Anaerobic Respiration (lack of oxygen)
Goal: to reduce pyruvate, thus generating NAD+
Where: the cytoplasm
Why: in the absence of oxygen, it is the only way to generate NAD+ (to use in glycolysis) and ATP.
The cell is a much more efficient energy converter when oxygen is available. What happens in a cell in the body that lacks oxygen? The breakdown of glucose cannot proceed beyond glycolysis. Without oxygen, cellular respiration could not occur because oxygen serves as the final electron acceptor in the electron transport system. The electron transport system would therefore not be available.
Recall that glycolysis takes place in the cytosol and involves the breakdown of glucose into pyruvic acid (pyruvate), producing a low yield of 2 molecules of ATP/glucose molecule. If there is an insufficient amount of oxygen present, pyruvate will be converted by lactate dehydrogenase to lactic acid (lactate) to regenerate NAD+ so that in the absence of oxygen, at least some ATP can be made by glycolysis.
To regenerate NAD+ from NADH, the electrons from NADH are added to pyruvate to produce alcohol (in plants and yeast) or lactate/lactic acid (animals, bacteria). The total ATP yield of anaerobic respiration comes from glycolysis with only 2 ATP molecules are produced per glucose.
Usefulness of Anaerobic Respiration
During vigorous exercise, oxygen is consumed faster than it is needed. Additional ATP energy is provided to the muscles by glycolysis and the result is a buildup of lactate in the muscles.When lactate builds up, the blood pH drops and the muscles fatigue. At rest, lactate is converted back to pyruvate (the oxygen debt is repaid). This is why you continue to breathe hard after you have finished running or rapid stair climbing.
- Glycolysis of glucose to pyruvic acid is allowed to continue, despite the fact that fewer ATP (two) are synthesized. This provides cells with a backup or emergency source of ATP.
- Some peripheral tissues such as skeletal muscles commonly function anaerobically without injury or cell damage for short periods of time.
- Red blood cells (RBCs) respire anaerobically, thus conserving the hemoglobin‑bound O2 for the tissues.