Question

When conditions in a muscle become anaerobic during strenuous exercise, why is it necessary to convert pyruvate to lactate? a. to decrease ${\mathrm{NAD}}^{+}$ and increase $\mathrm{NADH}$ b. to decrease NADH and increase NAD ${}^{+}$ c. to increase NADH and increase NAD ${}^{+}$ d. to decrease NADH and decrease NAD ${}^{+}$ e. to keep oxidative phosphorylation running

Short answer

The correct answer is b. The conversion of pyruvate to lactate during anaerobic conditions is necessary to decrease NADH and increase NAD+.

Step-by-step solution

Understanding of Anaerobic Respiration and Pyruvate

Under normal conditions, pyruvate formed in glycolysis is transported into the mitochondria and converted into Acetyl-CoA to enter the citric acid cycle (Krebs Cycle). However, under strenuous exercise, the muscle cells may not get enough oxygen, and thus conditions can become anaerobic. In these conditions, pyruvate is converted to lactate in the cytoplasm.

Role of NAD+ and NADH

The step where pyruvate is converted to lactate is actually a redox reaction where NADH is oxidized to NAD+. This is a critical reaction as the NAD+ produced can be recycled back into glycolysis to continue the production of ATP under anaerobic conditions. Anaerobic respiration produces 2 ATP/glucose, much less than the potential 36 ATP/glucose produced in aerobic respiration.

Identify the Correct Option

Based on the information, it's clear that the conversion of pyruvate to lactate serves to decrease NADH and increase NAD+. Therefore, option 'b' is the correct answer.

Key concepts

Pyruvate

Pyruvate is a key intermediate in several metabolic pathways. It is the end product of glycolysis, a process that breaks down glucose into smaller units. Under aerobic conditions, pyruvate enters the mitochondria where it is converted into Acetyl-CoA and fed into the citric acid cycle (Krebs Cycle) to produce energy. However, during anaerobic respiration, such as intense exercise when oxygen levels are low, pyruvate has a different fate. It remains in the cytoplasm of the cell and is converted into lactate. This conversion allows the cell to regenerate NAD+, a crucial component needed for glycolysis to proceed, ensuring that ATP, albeit in lesser amounts, continues to be synthesized.

Lactate

Lactate is produced from pyruvate during anaerobic respiration. In muscle cells, this process occurs due to the lack of adequate oxygen (anaerobic conditions). The conversion of pyruvate to lactate is catalyzed by the enzyme lactate dehydrogenase. This reaction is essential for regenerating NAD+, which is consumed during glycolysis to convert glucose into pyruvate. By regenerating NAD+, glycolysis can continue, even when the oxygen supply is limited. This means that muscles can still get some energy despite the challenging conditions.

NAD+

NAD+ stands for Nicotinamide Adenine Dinucleotide. It is an important coenzyme in cellular respiration. During glycolysis, NAD+ accepts electrons and is reduced to NADH. However, the continuous operation of glycolysis under anaerobic conditions requires the regeneration of NAD+ from NADH. Pyruvate's conversion to lactate accomplishes this by using NADH and releasing NAD+. This recycling is crucial as it allows glycolysis to persist in the absence of oxygen and ensures continuous ATP generation.

NADH

NADH is the reduced form of NAD+, carrying electrons and being an essential player in energy production. During glycolysis, NADH is produced, and it usually needs to be oxidized back to NAD+ to keep glycolysis going. In aerobic conditions, this oxidation occurs in the mitochondria. However, under anaerobic conditions, like intense exercise, the conversion of pyruvate to lactate helps oxidize NADH back to NAD+, permitting glycolysis to continue. This transformation is vital for maintaining energy production when oxygen is scarce.

Glycolysis

Glycolysis is the process that breaks down glucose into pyruvate, generating small amounts of ATP in the cytoplasm of cells. It is the first step in both aerobic and anaerobic respiration. Glycolysis does not require oxygen, making it an important energy source when oxygen levels are low. It produces 2 net ATP molecules per glucose molecule. The ATP and NADH produced in glycolysis provide necessary energy and reducing power for cells. Under anaerobic conditions, glycolysis can proceed by converting pyruvate to lactate, allowing regeneration of NAD+, essential for further glycolytic rounds to occur.