Animal cells, such as those in human muscle, primarily rely on cellular respiration to convert the chemical energy stored in glucose into adenosine triphosphate (ATP). Respiration is categorized based on whether it requires oxygen. Aerobic respiration, occurring in the mitochondria, is highly efficient and uses oxygen to produce a large amount of ATP. Anaerobic respiration, or fermentation, functions without oxygen, yielding a much smaller amount of ATP but doing so much more quickly. This oxygen-independent process becomes the temporary solution when energy demand exceeds the body’s ability to supply oxygen.
The Physiological Trigger for Anaerobic Metabolism
Animal cells are designed for aerobic metabolism, but circumstances can force a temporary switch to anaerobic pathways. This metabolic shift commonly occurs in skeletal muscle cells during high-intensity, short-duration activities like sprinting or heavy weightlifting. During these intense bursts of effort, the muscle’s demand for ATP outpaces the circulatory system’s capacity to deliver sufficient oxygen.
When oxygen is insufficient, the highly efficient stages of aerobic respiration in the mitochondria cannot operate fully. The cell activates anaerobic glycolysis, a faster but less efficient way to produce ATP, to prevent a complete energy shutdown. This temporary measure allows the muscle to continue contracting until the body can increase its oxygen supply. Anaerobic respiration provides an immediate energy boost, producing only about two molecules of ATP per molecule of glucose, compared to 32 or more from aerobic respiration.
The Immediate Product: Lactic Acid Fermentation
Anaerobic respiration begins with glycolysis, a sequence of enzyme-catalyzed reactions that breaks glucose into two molecules of pyruvate. This initial stage occurs in the cell’s cytoplasm and generates a net gain of two ATP molecules and two molecules of NADH. The fate of the pyruvate depends on the availability of oxygen.
When oxygen is scarce, pyruvate cannot enter the mitochondria for aerobic respiration. Instead, it is converted into lactate by the enzyme lactate dehydrogenase, a process known as lactic acid fermentation. This reaction simultaneously converts NADH back into \(\text{NAD}^+\).
The regeneration of \(\text{NAD}^+\) is the primary reason for this final step, as this electron carrier is necessary for glycolysis to continue. Recycling \(\text{NAD}^+\) ensures that glycolysis can continue to provide a small supply of ATP. Although the term “lactic acid” is often used, the product in the body is lactate, the ionic form at physiological pH.
Clearing Lactate: The Body’s Recovery Mechanism
Once intense activity ends and oxygen becomes available, the body must handle the accumulated lactate. Lactate is not a waste product but a valuable fuel source that can be recycled. The circulatory system transports lactate away from the muscle cells, primarily to the liver, where the Cori Cycle takes place.
In the liver, lactate is converted back into pyruvate, which is then used to synthesize new glucose through gluconeogenesis. This glucose can be released back into the bloodstream to fuel other tissues or converted into glycogen for storage, closing the cycle. The liver manages about 60% of lactate clearance, with the kidneys also contributing to its removal.
The common belief that lactate accumulation causes delayed onset muscle soreness (DOMS) felt a day or two after exercise is a misconception. Lactate is rapidly cleared from the muscles and bloodstream within minutes to an hour after exercise stops. Muscle soreness experienced days later is primarily caused by micro-tears in the muscle fibers and the resulting inflammatory response.