Structural Biochemistry/Energy coupling in chemical reactions
Many chemicals' reactions are not spontaneous and require energy to occur. The spontaneity of a chemical reaction is determined by its Gibbs free energy value. If negative, the reaction will proceed spontaneously; if positive, the reaction will not be spontaneous. Note that this is not equivalent to kinetics, or the speed of a reaction; Gibbs free energy only determines the spontaneity of a given reaction. How quickly the reaction proceeds is determined by other factors, such as the presence of enzymes, the amount of heat (energy) that the system has available, and the physical properties of the reacting molecules.
Spontaneous reactions occur without the need for extra energy, but they may happen slowly. In order to catalyze non-spontaneous reactions, such as the synthesis of macromolecules, enzymes and coupling are used by the cell. Almost all reactions that take place in a cell are catalyzed by enzymes that decrease the activation energy of the reaction. Essentially, this means that the enzyme opens up a more favorable "pathway" for the reaction, allowing it to initiate more easily than before, and with less energy. In addition, an unfavorable reaction can be coupled together with a favorable one to make the overall reaction favorable. For example, Glucose + Pi -> Glucose-6-phosphate has a positive G and is therefore unfavorable. But it can be coupled with ATP -> ADP + Pi (which has a negative G) to make the reaction favorable. The overall reaction thus becomes ATP + Glucose -> ADP + Glucose-6-phosphate and has a negative G. Therefore, ATP is considered the energy currency of the cell. However, it should be noted that other energy carrying molecules, such as GTP, do exist and are used for certain processes.
Anabolic and catabolic processes are examples of how the cell couples reactions together to create efficient energy exchange cycles. These processes are explained in more detail in their respective sections; however, it is appropriate to mention them here as they provide a relevant example of reaction coupling. Basically, catabolic reactions are those that convert chemical fuels to molecules that the cell can use for energy, such as ATP and other high-energy compounds. Anabolic reactions are those that require some amount of energy to occur. Thus, the cell can conveniently couple anabolic reactions with catabolic ones - the products of catabolic reactions can be used to drive anabolic reactions to completion. This allows the cell to link different types of reactions together efficiently; it's almost a type of "cellular recycling", as the products of one reaction (i.e. a catabolic one) can be re-used to help another reaction reach completion (i.e. an anabolic one).
Organisms are Energy Transducers
Organisms are transducers of energy, since in the transfer of energy they are less than 100% efficient. Organisms employ the energy harnessed to grow, repair, and maintain their bodies. The energy is also use to compete with other organisms, and to produce new organisms (offspring). In the process of doing these things, organisms generate waste, chemicals and heat. Organisms create local regions of order at the expense of using up some fraction of the total supply of useful energy found in the universe.