Summary
Highlights
Mitochondria are essential organelles within trillions of cells, responsible for converting food and oxygen into ATP (adenosine-triphosphate), the body's rechargeable energy currency. This process, cellular respiration, is vital for maintaining homeostasis, allowing organisms to adapt to changing conditions. Humans require over 100 pounds of ATP daily to function.
Cellular respiration is a continuous recycling process that produces ATP. Aerobic organisms, including humans, rely on oxygen to release energy from food and store it as ATP. Anaerobic respiration and fermentation are alternative processes that do not require oxygen, used by some bacteria and in making products like sourdough. Cellular respiration slowly breaks down glucose and oxygen, releasing energy in many controlled chemical reactions to produce ATP, unlike a rapid explosion.
Cellular respiration begins with glycolysis, which occurs in the cell's cytoplasm. This stage breaks down glucose into pyruvate, with an initial investment of ATP to start the process, but ultimately producing a net of two ATP molecules. This 'currency exchange' involves ten enzymes facilitating ten chemical reactions, converting glucose into pyruvate, which is crucial for subsequent stages. NAD+ is also converted to NADH, carrying electrons and protons for later use.
Pyruvate moves into the mitochondria, where it's oxidized, producing carbon dioxide and a two-carbon molecule. This molecule then enters the citric acid cycle (Krebs cycle). This complex cycle involves a series of reactions that further break down carbon compounds, releasing more carbon dioxide, one ATP, three NADH, and one FADH2 for every turn. The cycle regenerates its starting four-carbon molecule, allowing it to continue for each glucose molecule.
The final stage, oxidative phosphorylation, takes place in the mitochondria and is where the majority of ATP is generated. The inner mitochondrial membrane contains the electron transport chain, which accepts electrons from NADH and FADH2. As electrons move through the chain, chemical energy is converted into mechanical energy to pump protons into the intermembrane space, creating a proton gradient. This gradient drives ATP synthase, an enzyme that literally spins to produce about 30 ATP molecules per glucose molecule. Oxygen acts as the final electron acceptor, combining with protons to form water.
Cellular respiration begins with glycolysis in the cytoplasm, producing pyruvate, some ATP, and NADH. Pyruvate then enters the mitochondria for the citric acid cycle, generating more ATP, NADH, FADH2, and carbon dioxide. Finally, oxidative phosphorylation uses the electrons and protons carried by NADH and FADH2 to create a proton gradient, driving ATP synthase to produce a large amount of ATP. This entire complex process happens incredibly fast, with single cells generating millions of ATP molecules per second, powering all cellular functions. Mitochondria are indispensable for sustaining life.