Chapter 2, Section 2
What Is Respiration?
After you eat a meal, your body converts some of the food into glucose, a type of sugar. Respiration is the process by which cells obtain energy from glucose. During respiration, cells break down simple food molecules such as sugar and release the energy they contain. Because living things need a continuous supply of energy, the cells of all living things carry out respiration continuously. Plant cells, as well as animal cells, respire.
Figure 6Energy From Respiration All organisms need energy to live. The leopard frog uses energy to leap great distances. Although the mushrooms don’t move, they still need energy to grow and reproduce.
Storing and Releasing Energy
Energy stored in cells is something like money you put in a savings account in a bank. When you want to buy something, you withdraw some of the money. Cells store and use energy in a similar way. During photosynthesis, plants capture the energy from sunlight and “save” it in the form of carbohydrates, including sugars and starches. Similarly, when you eat a meal, you add to your body’s energy savings account. When cells need energy, they “withdraw” it by breaking down the carbohydrates in the process of respiration.
Breathing and Respiration
The term respiration has two meanings. You have probably used it to mean “breathing,” that is, moving air in and out of your lungs. To avoid confusion, the respiration process that takes place inside cells is sometimes called cellular respiration. The two meanings of the term respiration do point out a connection, however. Breathing brings oxygen, which is usually necessary for cellular respiration, into your lungs.
Predicting
During the winter months, some animals go into a state called hibernation. During hibernation, an animal does not eat and its body activities are greatly reduced. Predict what will happen to an animal’s rate of cellular respiration when the animal goes into hibernation. Explain your prediction.
The Two Stages of Respiration
Like photosynthesis, respiration is a two-stage process. The first stage takes place in the cytoplasm of the organism’s cells. There, molecules of glucose are broken down into smaller molecules. Oxygen is not involved, and only a small amount of energy is released.
The second stage of respiration takes place in the mitochondria. There, the small molecules are broken down into even smaller molecules. These chemical reactions require oxygen, and they release a great deal of energy. This is why the mitochondria are sometimes called the “powerhouses” of the cell.
Trace the steps in the breakdown of glucose in Figure 7. Note that energy is released in both stages. Two other products of respiration are carbon dioxide and water. These products diffuse out of the cell. In most animals, the carbon dioxide and some water leave the body during exhalation, or breathing out. Thus, when you breathe in, you take in oxygen—a raw material for respiration. When you breathe out, you release carbon dioxide and water—products of respiration.
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Figure 7Two Stages of Respiration Respiration, like photosynthesis, takes place in two stages. Interpreting Diagrams In which stage of respiration is oxygen used?
The Respiration Equation
Although respiration occurs in a series of complex steps, the overall process can be summarized in the following equation:
Notice that the raw materials for respiration are sugar and oxygen. Plants and other organisms that undergo photosynthesis make their own sugar. The glucose in the cells of animals and other organisms comes from the food they consume. The oxygen used in respiration comes from the air or water surrounding the organism.
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Comparing Photosynthesis and Respiration
Can you notice anything familiar about the equation for respiration? You are quite right if you said it is the opposite of the equation for photosynthesis. This is an important point. During photosynthesis, carbon dioxide and water are used to produce sugars and oxygen. During respiration, the sugar glucose and oxygen are used to produce carbon dioxide and water. Photosynthesis and respiration can be thought of as opposite processes.
Together, these two processes form a cycle that keeps the levels of oxygen and carbon dioxide fairly constant in Earth’s atmosphere. As you can see in Figure 8, living things use both gases over and over again.
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Figure 8Photosynthesis and Respiration You can think of photosynthesis and respiration as opposite processes. Comparing And Contrasting Which process uses oxygen? Which uses carbon dioxide?