How can a skeletal muscle produce additional ATP when there is not enough oxygen?

Skeletal muscle can produce additional ATP when there is not enough oxygen through several mechanisms:

Anaerobic glycolysis: When the oxygen supply is limited, muscles can break down glucose without using oxygen in a process called anaerobic glycolysis. This process occurs in the cytoplasm of muscle cells and results in the production of ATP, along with the by-products pyruvate and lactate.

Creatine phosphate breakdown: Creatine phosphate (CP) is a high-energy compound stored in skeletal muscles. When there is an immediate demand for energy and oxygen is limited, CP can be broken down to produce ATP. The enzyme creatine kinase facilitates this reaction, transferring a phosphate group from CP to ADP, generating ATP.

Substrate-level phosphorylation: In addition to anaerobic glycolysis, muscle cells can also use substrate-level phosphorylation to generate ATP without oxygen. This process involves the direct transfer of a phosphate group from a substrate molecule to ADP, resulting in the formation of ATP. An example of substrate-level phosphorylation in skeletal muscle is the conversion of glucose-6-phosphate to fructose-6-phosphate.

Fatty acid metabolism: While not a primary source of energy during high-intensity exercise, skeletal muscle can also utilize fatty acids as an energy source when oxygen is limited. Fatty acid metabolism occurs in the mitochondria and involves the breakdown of fatty acids into acetyl-CoA, which enters the citric acid cycle (Krebs cycle). Although the citric acid cycle requires oxygen, some ATP can be produced through substrate-level phosphorylation during fatty acid metabolism.

Muscle glycogen breakdown: Muscle glycogen, a stored form of glucose, can be broken down to release glucose-1-phosphate through a process called glycogenolysis. This glucose-1-phosphate can then enter anaerobic glycolysis or be converted to glucose-6-phosphate to undergo substrate-level phosphorylation, generating ATP.

These mechanisms allow skeletal muscle to continue generating ATP even when oxygen availability is limited, ensuring the maintenance of muscle function and the production of energy necessary for short-term, intense activities or during the transition to aerobic metabolism. However, it's important to note that these anaerobic processes produce lactate, which can contribute to muscle fatigue and must be cleared through subsequent recovery and oxygen supply.