Cellular respiration is the metabolic process that converts nutrients like glucose into ATP, the energy currency of cells. This process occurs primarily in the mitochondria, often called the powerhouse of the cell. The overall chemical equation shows that glucose plus oxygen yields carbon dioxide, water, and energy in the form of ATP. This is essentially how cells breathe - taking in oxygen and nutrients, and releasing carbon dioxide, water, and energy.
The first stage of cellular respiration is glycolysis, which occurs in the cytoplasm of the cell. During glycolysis, one glucose molecule is split into two pyruvate molecules through a series of enzymatic reactions. This process doesn't require oxygen, making it an anaerobic process. Glycolysis produces a small amount of energy in the form of 2 ATP molecules and also generates 2 NADH molecules, which carry electrons to later stages. Glycolysis is a universal process found in nearly all living organisms, from bacteria to humans, highlighting its evolutionary importance.
The Krebs Cycle, also known as the Citric Acid Cycle, is the second stage of cellular respiration. It takes place in the mitochondrial matrix, the inner compartment of the mitochondrion. After glycolysis, pyruvate enters the mitochondria and is converted to acetyl-CoA, which then enters the Krebs Cycle. This cycle is a series of chemical reactions that complete the oxidation of glucose. For each glucose molecule, the Krebs Cycle produces 2 ATP molecules directly, but more importantly, it generates 6 NADH and 2 FADH₂ molecules, which carry high-energy electrons to the next stage. The cycle also releases carbon dioxide as a waste product. Unlike glycolysis, the Krebs Cycle requires oxygen, making it an aerobic process.