs -cycle. Decarboxylation and dehydrogenation (Oxidative decarboxylation) takes place during formation of acetyl CoA. 

iii. Acetyl CoA is formed in the matrix by enzyme pyruvate dehydrogenase complex. 

4.0Krebs Cycle/TCA (Tricarboxylic Acid) Cycle/Ca (Citric Acid) Cycle

i. This cycle was discovered by H. A. Krebs’ (Nobel prize).

ii. TCA cycle occurs in mitochondrial matrix. All the enzymes of TCA cycle, except Succinate dehydrogenase (in the inner mitochondrial membrane) present in matrix.

iii. During Krebs’ cycle, acetyl CoA is completely oxidised into CO₂. 

iv. Krebs cycle is also called Citric acid (CA) cycle because 1^st Compound is Citric acid (6C). In this acid, 3 carboxylic groups (COOH) are found so process is also called TCA (Tricarboxylic Acid) cycle.

v. In Krebs’ cycle, oxaloacetic acid (OAA) is the first member and it also act as first acceptor of Acetyl Co-A. In the end of this cycle, OAA is re-formed.

vi. The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid. The reaction is catalysed by the enzyme citrate synthase and a molecule of CoA is released.

vii. Citrate is then isomerised to isocitrate. It is followed by two successive steps of decarboxylation, leading to the formation of α-ketoglutaric acid and then succinyl-CoA. 

viii. In the remaining steps of citric acid cycle, succinyl-CoA is oxidised to OAA allowing the cycle to continue. During the conversion of succinyl-CoA to succinic acid 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. 

ix. Also, there are three points in the cycle where NAD⁺ is reduced to NADH + H⁺ and one point where FAD⁺ is reduced to FADH₂.

x. The continued oxidation of Acetyl CoA via the TCA cycle requires the continued replenishment of oxaloacetic acid, the first member of the cycle. In addition, it also requires regeneration of NAD⁺ and FAD⁺ from NADH and FADH₂ respectively. 

xi. Oxidation or dehydrogenation occurs at 4 places in one Krebs cycle resulting in the formation of 3NADH, 1FADH₂ along with 1 GTP (ATP) produced by substrate level phosphorylation in each turn of TCA cycle. (=12 ATP) 

xii. Link reaction and Krebs’ cycle occurs two times during complete oxidation of 1 hexose molecule because by glycolysis one hexose converts into two pyruvic acid and both the molecules undergo separate link reaction and Krebs cycle. 

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