Cellular respiration involves a series of processes that are used to release energy from glucose in the form of ATP. Cellular respiration in organisms that have mitochondria involves two steps: anaerobic and aerobic. Anaerobic processes occur outside of mitochondria because they do not require oxygen. For the same reason, the anaerobic step is less energy-efficient and produces a small number of ATP molecules. Aerobic processes, such as the electron transport chain, use oxygen to make more ATP molecules. Glycolysis is the first step in cellular respiration, and it is an anaerobic process that takes place outside of mitochondria. During glycolysis, one glucose molecule is broken down into two smaller molecules called pyruvic acid.
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Glycolysis starts with the rearrangement of the glucose molecule, where it receives two extra phosphate groups from ATP. The modified molecule is called fructose-1,6-bisphosphate, and it has two three-carbon sugars with phosphate groups attached. This process uses 2 ATP molecules to produce phosphate groups. This step makes the sugar molecule less stable, allowing it to break down into two sugars, each bearing one phosphate group: one glyceraldehyde-3-phosphate and one DHAP, which is then converted into glyceraldehyde-3-phosphate (“Glycolysis”). This three-carbon molecule is then broken down further by releasing the phosphate group.
In this step, the phosphate group is detached and taken in by ADP, turning it into ATP. The second ATP is produced when the sugar loses one molecule of water and turns into phosphoenolpyruvate (“Glycolysis”). This compound also has a phosphate group that is donated to produce ATP, thus making it a pyruvate molecule. The process is repeated once for each three-carbon sugar molecule, and, therefore, 4 ATP molecules are produced in total. However, because the process uses 2 ATP molecules at the beginning stage, the net gain is only 2 ATP.
“Glycolysis.” Khan Academy, 2019. Web.