What is the major source of energy for cellular reactions in muscle tissue?

ATP, or adenosine triphosphate, is the primary energy carrier in all living cells, including muscle tissue. It is produced during cellular respiration and is used to power various biological functions and reactions within the cell. When muscles contract, ATP is required to provide the energy needed for the interaction between actin and myosin, the proteins responsible for muscle contraction.

While glucose is a critical source of energy and can be utilized by the body to produce ATP through glycolysis and subsequent metabolic pathways, it must first be broken down to generate ATP. NADH is involved in the electron transport chain and cellular respiration to help produce ATP, but it is not an energy source itself in the way ATP is. Creatine can help regenerate ATP from ADP during high-intensity exercise but is not the direct source of energy for cellular reactions.

Thus, ATP is the direct energy currency of cells and is essential for powering all activities, especially in muscle tissue where energy demands are high during contraction and movement.