Question

In alcoholic fermentation, each mole of pyruvate is converted to one mole of carbon dioxide and one mole of ethanol. In the process, about \(209 \mathrm{~kJ} / \mathrm{mol}\) of energy is produced. Under the most favorable conditions, more than one-half of this energy is stored as ATP. (a) What happens to the remaining energy produced in alcoholic fermentation? (b) Give two reasons why it would be nearly impossible to reverse the reaction that converts pyruvate to ethanol and carbon dioxide.

Short answer

(a) The remaining energy is dissipated as heat. (b) Reversing the reaction is nearly impossible due to energy barriers and the unidirectional nature of metabolic pathways.

Step-by-step solution

Introduction to Alcoholic Fermentation

Alcoholic fermentation is a biological process in which glucose is converted mostly into ethanol and carbon dioxide, involving the intermediate pyruvate. This process is anaerobic, meaning it does not require oxygen.

Analyzing Energy Distribution in Alcoholic Fermentation

When 1 mole of pyruvate is converted to ethanol and carbon dioxide, approximately 209 kJ/mol of energy is released. More than half of this energy is stored in the form of ATP, an energy carrier molecule, and fuels cellular processes. However, some energy is lost as heat due to the inefficiencies of biochemical transformations and the inability of ATP to capture all the energy released during fermentation.

Exploring the Fate of Remaining Energy (Part a)

The remaining energy, which is not captured as ATP, is dissipated as heat. This dissipation is a natural part of the metabolic process and contributes to the entropy of the system, complying with the second law of thermodynamics.

Challenges of Reversing the Reaction (Part b - Thermodynamic Barrier)

Reversing the reaction from ethanol and carbon dioxide back to pyruvate involves overcoming a significant energy barrier. The fermentation pathway releases energy, making the reverse process endothermic and unfavorable without an external energy source.

Issues of Reversing the Reaction (Part b - Biochemical Pathway Limitation)

Biochemical pathways are designed to be unidirectional under normal cell conditions to support efficiency and regulation. Enzymes specific to fermentation catalyze the forward reaction effectively, but the reverse reaction would require alternative pathways or conditions that do not naturally exist in cells.

Key concepts

Pyruvate Conversion

In alcoholic fermentation, pyruvate, a product of glycolysis, undergoes conversion into ethanol and carbon dioxide. This conversion is an essential part of the anaerobic pathway, which means it occurs in the absence of oxygen. Pyruvate is first transformed into a transitional compound, acetaldehyde, by releasing one molecule of carbon dioxide. Following this, acetaldehyde is reduced to ethanol.
The enzyme responsible for the initial conversion of pyruvate to acetaldehyde is pyruvate decarboxylase. This step is non-reversible in normal cellular conditions, contributing to the unidirectional flow of the fermentation process. Pyruvate conversion provides an alternative pathway for cells when oxygen is scarce, allowing for the continued breakdown of glucose even when aerobic processes cannot proceed.

Energy Distribution

During the process of alcoholic fermentation, the energy distribution is critical to understanding how cells maintain balance and function. When pyruvate is converted into ethanol and carbon dioxide, around 209 kJ/mol of energy is released. Most of this energy isn't directly usable for cellular work, but more than half of it is harnessed and stored in ATP molecules.
Energy distribution involves a trade-off where some amount is inevitably lost as heat. This energy loss happens because of the inefficiencies inherent in biochemical transformations and because ATP, while efficient, cannot capture all the released energy. The heat generated is a measure of entropy, reflecting the natural tendencies of energy flow as dictated by the second law of thermodynamics.

ATP Formation

ATP, or adenosine triphosphate, acts as the primary energy currency within cells. In alcoholic fermentation, the conversion of pyruvate into ethanol and carbon dioxide ultimately supports ATP formation. This is crucial for cellular processes that require energy.
The energy captured in ATP during fermentation is used for various cellular functions like biosynthesis, transport of molecules across cell membranes, and mechanical work - like muscle contraction. Formation of ATP in fermentation isn't as extensive as in aerobic respiration, yet it provides sufficient energy to fuel essential activities in anaerobic conditions. Nevertheless, this limited ATP production is a reason why organisms seek aerobic conditions for energy, where the yield is significantly higher.

Metabolic Processes

Metabolic processes encompass the vast arrangements of biochemical reactions within cells, involving pathways such as glycolysis and fermentation. Alcoholic fermentation is a subset of these processes, providing a crucial means of energy extraction and regeneration of necessary co-factors.
This specific metabolic pathway is critical for regenerating NAD extsuperscript{+} from NADH, an essential step that allows glycolysis to continue. Without this regeneration, glycolysis would halt, cutting off the cell's energy supply. The unidirectional nature of the metabolic pathways, especially fermentation, ensures that once pyruvate begins conversion under low oxygen availability, it follows through to ethanol formation. This prevents a buildup of pyruvate and allows cells to adapt to less favorable conditions, maintaining cellular operations even when oxygen is limited. By understanding these processes, we gain insight into the adaptability and efficiency of cellular energy management.