Question

Give an account of (Kreb’s cycle) Citric acid cycle.

Or

 Write the schematic representation of citric acid cycle with preparatory phase.

Answer 1

Kreb’s cycle or citric acid cycle is second step of respiration and takes place in the mitochondrial matrix. Pyruvic acid formed during glycolysis is first converted to acetyl CoA a 2 Carbon compound which in the mitochondrial matrix to initiate Kreb’s cycle.

Condensation:

acetyl CoA combines with oxaloacetate in presence of water to give a tricarboxylic acid citric acid. CoA is released.

Dehydration and Hydration: 

The citric acid then loses a molecule of water to form cis-aconitate which takes up a molecule of water and forms isocitrate.

Dehydrogenation:

Isocitrate undergoes dehydrogenation and forms oxalosuccinate. This yields 2 molecules of NADH2.

Decarboxylation:

By giving out CO₂ – oxalosuccinate is converted to αketoglutarate.

Dehydrogenation & decarboxylation: 

The a- ketoglutarate simultaneously shows dehydrogenation decarboxylation to form succinyl CoA, 2 molecules of CO₂, NADH₂ are released and CoA is added. Hydration & 

Phophorylation:

Succinyl CoA in presence of water forms succinate, CoA is released and 2 GTP’s are synthesised which are converted to ATP. 

Dehydrogenation:

Dehydrogenation of succinate yields Fumarate and 2 molecules of FADHO₂ are formed. 

Hydration:

Fumarate in presence of water yields Malate.

Malateby dehydrogenation gives 2 molecules of NADH₂ and is converted to oxaloacetate. ATP Calculation.

8 NADH2 = 8 x 3 = 24 ATP

2 FADH, = 2 x 2 = 4 ATP

2 GTP = 2 = 2ATP 

Total = 30 ATP