Botany - Respiration in Plants Question with Solution | TestHub

Kreb's cycle / TCA cycle:
Discovery: This cycle has been named after Sir Hans Kreb, the German biochemist who discovered this cycle in 1937. Kreb's cycle is also known as the citric acid cycle as it is the first stable product of the cycle. This cycle takes place in the matrix of mitochondria as it has a presence of various soluble enzymes here. Like glycolysis, it also involves 10 sequential steps in which the cycle is completed.

The sequential steps of the Kreb cycle are as follows:

(i) Condensation reaction: A 2C compound acetyl CoA combines with 4C compound oxaloacetate to form the 6C product called citrate. 

$Oxaloacetate + Acetyl CoA \stackrel{Citrate synthase}{\to } Citrate + CoA$

(ii) Dehydration reaction: A molecule of water is removed in this reaction.

$Citrate \underset{F{e}^{+2}}{\overset{Aconitase}{\to }} cis-Aconitate + H_{2}O$

(iii) Hydration reaction: This reaction involves the addition of water molecules to form isocitrate. 

$cis-Aconitate + H_{2}O \underset{F{e}^{+2}}{\overset{Aconitase}{\to }} Isocitrate$

(iv) Dehydrogenation: NADH₂ is formed in this step.

$Isocitrate + NA{D}^{+} \underset{M{n}^{+2}}{\overset{Isocitrate dehydrogenase}{\to }} Oxalosuccinate + NAD{H}_2$

(v) Decarboxylation: This reaction involves the removal of CO₂.

$Oxalosuccinate \underset{M{n}^{+2}}{\overset{Decarboxylase}{\to }} \alpha -Ketoglutarate$

(vi) Oxidative decarboxylation: This reaction is similar to a link reaction.$\alpha -Ketoglutarate + CoA + NA{D}^{+} \underset{M{g}^{+2}, TPP, Lipoic acid}{\overset{\alpha -Ketoglutarate dehydrogenase}{\to }} Succinyl CoA + NAD{H}_2 + C{O}_2$

(vii) Substrate-level phosphorylation: This step involves the formation of GTP which in presence of phosphokinase enzyme gets converted into ATP.$Succinyl CoA + GDP + Pi \stackrel{Succinyl CoA synthetase/Succinyl thiokinase}{\to } Succinate + GTP + CoA$

(viii) Dehydrogenation: This reaction involves the formation of FADH₂.$Succinate + FAD \stackrel{Succinic dehydrogenase}{\to } Fumarate + FAD{H}_2$

(ix) Hydration: This step involves the addition of water molecules to form malate.$Fumarate + H_{2}O \stackrel{Fumarase}{\to } Malate$

(x) Dehydrogenation: It involves the reformation of oxaloacetate.$Malate + NA{D}^{+} \stackrel{Malate dehydrogenase}{\to } Oxaloacetate + NAD{H}_2$

Hexokinase is the enzyme involved in the first step of glycolysis:

$Glu\cos e + ATP \underset{M{g}^{+2}}{\overset{Hexokinase}{\to }} Glu\cos e 6-Phosphate + ADP$