ATP is the "currency" of cellular energy
Adenosine triphosphate (ATP) is the principal cellular transporter of chemical energy. Energy from digested food has to be converted to a transportable, usable form so that it can get to where it's needed and be readily available to support crucial chemical processes. Organisms remove the energy stored in the bonds of food and store most of it in the third phosphate bond of ATP
Structure of ATP
The figure below shows the molecular structure of ATP.
Biological systems have evolved to be quite economical about the use of certain chemical compounds; notice that ATP is really just a slightly modified RNA nucleotide, consisting of the purine base adenine, the sugar ribose and three phosphate groups. ATP has two more phosphates than the nucleotide phosphate that builds RNA. It's that third phosphate linkage into which bond energy is loaded and from which energy is released.
Roll over/tap the image to identify the various parts of the molecule.
Energetics
There are millions of different kinds of organisms on Earth, and they take in energy in the form of nutrients in diverse ways; some eat one another, others eat only plant material, plants derive nutrients from the earth and air. But all organisms we know of take nutrients apart and store the energy they contain in ATP.
While primitive organisms use only the fermenting process called glycolysis to form
ATP, most complex organisms use a process called oxidative phosphorylation of ADP, adenosine di-phosphate. In this rather complicated process, the bonds of glucose molecules are systematically picked apart, and the energy stored in their bonds is transferred to the phosphate bond as a phosphate group is transferred to ADP:
What we mean when we say that we store energy in a bond
Strictly speaking, it isn't true that bonds store energy and that breaking them releases it, a misonception that sometimes persists.
In most cases when bonds are formed or broken, one or more other bonds are formed or broken in the process, and it takes energy both to form and break bonds. Whether the net result is that energy is input to or released from the system is a more complicated matter, the sum of bond breaking and making on the product side minus the sm on the reactant side. In the case of ATP hydrolysis to ADP, the overall process leads to a release of 32.7 KJ/mol of reaction (mole of ATP hydrolyzed).
Likewise, 32.7 KJ of energy is required to convert a mole of ADP into a mole of ATP, just the reverse of the reaction shown below.
Ea is the energy of activation of this reaction, the energy needed to get the reaction going. Typically, activation energies are high for biochemical reactions, and cells use catalysts (enzymes) to reduce those barriers and make the reactions faster.
ATP is a transient energy carrier
Millions of times per second, in each cell, ADP molecules are being loaded with energy by being converted to ATP, and ATPs are being recruited as energy donors when a cellular process requires that energy.
The ATP/ADP system is the universal currency of biological energy in all living things found on Earth.
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