How is ADP converted to ATP?
The correct answer and explanation is:
ADP (adenosine diphosphate) is converted to ATP (adenosine triphosphate) through a process called phosphorylation, which involves adding a phosphate group to ADP. This conversion occurs in several ways, but the primary mechanisms include substrate-level phosphorylation, oxidative phosphorylation, and photophosphorylation.
- Substrate-level phosphorylation: This occurs directly in the cytoplasm and mitochondria during glycolysis and the citric acid cycle. In glycolysis, the enzyme phosphoglycerate kinase catalyzes the transfer of a phosphate group from a high-energy substrate molecule, such as 1,3-bisphosphoglycerate, to ADP, forming ATP. Similarly, during the citric acid cycle (also known as the Krebs cycle), a similar process occurs in the mitochondria, where a high-energy phosphate is transferred to ADP to form ATP.
- Oxidative phosphorylation: This process occurs in the mitochondria, specifically in the inner mitochondrial membrane. It is the primary way cells produce ATP during aerobic respiration. During oxidative phosphorylation, electrons from molecules like glucose are transferred through the electron transport chain, generating a proton gradient across the inner mitochondrial membrane. This gradient drives the enzyme ATP synthase to add phosphate groups to ADP, forming ATP. Oxygen acts as the final electron acceptor, combining with protons to form water.
- Photophosphorylation: In plant cells, particularly in the chloroplasts, ATP is produced during photosynthesis through photophosphorylation. During the light-dependent reactions of photosynthesis, light energy is absorbed by chlorophyll, which excites electrons that move through the electron transport chain, ultimately leading to the formation of ATP. This process is similar to oxidative phosphorylation but occurs in plants and is driven by light energy rather than chemical energy.
In all these processes, the energy released from breaking bonds in nutrients, such as glucose, is captured and used to add a phosphate group to ADP, forming ATP. ATP is then used as the energy currency for various cellular functions, including muscle contraction, protein synthesis, and active transport.