This chapter discusses why cells require energy, and how they use energy and atoms from their environment to create the molecular order that makes life possible. It examines to understand how enzymes promote catalysis—the acceleration of the specific chemical reactions needed to sustain life. Substrates are not the only molecules that can influence how well or how quickly an enzyme works. In many cases, products, substrate lookalikes, inhibitors, and other small molecules can also increase or decrease enzyme activity. By serving as intracellular energy shuttles, activated carriers perform their function as go-betweens that link the release of energy from the breakdown of food molecules to the energy-requiring biosynthesis of small and large organic molecules. Living organisms are able to exist because of a continual input of energy. Part of this energy is used to carry out essential reactions that support cell metabolism, growth, movement, and reproduction; the remainder is lost in the form of heat.