Living organisms require a continual input of free energy for three major purposes: (1) the performance of mechanical work in muscle contraction and cellular movements, (2) the active transport of molecules and ions, and (3) the synthesis of macromolecules and other biomolecules from simple precursors. The free energy used in these processes is derived from the environment. Photosynthetic organisms, or phototrophs, obtain this energy by trapping sunlight in a chemical form, whereas chemotrophs, which include humans, obtain energy through the oxidation of carbon fuels. In this chapter, we will examine some of the basic principles that underlie energy flow in all living systems. These principles are as follows:
Fuels are degraded and large molecules are constructed step by step in a series of linked reactions called metabolic pathways.
An energy currency common to all life forms, ATP, links energy-
The oxidation of carbon fuels powers the formation of ATP.
Although there are many metabolic pathways, a limited number of types of reactions and particular intermediates are common to many pathways.
Metabolic pathways are highly regulated to allow the efficient use of fuels and to coordinate biosynthetic processes.