transformed into heat. The human body cannot produce energy on its own and must,
therefore, continuously take in a fresh supply of energy.
b. Except for a few special situations, all of the energy for living matter on Earth
is received from the Sun through solar radiation. Green plants trap and bind this solar
energy in molecules of glucose by the process of photosynthesis.
c. Humans take this glucose into their bodies directly by eating green plants or
indirectly by eating the flesh of plant-eating animals. The human body releases the
trapped energy from glucose by a process known as metabolic oxidation.
d. The released energy is used to form the compound ATP (adenosine triphos-
phate) from ADP (adenosine diphosphate). ATP is like a charged battery; the
"discharged battery" is called ADP. Molecules of ATP are present in all of the living
cells of the body. Within each cell, molecules of ATP are "discharged" to release a
large quantity of energy to drive the various life processes. Through further metabolic
oxidation, the resulting ADP molecules are "recharged" to form ATP molecules once