During photosynthesis, "producers" like green plants, algae and some bacteria convert light energy from the sun into chemical energy. Photosynthesis produces chemical energy in the form of glucose, a carbohydrate or sugar. The glucose produced by photosynthesis is an integral part of the food chain because a great deal of energy is stored in the chemical bonds in the glucose molecule, and this energy can be released during digestion and chemical processing by other organisms.
Photosynthetic organisms are autotrophs, or organisms that can make energy from inorganic compounds. Autotrophs are also called "producers." All non-autotrophic organisms, including humans, are heterotrophs, and rely on organic sources of chemical energy. Essentially, all heterotrophic organisms thus rely in some sense on the energy made by autotrophs through photosynthesis.
The term "chemical energy" refers to the energy stored in the chemical bonds between atoms in molecules. Chemical bonds are a form of stored or "potential" energy, because when the bonds are broken, energy is released.
Photosynthesis uses light energy to convert carbon dioxide and water into glucose and oxygen gas. Each molecule of glucose essentially "stores" up to 38 molecules of ATP which can be broken down and used during other cellular reactions. ATP, or adenosine triphosphate, is the form of chemical energy cells use to function. Cellular respiration is the complementary reaction to photosynthesis, because it is the reaction that cells use to break down glucose molecules and release ATP. The potential energy stored in the molecular bonds of glucose becomes kinetic energy after cellular respiration that cells can use to do work like move muscles and run metabolic processes.
Approximately 176 billion tons of carbohydrate in the form of glucose is produced by photosynthesis every year. This carbohydrate energy constitutes the "producer" level of the food chain which then sustains organisms at other trophic levels.
Additionally, just about all of the oxygen in the atmosphere is produced by photosynthetic organisms. Evidence in the geological record has long suggested that the first photosynthetic organisms oxygenated the atmosphere and paved the way for more complicated, oxygen-requiring organisms early in the history of life on Earth. According to an April 11, 2009 article in "Science News," photosynthetic organisms may have begun to oxygenate the atmosphere as long as 3.46 billion years ago.