ATP is produced by both anaerobic and aerobic cellular respiration. Anaerobic respiration, which does not require O2, results in the breakdown of glucose to yield ATP and lactic acid. During intense exercise, such as during a spint, smaller amounts of ATP are produced by anaerobic respiration. Aerobic respiration requires O2 and breaks down glucose to produce ATP, CO2, and H2O. Anaerobic respiration occurs in the cytoplasm of cells, whereas most of aerobic metabolism occurs in the mitochondria. Cells with a high metabolic rate, such as muscle fibers, which depend on large amounts of O2 and carry out primarily aerobic metabolism, contain large