4.12–4.16: Living organisms extract energy through cellular respiration.

A pit stop at the toddler refueling station.

Food is fuel. And all the activities of life—growing, moving, reproducing—require fuel. Plants, most algae, and some bacteria obtain their fuel directly from the energy of sunlight, which they harness through photosynthesis. Less self-sufficient organisms, such as humans, alligators, and insects, must extract the energy they need from the food they eat. This energy comes from photosynthetic organisms either directly (from eating plants) or indirectly (from eating animals that eat plants) (FIGURE 4-27).

Figure 4.27: Living organisms require fuel (in one form or another).

All living organisms—including plants (a fact that is often overlooked)—extract energy from the chemical bonds of molecules (which can be considered “food”) through a process called cellular respiration (FIGURE 4-28). This process is a bit like photosynthesis in reverse. In photosynthesis, the energy of the sun is captured and used to build molecules of sugars, such as glucose. In cellular respiration, plants and animals break down the chemical bonds of sugar and other energy-rich food molecules (such as fats and proteins) to release the energy that went into creating them. (Don’t confuse cellular respiration with the act of breathing, which is also called respiration.) As energy is released, cells capture and store it in the bonds of ATP molecules. This plentiful, readily available stored energy can then be tapped as needed to fuel the work of the life-sustaining activities and processes of all living organisms.

Figure 4.28: Cellular respiration: the big picture.

In humans and other animals, cellular respiration starts after we eat food, digest it, absorb the nutrient molecules into the bloodstream, and deliver them to the cells of our bodies. At this point, our cells begin to extract some of the energy stored in the bonds of the food molecules. We focus here on the breakdown of glucose, but later in this chapter we’ll see that the process is similar for the breakdown of fats or lipids. Ultimately, when a food molecule has been completely processed, the cell has used the food molecule’s stored energy (along with oxygen) to create a large number of high-energy-storing ATP molecules (which supply energy to power the cell’s activities), water, and carbon dioxide (which is exhaled into the atmosphere).