Chapter 3 • Lesson 15



Chapter 3 • Lesson 15

Biogeochemical Cycles

Objective: 2,1,1

Key Terms

carbon-oxygen cycle • nitrogen cycle • nitrogen fixation • denitrification • water cycle evaporation • transpiration • condensation • precipitation

Getting the Idea

Earth's organisms need many elements, including carbon, oxygen, and nitrogen, to carry out their life processes. These elements exist both within organisms and in water, air, and land. Like all matter, the elements needed by organisms are present in limited, or finite, amounts on Earth. The planet does not create or receive more of these elements when needed. Instead, they move between the biotic (living) and abiotic (nonliving) parts of the biosphere in biogeochemical cycles.

The Carbon-Oxygen Cycle

All organisms contain carbon and oxygen. Both these elements exist in Earth's atmosphere, primarily in the form of carbon dioxide (CO2) and oxygen gas (O2). As you have learned, plants use photosynthesis to convert carbon dioxide from the atmosphere into carbohydrates (sugars), which supply the energy the plants need. In this process, plants release oxygen back into the atmosphere.

In the last lesson, you saw how energy moves between organisms through the feeding relationships that make up food webs. This energy is stored in the chemical bonds of the carbohydrates made during photosynthesis. The carbon and oxygen in those carbohydrates also move between organisms through these feeding relationships.

The carbon in carbohydrates and other compounds that make up organisms is returned to the atmosphere when decomposers break down wastes and the remains of dead organisms. Organisms also release carbon to the atmosphere when they break down organic compounds, such as glucose, into carbon dioxide and water through respiration. Recall that this process also uses the oxygen that was released during photosynthesis. The continuous transfer of carbon and oxygen between Earth's living organisms and the nonliving parts of the environment is known as the carbon-oxygen cycle.

As the diagram shows, photosynthesis, cellular respiration, and decomposition are not the only processes that move carbon and oxygen between organisms and the nonliving environment. The carbon-oxygen cycle also involves an important geological process—the formation of fossil fuels—as well as the combustion, or burning, of fuels. Volcanic eruptions (not shown) are another geological process that has played an important part in the carbon-oxygen cycle over time by releasing large amounts of carbon dioxide and carbon monoxide into the atmosphere.

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Coal, oil, and natural gas are fossil fuels formed by geological processes over millions of years. Ancient plants and microorganisms were gradually changed into fossil fuel by high pressures and temperatures. The carbon in their bodies was stored in the fossil fuel. Fossil fuel deposits beneath Earth's surface store huge amounts of carbon. The carbon is released in the form of carbon dioxide when the fuels are burned. Other fuels derived from plants, such as wood and charcoal, also contain stored carbon that is returned to the air by combustion.

Forests are a major carbon reservoir. Trees store large amounts of carbon in their tissues, such as wood. This carbon may be released into the atmosphere as carbon dioxide by natural or human-caused forest fires. The use of trees as firewood also releases carbon dioxide to the bustion requires oxygen. Therefore, as fuels burn, oxygen is removed from the atmosphere.

The oxygen joins with matter in the fuel to form waste products that include carbon dioxide and water vapor. Humans burn large amounts of fossil fuels to meet their energy needs. Burning these and other carbon-based fuels increases the amount of carbon dioxide that enters the atmosphere. You will learn more about how burning fossil fuels affects the environment in Lesson 18.

The Nitrogen Cycle

The nitrogen cycle is the continuous movement of nitrogen among Earth's organisms, waters, rocks, minerals, and atmosphere. Recall from Lesson 3 that nitrogen is an essential component in amino acids and proteins as well as in nucleic acids and ATP.

The atmosphere contains about 78 percent nitrogen in the form of nitrogen gas (N2). However, most organisms cannot use nitrogen in this form. The atmospheric nitrogen is changed in form by nitrogen fixation. Nitrogen fixation is a process in which certain soil bacteria break down nitrogen gas from the atmosphere and convert it into a form that plants can use. Some soil bacteria have a relationship with certain plants in which the bacteria convert nitrogen gas in exchange for carbohydrates. When animals consume the plants, the animals get the nitrogen, which is passed through the food chain to other animals, and then to decomposers.

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When organisms die, the nitrogen in their cells and tissues is returned to the environment. Under certain low-oxygen conditions in the soil and in marine environments, bacteria convert nitrogen compounds into nitrogen gas in a process called denitrification. This process returns nitrogen to the atmosphere.

The Water Cycle

Water is vital to all organisms. The water cycle, or hydrologic cycle, is the continuous circulation of water among Earth's organisms, atmosphere, and surface. This cycle is driven by energy from the sun. The diagram on the next page summarizes the main processes involved in the water cycle.

Water enters the atmosphere through three basic processes: evaporation, cellular respiration, and transpiration. Evaporation is the change of a liquid to a gas at the surface of the liquid. In the water cycle, heat from the sun changes liquid water at Earth's surface into water vapor. As you learned in Lessons 7 and 10, water is a waste product of cellular respiration. Many animals release the water vapor formed during cellular respiration to the atmosphere when they exhale. Plants release extra water by transpiration. Transpiration is the release of water vapor through the stomata, or openings, in a plant's leaves. Plants also use transpiration to cool themselves.

Water vapor that enters the atmosphere may cool and condense. Condensation is the process in which a gas changes to a liquid when it loses thermal energy. In the water cycle, water vapor condenses to form liquid water, much of which is stored in clouds. This water can then return to Earth as precipitation. Precipitation is water that falls from the atmosphere to Earth as rain, snow, sleet, or hail.

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Water is stored on Earth in many different places, including oceans, lakes, glaciers, and plants. Water from Earth's surface can travel deep into the ground and be stored as groundwater. Most of Earth's freshwater is stored in glaciers and ice caps or below Earth's surface as groundwater.

Surface water, water that flows over Earth's surface, can run off the land or flow underground into nearby bodies of water.

Organisms can take in and use liquid water that collects on or beneath Earth's surface. Most plants take in water from soil through their roots. Animals, including humans, take in much of the water they need by drinking. Animals also take in water in the food they eat. Organisms use water to carry out their life processes. Other materials dissolved in the water that organisms take in may be carried into cells and used for their life processes. In turn, wastes may exit cells by dissolving in water. These wastes, along with excess water, are removed from an animal's body in the form of urine. Some animals, including humans, also remove water from their bodies by sweating, or perspiring. This water reenters the water cycle as it evaporates from the skin. Recall that plants release excess water to the air by transpiration.

When the sun heats surface water (such as lakes, oceans, and glaciers), it is converted into water vapor, which enters the atmosphere. In time, this water vapor can cool and condense to form liquid water. The liquid water returns to Earth's surface as precipitation.

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