ATP (adenosine triphosphate) is formed primarily through cellular respiration, a process that converts glucose and oxygen into energy within the mitochondria of cells. This process occurs in several stages:
- Glycolysis: Glucose is broken down into two molecules of pyruvate in the cytoplasm, producing a small amount of ATP and NADH (an electron carrier). This step does not require oxygen and yields 2 ATP molecules by substrate-level phosphorylation
- Krebs cycle (Citric acid cycle): Pyruvate is transported into the mitochondria and converted into acetyl-CoA, which enters the Krebs cycle. This cycle produces carbon dioxide, ATP, and high-energy electron carriers NADH and FADH2
- Electron transport chain and oxidative phosphorylation: The NADH and FADH2 donate electrons to the electron transport chain in the inner mitochondrial membrane. This drives the pumping of protons across the membrane, creating an electrochemical gradient. ATP synthase uses this gradient to synthesize ATP from ADP and inorganic phosphate. One NADH can produce about 2.5 ATP molecules, and one FADH2 about 1.5 ATP molecules
Besides aerobic respiration (which requires oxygen), ATP can also be produced anaerobically through fermentation or anaerobic respiration in some organisms, though these yield less ATP
. In plants and cyanobacteria, ATP is also formed via photophosphorylation during photosynthesis, where sunlight energy is used to convert ADP to ATP
. Overall, ATP formation is a continuous cycle where ATP releases energy by losing a phosphate group to become ADP, and ADP is recycled back into ATP by adding a phosphate group using energy derived from metabolic processes
. In summary, ATP is formed mainly by:
- Breaking down glucose through glycolysis and the Krebs cycle
- Using high-energy electrons in the electron transport chain to power ATP synthase
- Recycling ADP to ATP continuously to supply energy for cellular functions
