Cellular respiration that occurs in the presence of oxygen is called aerobic respiration. Most of the time, the cells in our bodies use aerobic respiration. The following chemical equation shows the chemical changes that occur during cell respiration. You can see how the monosaccharide glucose reacts in the presence of oxygen to produce carbon dioxide gas, water, and ATP energy.
C6H12O6 + 6O2 -> 6CO2 + 6H2O + ATP Energy
Mitochondria
The process of cellular respiration involves many steps that allow the cell to use the energy in each glucose molecule to make as many ATP molecules as possible. The steps in this process are different if cells do not have enough oxygen available. In this reading, we will look at the basic steps of glycolysis, aerobic respiration, and anaerobic respiration.
Glycolysis is always the first phase of cellular respiration and it occurs in all organisms. The word glycolysis literally means "sugar breaking". "Gly" stands for sugar (glucose) and "lysis" means to break. This stage takes place in the cytoplasm of the cell. During glycolysis, glucose is broken down into 2 molecules of pyruvic acid, NADH, and 2 molecules of ATP. Although only 2 molecules of ATP are produced, the process of glycolysis is so fast that cells can produce thousands of ATP molecules in only a few milliseconds! The speed of glycolysis is a huge advantage when the energy demand of cells suddenly increases. Another advantage of glycolysis is that the process does not need oxygen to occur, and it can quickly supply chemical energy to cells when oxygen is not available.
What happens in the next stage of cellular respiration depends on if there is oxygen available. If oxygen is present, the pyruvic acid produced in glycolysis is used in the second stage of cellular respiration, called the Krebs Cycle. This cycle is sometimes called the citric acid cycle because the first molecule formed during these reactions is citrate, a type of citric acid. During the Krebs cycle, pyruvic acid from glycolysis is used to make carbon dioxide, 2 molecules of ATP, and electron carriers. The carbon dioxide is exhaled out of the organism through the respiratory system. The 2 ATP molecules are used up as energy and the electron carriers are later used to generate huge amounts of ATP during the electron transport chain. The electron transport chain uses those high-energy electrons transported by the carrier molecules to convert ADP to ATP molecules. This creates a high amount of ATP, up to 34 molecules are produced. The Krebs Cycle and electron transport chain both take place in the mitochondria.