Cellular Respiration

Everything that is alive requires an energy source to fuel its activities. Ultimately, that source of energy is the sun. How then did the organisms on Earth harness this energy? It all starts with the photosynthesizers. These organisms are able to take carbon dioxide and water and then use energy captured from the sun as photons to force these molecules together, producing glucose and oxygen. The glucose is the key to the next critical step, one that nearly all organisms use in one form or another - cellular respiration. Cellular respiration is carried out by enzymes, either in the cell membrane in prokaryotes or in the mitochondria in eukaryotes.

The chemical reaction begins by breaking down the glucose using oxygen to create carbon dioxide and water again, but in the process, the energy that originally went into making the glucose molecule goes to two new places. One is into synthesizing ATP, or adenosine triphosphate molecules, an energy source that cells can readily use. The rest is lost as heat. So far, we have been talking about a kind of cellular respiration that uses oxygen, and this is referred to as aerobic respiration, but some organisms and cells are capable of doing re