The cells which make up all living organisms fall into two categories: simple prokaryotic cells which lack a membrane-bound nucleus and the more complex eukaryotic type, which contains a nucleus that holds all components needed for the workings of the cell and the organism. Animal and plant cells are eukaryotic, carrying the organelles which perform functions such as synthesizing protein and processing ATP for energy.
Adenosine triphosphate (ATP) is a high-energy molecule contained in the nucleus of eukaryotic cells. This molecule provides all energy needed for various physiological processes, and is renewed through oxidation, so a cell always contains a supply of ATP to fuel the energy demands of the organism. While both plant and animal cells utilize ATP for energy, plants also contain chloroplasts, a type of specialized organelle for producing energy through photosynthesis.
Mitochondria, an organelle found in both plant and animal cells, contains protein-rich components called cristae, which produce ATP. ATP is converted to adenosine diphosphate (ADP), a process which releases energy. Through the action of food energy stored in the mitochondria, ADP is converted back to ATP and the cycle continues.
In addition to mitochondria, plants have another source of ATP in chloroplasts, which synthesize solar energy through the action of chlorophyll. This process produces glucose, an essential energy source. Because chloroplasts provide an additional source of ATP through photosynthesis, a plant's reserves of ATP produced by the mitochondria are not depleted by producing glucose.