Fermentation occurs under anaerobic conditions when cells do not have access to oxygen, allowing glucose to be converted to energy when oxygen-requiring pathways of ATP synthesis are not operating. This process occurs after glycolysis, in the cytosol. Cells undergoing fermentation generate all of their ATP through glycolysis, which is less efficient than oxidative phosphorylation. The main purpose of fermentation is to regenerate NAD+ so that glycolysis can continue, enabling the production of small amounts of ATP in oxygen-deprived cells.
There are two types of fermentation: alcoholic fermentation, which occurs in yeast cells and produces ethanol, and lactic acid fermentation, which happens in animal cells and some bacteria. In lactic acid fermentation, an enzyme called lactate dehydrogenase converts 2 pyruvate and 2 NADH into 2 lactic acid molecules and 2 NAD+'s. Alcoholic and lactic acid fermentation both enable cells to generate ATP through glycolysis indefinitely by regenerating NAD+ from NADH.
<ul> <li>Cellular fermentation</li> <ul> <li>Occurs under anaerobic conditions</li> <li>Occurs after glycolysis</li> <li>Occurs in the cytosol</li> <li>Enables continuous glycolysis by converting NADH into NAD+</li> </ul> <li>Two types of fermentation</li> <ul> <li>Alcoholic fermentation</li> <ul> <li>Occurs in yeast cells</li> <li>Produces ethanol and carbon dioxide</li> </ul> <li>Lactic acid fermentation</li> <ul> <li>Occurs in oxygen-deprived animal cells</li> <li>Some bacteria also carry out lactic acid fermentation</li> <li>Conversion of 2 pyruvate and 2 NADH into 2 lactic acid molecules and 2 NAD+</li> <li>Rate limiting enzyme: lactate dehydrogenase</li> </ul> </ul> </ul>
Anaerobic glycolysis is an essential part of cellular metabolism, as it allows cells to produce energy (ATP) in the absence of oxygen. This process breaks down glucose into pyruvate, which is then converted into lactic acid through lactic acid fermentation. Anaerobic glycolysis is especially important for cells subjected to low oxygen conditions, such as muscles during strenuous exercise, providing them with a rapid energy source.
NAD+ (Nicotinamide adenine dinucleotide) is an essential electron carrier in both glycolysis and lactic acid fermentation. During glycolysis, NAD+ gets reduced to NADH by accepting electrons. In anaerobic conditions, NADH transfers electrons to pyruvate during lactic acid fermentation, regenerating NAD+ in a process catalyzed by lactate dehydrogenase. This regeneration of NAD+ allows glycolysis to continue, providing a continuous energy supply to the cell in anaerobic conditions.
Both the TCA (tricarboxylic acid) cycle and oxidative phosphorylation require the presence of oxygen to generate ATP. Under anaerobic conditions, such as intense exercise, the cells cannot rely on these pathways as oxygen is limited. Therefore, anaerobic glycolysis and lactic acid fermentation take place to produce ATP without the need for oxygen. Converting pyruvate to lactate ensures the cell can continue to produce energy in these oxygen-deprived situations.
Lactate dehydrogenase (LDH) is an enzyme that plays a crucial role in lactic acid fermentation during anaerobic glycolysis. It catalyzes the conversion of pyruvate to lactate while transferring electrons from NADH to NAD+, which allows the regeneration of NAD+ for use in glycolysis. This enables the cell to continue producing energy under anaerobic conditions even when the TCA cycle and oxidative phosphorylation cannot be utilized.
ATP synthesis in anaerobic glycolysis is less efficient compared to aerobic respiration, which includes the TCA cycle and oxidative phosphorylation. In anaerobic glycolysis, only a net gain of 2 ATP molecules is produced per glucose molecule due to lactic acid fermentation. In contrast, aerobic respiration generates a total of up to 36-38 ATP molecules per glucose molecule. Although anaerobic glycolysis generates ATP more rapidly, the trade-off is a lower overall yield of ATP compared to aerobic respiration.