Cellular respiration begins by breaking down sugars known as glucose during a process called glycolysis. Both types of cellular respiration require pyruvate to function, which is the initial product of glycolysis. Glycolysis can begin with or without the intake of oxygen by the cell. Aerobic respiration, which has oxygen present, mostly takes place in organelles known as mitochondria, which are found in complex eukaryotic cells. Anaerobic respiration, which lacks oxygen, occurs in the fluid layer of a cell known as the cytoplasm and produces more energy than anaerobic respiration.