Name and describe the three ways to add phosphate to ADP to make ATP

There are three main ways to add phosphate to ADP to make ATP:

- Substrate-level phosphorylation: This is a process in which a phosphate group is transferred from a high-energy organic molecule (substrate) to ADP, forming ATP. This occurs in some metabolic pathways, such as glycolysis and the citric acid cycle, where intermediate compounds donate phosphate groups to ADP.

- Oxidative phosphorylation: This is a process in which a phosphate group is added to ADP using the energy derived from the electron transport chain. The electron transport chain is a series of protein complexes and electron carriers that transfer electrons from reduced molecules (such as NADH and FADH2) to oxygen, generating a proton gradient across the inner mitochondrial membrane. The energy derived as a result of the chemical gradient is then used to synthesize ATP by coupling the reaction of inorganic phosphate to ADP in the active site of the ATP synthase enzyme.

- Photophosphorylation: This is a process in which a phosphate group is added to ADP using the energy derived from light. This occurs in photosynthetic organisms, such as plants, algae, and some bacteria, that capture light energy and use it to generate ATP and NADPH. There are two types of photophosphorylation: cyclic and noncyclic. In cyclic photophosphorylation, electrons from photosystem I are recycled back to the same photosystem, generating ATP but not NADPH. In noncyclic photophosphorylation, electrons from photosystem II are transferred to photosystem I, generating both ATP and NADPH.