Energy for Life
What Is the Role of Photosynthesis and Respiration in the Carbon Cycle?
The Carbon Cycle
Carbon is constantly circulating between living things and the environment. As it circulates, it changes form—from gas to solid and back again. There are several ways in which carbon is stored on Earth. A large quantity of stored carbon is called a reservoir. Carbon reservoirs include fossil fuel deposits, biomass (living plants and animals), oceans, and rocks. Carbon is essential to life on Earth, and the carbon cycle helps keep the amount of carbon in the atmosphere relatively constant.
The exchange of carbon between biomass and the atmosphere is also referred to as the fast carbon cycle, because this cycle occurs relatively quickly. Carbon is taken from the atmosphere when plants use it to produce food. Then, animals consume the food and use its energyglossary term (opens in a new window) for the animal’s life processes. When the animal dies, it decomposes and releases the carbon into the atmosphere or into the soil.
Photosynthesis and Respiration
Photosynthesisglossary term (opens in a new window) and respiration are two processes that exchange carbon between the atmosphere and living things in the environment. They are essential to the functioning of the fast carbon cycle.
Photosynthesis is the biochemical process by which plants use energy from the sun to produce food from carbon dioxide and water. In this way, carbon moves from the atmospheric reservoir to the biomass reservoir.
Cellular respirationglossary term (opens in a new window) is the set of metabolic processes by which plants and animals convert the chemical potential energyglossary term (opens in a new window) in glucose and other nutrients into adenosine triphosphate (ATP), which is then used to fuel other biochemical processes. The overall reaction for cellular respiration converts glucose into carbon dioxide (which is exhaled) and water. Thus, carbon moves from biomass back into the atmosphere.
Photosynthesis and cellular respiration do not only cycle matter in the form of carbon through the carbon cycle, they also cycle energy through the carbon cycle. Photosynthesis converts solar energy into the chemical potential energy contained in glucose and other carbohydrates. Then, cellular respiration converts the chemical potential energy into ATP, which fuels other biochemical processes.
Teacher Note: Connections
This item assesses students’ ability to analyze the cause-and-effect relationships in the carbon cycle. Students will use what they have learned about the smaller scale processes within the carbon cycle to predict how changes in the quantities of inputs and the number of organisms would affect the overall quantity of carbon in the atmosphere. To provide extra support, have students write the overall reactions for photosynthesis and respiration. Then, ask leading questions to help students determine the effect of changing quantities of reagents and populations of organisms. For example, Would increasing the population organisms increase or decrease the rate of respiration? Would increasing the rate of respiration increase the quantity of reagents or products in the reaction? How would this affect the amount of carbon in the atmosphere?
