Question

How many carbon dioxide molecules are produced when two molecules of pyruvic acid are fed in Krebs cycle?
a. Two
b. Three
c. Four
d. Six

Answer 1

Hint: Krebs cycle is a tricarboxylic acid cycle in which glucose and other molecules are broken down into simpler compounds like carbon dioxide and water in the presence of oxygen in order to release energy. The cycle takes place in aerobic organisms as it is the process of aerobic cell metabolism.

Complete answer:
Krebs cycle:
It is also known as TCA (Tri-Carboxylic Acid) or CAC (Citric Acid Cycle). It is the second stage of aerobic respiration. Aerobic respiration is a form of respiration which takes place in the presence of oxygen. Another form of respiration is anaerobic respiration.
Step 1: In the first step, acetyl CoA which is composed of two carbon molecules combines with oxaloacetate which is a four carbon molecule to produce citrate, a six carbon molecule.
Step 2: This six carbon citrate is isomerised to isocitrate.
Step 3: Isocitrate, an isomer of citrate is oxidized to a five carbon molecule known as alpha-ketoglutarate. The oxidation process results in carbon dioxide release. Here, one NADH (Nicotinamide adenine dinucleotide) is also formed. In this step, the first molecule of carbon dioxide is released.
Step 4: After this step, alpha-ketoglutarate oxidised to form a four carbon molecule which combines to succinyl CoA. With this, another NADH molecule is released along with another molecule of carbon dioxide.
Step 5: Succinyl CoA converts to a four carbon molecule called succinate and one GTP molecule.
Step 6: The succinate formed in the previous step is converted to fumarate which is a four carbon molecule. This fumarate is further converted to malate, again a four carbon molecule. Hence, malate converts to oxaloacetate and releases a third molecule of NADH.

From the cycle above, we infer that there are two carbon dioxide molecules released during Krebs cycle. Pyruvic acid is an organic acid. During the glycolysis process of two pyruvic acid molecules one molecule of carbon dioxide is produced. Therefore, the total number of molecules produced is four.

So if two carbon dioxide molecules are produced from a single cycle having one mole of pyruvic acid then two molecules of pyruvic acid will give us four carbon dioxide molecules.

Hence, the correct answer is option (C).

Note: Students may commit a mistake while reading the question and mark the answer as two carbon dioxide molecules- the number of molecules released during the kreb’s cycle. It is important to consider the presence of pyruvic acid in the kreb’s cycle as it also produces a carbon dioxide molecule.