Energy derived from foodstuffs is transferred first into the phosphate bonds of ATP and similar compounds and stored there. These phosphate bonds are said to be energy-rich. When hydrolysis occurs during the process of polymerization, the energy is released.
In polymerization the energy released by breaking the energy-rich phosphate bonds is not lost. For example, in the polymerization of glucose molecules, each hexose molecule (or other monomer) is first activated or energized by combination with one energy-rich phosphate group.
The transfer of the phosphate to the monomer is achieved via several intermediate steps involving transphosphorylases and phosphokinases (Gr. kinema = activity) or synthetases. In the case of starch formation glucose-1-phosphate is formed by uridine diphosphate (UDP) via uridine triphosphate (UTP).
UDP and UTP constitute a nucleotide energy-transferring system working via, and analogous to, ADP and ATP. As each glucose-1-phosphate unit is added to the polymer chain, UDP is liberated and the energy of the phosphate bond goes into the making of the glycoside linkage between the glucosyl groups.
UDP then receives energy-rich phosphate from ATP, becoming UTP, ATP become ADP. The ADP then receives new phosphate from substrate-level and oxidative-chain phosphorylations (and from photo- phosphorylations in photosynthetic cells) and is re-energized as ATP.
The role of ATP-A DP and UTP-UDP in the formation of starch furnishes an illustration of one common way in which energy from foodstuffs is transferred, by means of energy-rich bonds, into polymers. With some modifications to be discussed, the starch model holds also for the synthesis of proteins, RNA, DNA and lipids as described in the following paragraphs.