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The Presence of Oxygen
There are two types of cellular respiration (see "Cellular Respiration" concept): aerobic and anaerobic. One occurs in the presence of oxygen (aerobic), and one occurs in the absence of oxygen (anaerobic). Both begin with glycolysis - the splitting of glucose.
Glycolysis (see "Glycolysis" concept) is an anaerobic process - it does not need oxygen to proceed. This process produces a minimal amount of ATP. The Krebs cycle and electron transport do need oxygen to proceed, and in the presence of oxygen, these process produce much more ATP than glycolysis alone.
Scientists think that glycolysis evolved before the other stages of cellular respiration. This is because the other stages need oxygen, whereas glycolysis does not, and there was no oxygen in Earth’s atmosphere when life first evolved about 3.5 to 4 billion years ago. Cellular respiration that proceeds without oxygen is called anaerobic respiration.
Then, about 2 or 3 billion years ago, oxygen was gradually added to the atmosphere by early photosynthetic bacteria (cyanobacteria). After that, living things could use oxygen to break down glucose and make ATP. Today, most organisms make ATP with oxygen. They follow glycolysis with the Krebs cycle and electron transport to make more ATP than by glycolysis alone. Cellular respiration that proceeds in the presence of oxygen is called aerobic respiration.
- Cellular respiration always begins with glycolysis, which can occur either in the absence or presence of oxygen.
- Cellular respiration that proceeds in the absence of oxygen is anaerobic respiration.
- Cellular respiration that proceeds in the presence of oxygen is aerobic respiration.
- Anaerobic respiration evolved prior to aerobic respiration.