Answer:
In order to produce ATP in a cell, the reactants that must be available are ADP (adenosine diphosphate) and phosphate (Pi). ADP is formed when ATP is used to provide energy for cellular processes, and it can be converted back to ATP through the addition of a phosphate group, which requires energy input. This process is known as phosphorylation, and it can occur through two main mechanisms: substrate-level phosphorylation and oxidative phosphorylation.
In substrate-level phosphorylation, a phosphate group is transferred from a donor molecule to ADP, forming ATP. This occurs in the cytoplasm during glycolysis and in the mitochondrial matrix during the Krebs cycle. In oxidative phosphorylation, ATP is generated through a series of electron transport chain reactions that occur in the inner mitochondrial membrane, and the energy released by these reactions is used to add a phosphate group to ADP, forming ATP.
Therefore, in order to produce ATP, the cell must have a supply of ADP and phosphate (Pi), which can be obtained through the breakdown of other molecules or through the import of these molecules from the extracellular environment. Additionally, a source of energy, such as glucose or other nutrients, is also required to drive the phosphorylation reactions.