Question

Why is the Kreb cycle also known as citric acid cycle?
A. Due to first stable product as citric acid
B. First stable product citric acid contain two carboxyl group
C. First stable product citric acid contain 3-carboxyl group
D. Citric acid is devoid of any –COOH group

Answer 1

Hint: Kreb cycle is also known as Tricarboxylic acid cycle (TCA) or Citric acid cycle. This cycle is a part of aerobic respiration and a main resource for providing energy to the cells. It was discovered by a German scientist Hans Adolf Kreb in 1937, hence the name Kreb cycle.

Complete answer: Kreb cycle is an element of a large metabolic activity starting from glycolysis in which glucose is oxidised into pyruvic acid, which in turn is oxidised to form Acetyl-CoA.
The Kreb cycle converts the energy of Acetyl-CoA into Nicotinamide adenine dinucleotide (NADH). It starts with the reaction of Acetyl-CoA and oxalo acetic acid and water, to form the first stable product – Citric Acid.
Therefore, this cycle is also known as citric acid cycle. So the correct option is (A) Due to the first stable product as citric acid.

Additional Information: The cycle begins with addition of Acetyl-CoA to oxalo acetic acid forming Citric acid which is then isomerised into isocitrate. Isocitrate is then converted into alpha-ketoglutaric acid through oxidation and decarboxylation. Alpha-ketoglutaric acid undergoes decarboxylation and oxidation again to produce Succinic acid along with phosphorylation of GDP into GTP. Succinic acid is then converted into Malic acid, which transforms into oxalo acetic acid that again starts the cycle. There are three parts where NADH is formed and one where FADH2 is formed.

Note: The Kreb cycle occurs in the cells of all the eukaryotes and prokaryotes. It was observed for the first time in the muscle tissue of pigeons. It occurs in the mitochondria of a eukaryotic cell, and in cytosol of a prokaryotic cell.