What you’ll learn to do: Identify the reactants and products of cellular respiration and where these reactions occur in a cell

Figure 1. A hummingbird needs energy to maintain prolonged flight. The bird obtains its energy from taking in food and transforming the energy contained in food molecules 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, but humans also use energy while thinking, and even during sleep. In fact, the living cells of every organism constantly use energy.

Nutrients and other molecules are imported into the cell, metabolized (broken down) and possibly synthesized into new molecules, modified if needed, transported around the cell, and possibly distributed to the entire organism. For example, the large proteins that make up muscles are built from smaller molecules imported from dietary amino acids. Complex carbohydrates are broken down into simple sugars that the cell uses for energy.

Just as energy is required to both build and demolish a building, energy is required for the synthesis and breakdown of molecules as well as the transport of molecules into and out of cells. In addition, processes such as ingesting and breaking down pathogenic bacteria and viruses, exporting wastes and toxins, and movement of the cell require energy. From where, and in what form, does this energy come? How do living cells obtain energy, and how do they use it? This chapter will discuss different forms of energy and the physical laws that govern energy transfer. This chapter will also describe how cells use energy and replenish it, and how chemical reactions in the cell are performed with great efficiency.

In the process of photosynthesis, plants and other photosynthetic producers create glucose, which stores energy in its chemical bonds. You will actually study photosynthesis in more detail a bit later. But once photosynthesis has created glucose to store energy, both plants and consumers, such as animals, undergo a series of metabolic pathways, collectively called cellular respiration, to use that energy. Cellular respiration extracts the energy from the bonds in glucose and converts it into a form that all living things can use. Now let’s take a more detailed look at how all eukaryotes—which includes humans!—make use of this stored energy.