Question

During lactic acid fermentation, which of the following events is least likely to occur? (A) An oxygen debt builds. (B) NADH is reduced. (C) Lactate accumulates in the muscle tissue. (D) ATP is produced. (E) \(\mathrm{NAD}^{+}\) is recycled and returns to the Krebs cycle.

Short answer

The answer is (B). During lactic acid fermentation, NADH doesn't get reduced but oxidized.

Step-by-step solution

Understand lactic acid fermentation

Lactic acid fermentation is an anaerobic process carried out by some bacteria and muscles cells when oxygen is scarce. The process involves the NADH produced during glycolysis reducing pyruvate into lactate, regenerating NAD+ which returns to glycolysis allowing this to continue creating ATP. An oxygen debt can build in muscles performing this fermentation as they require oxygen to purge the lactate and repay the 'debt'.

Analyze the options

Now, let's go through each option: (A) is true because lactic acid fermentation occurs during oxygen scarcity, leading to an oxygen debt. (C) is true because lactic acid fermentation results in lactate's accumulation. (D) is true since ATP is indeed produced in the process. (E) is true as the regeneration of NAD+ is crucial for the resumption of glycolysis.

Identify the least likely event

From the analysis, we can see that all the statements are in line with the principles of lactic acid fermentation, except for (B) NADH is reduced. In fact, during this process, NADH acts as an electron donor, meaning it gets oxidized, not reduced.

Key concepts

Anaerobic Respiration

Anaerobic respiration is a type of respiration that does not use oxygen. This process is critical for organisms in environments where oxygen is scarce or unavailable. Unlike aerobic respiration, which depends on oxygen to produce energy efficiently, anaerobic respiration relies on different biochemical pathways.

In lactic acid fermentation, a form of anaerobic respiration, cells convert glucose into lactate. This conversion allows for the regeneration of NAD\(^+\), which is essential to keep glycolysis going and produce energy in the form of ATP.

Lactic acid fermentation is particularly important in muscle cells during intense exercise when oxygen delivery to cells is limited. This process, while less efficient than aerobic respiration, still supports vital functions by providing a rapid way to produce energy without the need for oxygen. However, it results in the buildup of lactate, often leading to muscle fatigue.

Glycolysis

Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. This process takes place in the cytoplasm of cells. It is anaerobic, meaning it does not require oxygen to proceed.

During glycolysis, one molecule of glucose is converted into two molecules of pyruvate. Along the way, glycolysis generates two molecules of ATP and two molecules of NADH. These molecules are essential for cellular energy management. Pyruvate, on the other hand, can serve different roles based on the cell's oxygen availability. In conditions where oxygen is scarce, pyruvate undergoes further processing, like in lactic acid fermentation, instead of entering the mitochondria for aerobic respiration.

Glycolysis is an ancient metabolic pathway, one of the most conserved processes in biology, suggesting its vital role in energy production for the survival of cells.

NADH Oxidation

NADH oxidation is a critical step in energy production, especially in processes like lactic acid fermentation. In this process, NADH donates electrons to the pyruvate converts it into lactate. This donation means that NADH is oxidized into NAD\(^+\).

The regeneration of NAD\(^+\) is a crucial step in continuing glycolysis, as it allows cells to maintain their energy production even when oxygen is not available. Without NAD\(^+\), the glycolytic pathway would stall, and cells would be unable to produce ATP.

This recycling of NAD\(^+\) is what facilitates ongoing energy production, ensuring cells have the resources they need to function, despite limited oxygen availability. The process of oxidation and recycling is essential for the balance of cellular respiration under anaerobic conditions.

ATP Production

ATP production is the ultimate goal of both anaerobic and aerobic respiration, as ATP serves as the main energy currency of the cell. During lactic acid fermentation, ATP is generated through glycolysis. This process produces a small amount of ATP quickly, which is crucial for cells like muscle cells during high-intensity activities.

Though yields of ATP through anaerobic processes like fermentation are lower compared to aerobic respiration, they provide immediate energy when oxygen levels are too low for aerobic pathways to operate effectively.

Each glucose molecule metabolized in glycolysis gives a net gain of two ATP molecules. Though small, this ATP production is vital for sustaining cellular activities in the short term, enabling organisms to survive in challenging, low-oxygen environments.