When a molecule of pyruvic acid is subjected to anaerobic oxidation and forms lactic acid, there is
A. Loss of 3 ATP molecules
B. Loss of 6 ATP molecules
C. Gain of 2 ATP molecules
D. Gain of 4 ATP molecules

Hint: Anaerobic oxidation of pyruvic acid regenerates NAD+. NAD+ is required as an electron acceptor during the reactions of glycolysis. If the supply of NAD+ is exhausted, glycolysis can not continue.

Complete step by step answer: Cellular respiration is the process during which nutrients such as simple sugar molecules are oxidized and the energy stored in their chemical bonds is used to synthesize the energy currency of the cells. The energy currency of the cells is ATP molecules. Cellular respiration breaks down the glucose molecule step by step to retrieve maximum energy stored in it.
The first phase of cellular respiration splits a molecule of glucose into two molecules of pyruvic acid by a sequence of ten reactions. This phase is called glycolysis.
In the presence of oxygen, the oxidative decarboxylation of pyruvate forms acetyl CoA which then enters into the Krebs’ cycle in the mitochondrial matrix. Krebs’ cycle oxidizes acetyl CoA into CO2 and H2O and the chemical energy is stored in the form of NADH and FADH2 (reducing powers). These reducing powers enter into the electron transport chain to become oxidized.
When the cells are not supplied with sufficient oxygen to facilitate the entry of pyruvate into the aerobic phase of cellular respiration, anaerobic respiration occurs. One mechanism of anaerobic respiration is the reduction of pyruvic acid into lactic acid. Here, NADH serves as a source of electrons. The reduction of each molecule of pyruvic acid consumes one NADH which in turn is oxidized into NAD+.
Since the oxidation of one molecule of NADH by electron transport chain during the aerobic respiration produces around 3 ATP molecules, the formation of lactic acid from pyruvic acid results in the loss of 3 ATP molecules.
So, the correct answer is option A. Loss of 3 ATP molecules.
So, the correct answer is “Option A”.

Note: Glycolysis is an oxygen-independent stage of cellular respiration. It occurs in the cytoplasm. It is common to both aerobic and anaerobic respiration. Oxidation of reducing powers by electron transport chain during aerobic cellular respiration produces the electrochemical gradient across the inner mitochondrial membrane to support the phosphorylation of ADP into ATP.