Bioenergetics

Bioenergetics is the study of energy transfer within living systems. For living organisms to function, they must transfer energy within their systems and between other organisms. These transfers happen following the Laws of Bioenergetics, which are themselves restatements of laws of thermodynamics. 
The first law states that energy cannot be created or destroyed. The total amount of energy must remain constant. Organisms cannot create energy, merely transform it from one form to another. For instance, plants do not create energy through photosynthesis, they convert energy from the sun into stored energy in food that can the be used to perform life functions. 
Energy is able to move through the systems by undergoing these energy conversion. As an example, energy from the sun is converted from thermal energy into chemical energy by a producer; the producer is then eaten by a consumer; and the energy travels up the food chain. At each level the amount of energy is reduced, following the second law of bioenergetics that states that energy transfer will always proceed in the direction of increased entropy, and the release of “free energy.” In this situation, most energy is lost as heat as it travels through the food chain. 
In living organisms, the molecule that is the source of the stored energy used for cell functions comes in the form of Adenosine Triphosphate, or ATP. The energy is stored in the bonds between the phosphates in the ATP. When energy is needed to do work, for instance moving materials across the cell membrane or for chemical reactions, the bonds are broken and energy is released. Cells therefore have to produce the ATP through the processes of photosynthesis and cellular respiration.