4.6. ATP, THE ENERGY SORCE: ADENOSINE BASED NUCLEOTIDES
Adenosine triphosphate (ATP), the energy currency or coin of the cell pictured in Figures 1 and 2, transfers energy from chemical bonds to endergonic (energy absorbing) reactions within the cell. Structurally, ATP consists of the adenine nucleotide (ribose sugar, adenine base, and phosphate group, PO4-2) plus two other phosphate groups. Energy is stored in the covalent bonds between phosphates, with the greatest amount of energy (~ 7 kcal/mole) in the bond between the second and third phosphate groups. This covalent bond is known as a pyrophosphate bond.
We can write the chemical reaction for the formation of ATP as:
a) ADP + Pi + energy → ATP
b) Adenosine diphosphate + inorganic Phosphate + energy à Adenosine Triphosphate
The chemical formula for the expenditure/release of ATP energy can be written as:
a) ATP → ADP + energy + Pi
b) Adenosine Triphosphate produces Adenosine diphosphate + energy + inorganic Phosphate
An analogy between ATP and rechargeable batteries is appropriate. The batteries are used, giving up their potential energy until it has all been converted into kinetic energy and heat/unusable energy. Recharged batteries (into which energy has been put) can be used only after the input of additional energy. Thus, ATP is the higher energy form (the recharged battery) while ADP is the lower energy form (the used battery). When the terminal (third) phosphate is cut loose, ATP becomes ADP (Adenosine diphosphate; di = two), and the stored energy is released for some biological process to utilize. The input of additional energy (plus a phosphate group) "recharges" ADP into ATP (as in my analogy the spent batteries are recharged by the input of additional energy).
4.7. SUMMARY TABLE OF NUCLEIC ACID UNITS
