Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Biosynthesis of Carbohydrates in Animal Tissues
Gluconeogenesis occurs during the recovery period following muscular activity

The Synthesis of glucose from small precursor molecules proceeds at an exceptionally high rate during the recovery period following exhaustive Muscle exertion, such as a 100-meter sprint (Box 15-1). During such intensive muscular activity, the demand for ATP in skeletal Muscles increases immensely, and the Circulatory system can no longer deliver glucose and oxygen quickly enough to meet this demand. In this case, muscle Glycogen serves as a reserve fuel, rapidly breaking down via Glycolysis to yield lactate; this process is accompanied by the synthesis of ATP, which provides the energy required for Muscle contraction. Because oxygen is in short supply under these conditions, lactate cannot undergo further transformation within the muscles and diffuses into the bloodstream, causing Blood lactate levels to rise significantly. A sprinter who has just finished the 100-meter dash initially breathes very heavily, but gradually their breathing settles down and returns to normal after some time. During this recovery period, blood lactate levels also return to their normal low baseline. A significant portion of the excess oxygen consumed during recovery—an excess that serves as a measure of the so-called oxygen debt—is utilized to generate the ATP necessary to resynthesize blood glucose and muscle glycogen from the lactate produced anaerobically during the sprint. Over the course of recovery (which may take up to 30 minutes for complete restoration), lactate is cleared from the blood by The Liver and converted back into blood glucose via The Gluconeogenesis pathway described above. This blood glucose is then transported back to the muscles, where it is converted into glycogen (Fig. 20-5). Because The formation of a single glucose molecule from two lactate molecules consumes a total of six high-energy ATP phosphate groups, whereas The breakdown of a glucose molecule in muscles yields only two ATP molecules, utilizing muscle glycogen as an anaerobic fuel is metabolically very costly for the Cells. Furthermore, replenishing glycogen reserves requires extended recovery periods (Box 15-1).

One might, of course, consider this an excessive price to pay for victory in a short-distance race. However, it should not be forgotten that this capacity for instant mobilization—even if short-lived—confers a tremendous evolutionary advantage, ensuring survival in the predatory struggle between predator and prey (Box 15-1).

Large marine mammals, such as seals and walruses, as well as amphibious reptiles like alligators and turtles, can remain submerged for extended periods not only because their bodies store significant amounts of oxygen as oxymyoglobin, but also due to their ability to generate ATP through glycogen breakdown via glycolysis.