Energy is a requirement of life. Living is hard work; energy must be expended to perform all the processes that occur in organisms. Chemical reactions, cellular functioning, running, growing, reproducing-all require energy. Without a steady source of usable energy, organisms would die. As we will see in the next two chapters, photosynthetic organisms use the sun as their energy source and convert solar energy to chemical energy. That chemical energy is then used by the photosynthesizers themselves and by essentially all other organisms. In this chapter we learn the fundamentals of energy and metabolism: what energy is, how it can be transformed and transferred, how chemical energy and reactions operate in living organisms, and how such systems are controlled.