Figure 1. A hummingbird needs energy to maintain prolonged flight. The bird obtains its energy from taking in food and transforming it into forms of energy to power its flight through a series of biochemical reactions. (credit: modification of work by Cory Zanker)

Virtually every task performed by living organisms requires energy. Energy is needed to perform heavy labor and exercise.  Humans use energy during thought processes, and even during sleep.  The living cells of every organism constantly use energy.

Nutrients and other molecules are imported into the cell.  Once inside the cell, they are broken down into new molecules, sometimes modified, and distributed into the entire organism.  These new molecules are then used to produce energy.

Energy is required for the synthesis and breakdown of molecules and for their transport through an organism.  Most processes a cell takes on require energy.  Where does this energy come? How is energy obtained?  How is it used? This chapter will discuss different forms of energy and the physical laws that govern energy transfer. It will also discuss how cells use and replenish energy and the importance of the energy efficiency.