The first stage of respiration, glycolysis, happens in the cytoplasm of the cell, outside the mitochondria. The enzymes that catalyse its steps are dissolved in the cytoplasm, and glucose that enters the cell is broken down there.Glycolysis does not need oxygen. It is therefore the one stage shared by aerobic and anaerobic respiration, and it is found in almost all living organisms, which suggests it evolved very early. Its end product is pyruvate, a 3-carbon molecule, along with a small net gain of ATP (2 per glucose) and reduced NAD.What happens next to pyruvate depends on oxygen:
• With oxygen, pyruvate is actively transported from the cytoplasm into the mitochondrial matrix, where the remaining stages of aerobic respiration release much more energy.
• Without oxygen, pyruvate stays in the cytoplasm. It is converted to lactate (in animals) or to ethanol and carbon dioxide (in plants and fungi). This uses the hydrogen carried by reduced NAD, so NAD is regenerated and glycolysis can keep going.Some cells rely completely on the cytoplasm for respiration. Mammalian red blood cells have no mitochondria, so they make all their ATP by glycolysis. Prokaryotes also lack mitochondria: in bacteria that respire aerobically, glycolysis and the Krebs cycle take place in the cytoplasm and the electron transport chain is in the cell surface membrane.
• With oxygen, pyruvate is actively transported from the cytoplasm into the mitochondrial matrix, where the remaining stages of aerobic respiration release much more energy.
• Without oxygen, pyruvate stays in the cytoplasm. It is converted to lactate (in animals) or to ethanol and carbon dioxide (in plants and fungi). This uses the hydrogen carried by reduced NAD, so NAD is regenerated and glycolysis can keep going.Some cells rely completely on the cytoplasm for respiration. Mammalian red blood cells have no mitochondria, so they make all their ATP by glycolysis. Prokaryotes also lack mitochondria: in bacteria that respire aerobically, glycolysis and the Krebs cycle take place in the cytoplasm and the electron transport chain is in the cell surface membrane.