Question

If one molecule of acetyl-CoA oxidized inside a eukaryotic cell then the gain of energy is equal to
A. 12ATP
B. 9ATP
C. 18ATP
D. 15ATP

Answer 1

Hint: Cellular respiration is the process in which the energy-rich molecule ATP is formed with the help of Glucose and oxygen. ATP stands for adenosine triphosphate is an organic compound that provides energy to carry out several metabolic actions.

Complete answer: Cellular respiration is a set of several metabolic reactions. It is a catabolic process where the breakdown of glucose takes place with the help of oxygen. Carbon dioxide and water are released. The energy-rich compound formed is ATP. Hydrolysis of ATP gives ADP i.e. adenosine diphosphate and phosphate which is used to carry out cellular work.
Overall reaction: \[Glucose{\text{ }} + {\text{ }}oxygen \to ATP{\text{ }} + {\text{ }}water{\text{ }} + {\text{ }}carbon{\text{ }}dioxide\].
All the reactions of cellular respiration are grouped into three stages-
1. Glycolysis
2. The Krebs cycle is known as the citric acid cycle because its first stable compound is citric acid.
3. ETS i.e. electron transport system.
Glycolysis- Glycolysis is the set of reactions in which glucose is converted into pyruvate. It is a ten-step pathway that occurs in the cytoplasm of a cell.
The Krebs cycle is also called the citric acid cycle. In the Krebs cycle, Pyruvate further enters mitochondria. It is an 8 step pathway that forms NADH, FADH molecules. These molecules are further transported to ETS. Acetyl-CoA enters the Krebs cycle and undergoes oxidation. The net gain of the Krebs cycle is three molecules of NADH, one GTP molecule, and one FADH$_2$ molecule.​ When one NADH molecule enters ETS it gains energy equivalent to three ATP while one FADH2 ​is equivalent to two ATP.
Therefore, the total ATP produced by Krebs cycle is – $\left( {3 \times 3} \right) + 2 + 1 = $ 12ATP
So, the correct answer to the given question is option A, i.e., 12ATP.

Note: Krebs cycle is sometimes also referred to as TCA cycle tricarboxylic acid cycle. ETS (Electron transport chain)-NADH, FADH act as electron donors. These electrons pass through membrane-associated protein complexes and result in the formation of ATP.