If all the energy in a reaction were released at the same time, most of it would be lost as heat—burning up the cells—and little could be captured to do metabolic (or any other kind) of work. Organisms have evolved a multitude of materials and mechanisms—such as enzymes—that control and permit the stepwise use of the released energy.
Enzymes control the state of energy a molecule must attain before it can release energy and are the chief catalysts of biochemical reactions. They are neither consumed nor changed in the reactions. Basically, enzymes reduce the activation energy needed to start a reaction by temporarily bonding with the reacting molecules and, in so doing, weakening the chemical bonds.
Almost all of the over 2,000 known enzymes are proteins, nearly all of which operate with cofactors—metal ions or organic molecules ( coenzymes). The enzymes act in series with each enzyme catalyzing only a part of the total reaction (which is why there are so many of both enzymes and cofactors). If the same type of reaction occurs in two different processes, each requiring the same enzyme, two different but structurally similar enzymes are used. These are called isozymes, and each is specific for its own process.
Two different structural models are used to explain why enzymes work so efficiently. According to the lock‐and‐key model, there is a place in the enzyme molecule, the active site (the lock), into which the substrate (the key) fits by virtue of the latter's electrical charge, size, and shape. In actuality, however, the connection appears to be much more flexible than this model permits. The induced‐fit model takes this into account and states that although size and shapes are comparable, the active site is flexible and appears to adjust to fit the substrate. In so doing, it tightens the connection when the molecules come together and initiates the enzymatic reaction. However it works physically, chemically the enzyme‐substrate relationship is exact and specific, one enzyme for each substrate.\
ATP (adenosine triphosphate)
Energy is the currency of the living world and ATP, like the coins that change hands in our economy, is the means through which energy is circulated in and among cells; it is the most common energy carrier. ATP is a nucleotide composed of adenine, the sugar ribose, and three phosphate groups. Its value as an energy carrier lies in the two easily broken bonds that attach the three phosphate groups to the rest of the molecule. These bonds are inappropriately called high energy bonds; they have ordinary energy values, but are weak and so easily split. Hydrolysis of the molecule (catalyzed by ATPase) breaks the terminal weak bond releasing energy, an inorganic phosphate (P i) and ADP (adenosine diphosphate). Sometimes the reaction repeats, and the second bond also is broken releasing more energy, another P i and ADM (adenosine monophosphate). The ADP is recharged back to ATP in cellular respiration. ATP is also made during photosynthesis.
ATP is indispensable for short‐term energy use, but not useful either for long‐term energy storage or for processes requiring large amounts of energy. The former needs are met in plants chiefly by starch and lipids, the latter by sucrose.