Unlike photosynthesis, which only occurs in photosynthetic cells, the process of cellular respiration occurs in all living cells. Cellular respiration takes place in the cell’s mitochondria and is a process that releases energy that will later be used by the cell. While photosynthesis takes in carbon dioxide, water, and light energy to produce glucose and oxygen, cellular respiration has the exact opposite equation. Cellular respiration’s formula is as follows:
6O2 + C6H12O6 6CO2 + 6 H2O + E
Photosynthesis is used only by plants and other photosynthetic organisms to create the sugars that are necessary to carry out cellular respiration and the production of energy molecules called ATP. In animals, sugars must first be consumed in order for the glucose needed to be present in the cells. Cellular respiration most often involves the complete breakdown of glucose to carbon dioxide and water. The process of breaking down glucose is an exergonic reaction that releases energy and heat. As the breakdown of glucose occurs, electrons will be removed from substrates and will eventually be received by oxygen atoms. These oxygen atoms will combine with hydrogen ions to form water. When hydrogen atoms are initially removed from glucose during breakdown, electrons are removed. The removal of electrons is known as oxidation. When oxygen atoms are added to the hydrogen atoms to form water, reduction, or the gaining of electrons, has occurred. Therefore, cellular respiration is known as an oxidation-reduction reaction (Mader 133).
The oxidation of glucose by removal of hydrogen atoms involves four complex phases that take place both inside of and outside of the mitochondria. The first phase is known as glycolysis. Glycolysis takes place outside of the mitochondria in the cytoplasm and can be completed without oxygen (fermentation). In this phase, glucose is converted into two 3-carbon compounds called PGAL. This...