How is ATP produced during aerobic respiration?

ATP is produced during aerobic respiration through the processes of glycolysis, the Krebs cycle, and oxidative phosphorylation.

Aerobic respiration is a series of reactions that convert glucose into ATP, the main energy currency of cells. This process occurs in three main stages: glycolysis, the Krebs cycle, and oxidative phosphorylation, all of which contribute to ATP production.

Glycolysis is the first stage of aerobic respiration and occurs in the cytoplasm of the cell. One molecule of glucose is broken down into two molecules of pyruvate, producing a net gain of two ATP molecules. This process does not require oxygen, hence it is also a part of anaerobic respiration.

The second stage, the Krebs cycle, occurs in the mitochondria. Each pyruvate molecule is further broken down, releasing high-energy electrons. These electrons are captured by carrier molecules NAD+ and FAD, forming NADH and FADH2. Although the Krebs cycle itself does not produce ATP directly, the high-energy electrons carried by NADH and FADH2 are used in the next stage to generate ATP.

The final stage, oxidative phosphorylation, is where the majority of ATP is produced. This process also takes place in the mitochondria, specifically in the inner mitochondrial membrane. The high-energy electrons from NADH and FADH2 are passed along the electron transport chain, a series of protein complexes. As the electrons move along the chain, they release energy which is used to pump protons across the membrane, creating a proton gradient. This gradient drives the synthesis of ATP from ADP and inorganic phosphate, a process known as phosphorylation. Oxygen is the final electron acceptor in this process, combining with electrons and protons to form water. This is why oxygen is essential for aerobic respiration.

In total, the complete oxidation of one glucose molecule during aerobic respiration can produce up to 38 ATP molecules, although in eukaryotic cells the yield is often closer to 30-32 ATP due to the energy cost of transporting ATP out of the mitochondria.