Photosynthesis happens when green plants and photosynthetic algae use sunlight to form chemical reactions that result in the production of carbohydrates (sugars) and the release of oxygen. In order to go through photosynthesis, "plants must possess chlorophyll (molecule that absorbs sunlight) and have a supply of carbon dioxide and water" (Primary Producers, 2003, Photosynthesis section). There are two chemical reactions that are involved with photosynthesis. "During the light reaction, sunlight is used to split water into molecular oxygen, hydrogen ions and electrons, with oxygen given off as a byproduct. In the dark reaction, for which sunlight is not required, the hydrogen ions and electrons are used to convert carbon dioxide into carbohydrates" (Primary Producers, 2003, Photosynthesis section). 

Joseph Priestley conducted an experiment to determine the importance of oxygen. He had two jars and in one jar, he had a lit candle. The second jar had a lit candle and a plant. The candle in the second jar burned longer than the first one because the plant provided the candle with oxygen.

carbon dioxide + water  --> sugars + oxygen   

Cellular Respiration

 A majority of living organisms need oxygen to break down sugars, their main sources of energy. During this process, carbon dioxide and water are released (Ritter, 2001, pg.61). "These reactions release energy from food substances and make it available so that the cells" of the organisms can function (World Book, 2008, Cellular Respiration). It is possible for cells to obtain energy without oxygen, by the chemical process of glycolysis, which is the converting of molecules of glucose (simple sugars) into smaller molecules that become pyruvic acid. This event releases energy, "which is captured in a compound known as adenosine triphosphate (ATP)" (World Book, 2008, Cellular Respiration). While ATP is very important because it provides energy to all cells, glycolysis produces a very small amount of energy. Oxygen is a necessary component that is required by cells to obtain large amounts of ATP. Therefore, while organisms take in oxygen during cellular respiration, carbon dioxide and water are released, and a lot of energy is stored as ATP (World Book, 2008, Cellular Respiration). 

Joseph Priestley conducted a second experiment that portrayed how living organisms help each other, and the experiment revealed the relationship between photosynthesis and cellular respiration. There were three mice in different jars. One jar had a full-grown plant in it; another had a plant with few leaves; the third jar had no plant. The mouse in the jar with the full plant survived the longest, while the lone mouse in the jar died first. These results state that plants produce oxygen that are needed by living organisms in order to function.

sugars + oxygen --> carbon dioxide + water