CHAPTER 20 LESSON 2 Key Concept How does matter move in ecosystems? Matter and Energy in the Environment Cycles of Matter What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 3. Carbon, nitrogen, and other types of matter are used by living things over and over again. 4. Clouds are made of water vapor. Identify the Main Point As you read, highlight the main point of each paragraph. Then use a second color to highlight a detail or example to help explain the main point. Reading Check 1. Define In your own words, write a definition of the water cycle. How does matter move in ecosystems? The water that you use to wash your hands might once have traveled through the roots of a tree in Africa. It might also have been part of a glacier in Alaska. How can this be? Water is used over and over again in ecosystems. It never stops moving. The same is true of carbon, oxygen, nitrogen, and other types of matter. Elements that move through one matter cycle might play a role in another cycle. The Water Cycle Water covers about 70 percent of Earth s surface. Water surrounds all of Earth s landmasses. Most of Earth s water is in the oceans. Water is in rivers, streams, lakes, and underground. Water is also in the atmosphere, in icy glaciers, and in living things. Water is always moving from Earth to the atmosphere and back again. This movement is called the water cycle. The water moves by three processes: evaporation, condensation, and precipitation. 336 Matter and Energy in the Environment Reading Essentials
Transpiration Precipitation Condensation Evaporation 2. Identify Highlight the arrow showing which way water goes during transpiration. Groundwater Evaporation In the water cycle, as shown above, the Sun supplies the energy. As the Sun heats water on the surface of Earth, evaporation occurs. Evaporation (ih va puh RAY shun) is the process during which liquid water changes into a gas called water vapor. This water vapor rises into the atmosphere. Temperature, humidity, and wind affect how quickly water evaporates. Water is also released from living things. Cellular respiration occurs in many cells. Water is a by-product of cellular respiration. This water leaves cells and enters the environment and atmosphere as water vapor. Transpiration is the release of water vapor from the leaves and stems of plants. Condensation The higher in the atmosphere you are, the cooler the temperature is. As water vapor rises in the atmosphere, it cools. This cooling causes condensation. Condensation (kahn den SAY shun) is the process during which water vapor changes into liquid water. Clouds form as a result of condensation. Clouds are made of millions of tiny water droplets or ice crystals. These form when water vapor condenses on particles of dust and other substances in the atmosphere. 3. Evaluate Why is condensation the opposite of evaporation? Reading Essentials Matter and Energy in the Environment 337
Key Concept Check 4. Describe What form does water take as it falls to Earth? 5. Identify What changes nitrogen gas in the air to nitrogen compounds that fall to the ground? Precipitation Precipitation (prih sih puh TAY shun) is water that falls from clouds to Earth s surface. This water enters bodies of water or soaks into soil. Precipitation can be rain, snow, sleet, or hail. Precipitation forms as water droplets or ice crystals join together in clouds. The droplets or crystals get so large and heavy that they fall to Earth as precipitation. Over time, living things use this precipitation, and the water cycle continues. Bacteria in soil convert nitrogen compounds into nitrogen gas, which is released into the air. Decaying organic matter and animal waste return nitrogen compounds to the soil. Nitrogen gas in atmosphere Animals eat plants. Plants take in and use nitrogen compounds from the soil. Lightning changes nitrogen gas in the atmosphere to nitrogen compounds. The nitrogen compounds fall to the ground when it rains. Nitrogen compounds in soil Nitrogen-fixing bacteria on plant roots convert unusable nitrogen in soil to usable nitrogen compounds. Reading Check 6. Define What is nitrogen fixation? The Nitrogen Cycle You know that water is necessary for life on Earth. The element nitrogen is also necessary for life. Nitrogen is part of proteins, which all organisms need to stay alive. It also is part of DNA, the molecule that contains genetic information. Like water, nitrogen cycles between Earth and the atmosphere and back again. The nitrogen cycle is shown above. From the Environment to Organisms You learned earlier that the atmosphere is mostly nitrogen. Plants and animals cannot use the form of nitrogen that is in the atmosphere. How do organisms get nitrogen into their bodies? The process that changes nitrogen in the atmosphere into nitrogen compounds that living things can use is called nitrogen fixation (NI truh jun fihk SAY shun). Nitrogen from the atmosphere is changed into a different form with the help of bacteria that live in soil and water. These bacteria take in nitrogen from the atmosphere and change it into nitrogen compounds that other organisms can use. Plants and some other organisms take in this changed nitrogen from the soil and water. Then animals take in nitrogen when they eat these plants and other organisms. 338 Matter and Energy in the Environment Reading Essentials
From Organisms to the Environment How does nitrogen from living things return to the environment? Some types of bacteria can break down the tissues of dead organisms. These bacteria help return the nitrogen in the tissues of those organisms to the environment. You ve seen these bacteria at work in a decaying log or a rotten apple. Nitrogen also returns to the environment in the waste products of organisms. Manure is a waste product of organisms. Farmers often spread manure on their fields as a way to provide nitrogen for crops. The Oxygen Cycle You learned that organisms need water and nitrogen. Almost all organisms also need oxygen for cellular processes that release energy. Oxygen is also a part of carbon dioxide and water, substances that are important to life. The picture below shows how oxygen cycles through an ecosystem. ACADEMIC VOCABULARY release (verb) to set free or let go During photosynthesis, plants release oxygen gas into the air. During photosynthesis, plants take in carbon dioxide gas from the air. O2 (oxygen) Animals and other organisms release carbon dioxide gas into the air. CO2 (carbon dioxide) Animals and other organisms take in oxygen gas from the air. Most of the oxygen in the atmosphere comes from photosynthesis. Earth s early atmosphere probably did not contain oxygen gas. Certain bacteria evolved that made their own food through photosynthesis. Oxygen gas is a byproduct of photosynthesis. Over time, other photosynthetic organisms evolved. The amount of oxygen in Earth s atmosphere increased. Photosynthesis is the main source of oxygen in Earth s atmosphere today. Humans and many other living organisms take in oxygen and release carbon dioxide during cellular processes. There are many other relationships between different types of matter in ecosystems. 7. Explain What gas do humans take in and what gas do they release as part of the oxygen cycle? 8. Generalize Imagine that there are no plants on Earth. Would the planet have oxygen in its atmosphere? Why or why not? Reading Essentials Matter and Energy in the Environment 339
The Carbon Cycle All organisms contain carbon. Some organisms, including humans, get carbon from food. Other organisms, such as plants, get carbon from the atmosphere or bodies of water. Carbon cycles through ecosystems like water, nitrogen, and oxygen do. It is used over and over again. Carbon compounds in atmosphere Photosynthesis Combustion 9. Explore Circle the processes that return carbon to the atmosphere. Photosynthesis Cellular respiration Cellular respiration Plants, certain protists, and bacteria on land Cellular respiration Decomposition Animals Plants, certain protists, and bacteria in water CO2 in water Decomposition Sediments Carbon compounds in soil Decomposition Fossil fuels Make a half-book to organize information about the biotic and abiotic parts of one cycle of matter. Cycles in Nature Carbon in Soil All organisms return carbon to the environment, as shown above. Like nitrogen, carbon can enter the environment when organisms die and decompose. Decomposition is the breaking down of dead plants and animals. This process returns carbon compounds to the soil and releases carbon dioxide (CO 2 ) into the atmosphere. Other organisms then use carbon dioxide. Carbon also is found in fossil fuels. Carbon in Air How do living things use the carbon dioxide that is in the air? Plants and other organisms make their own food through photosynthesis. They take in carbon dioxide and water to make sugars. When other organisms eat plants, they get carbon and energy. Carbon dioxide is a by-product of the cellular processes that break down sugars to release energy. Carbon dioxide enters the atmosphere to be used again. 340 Matter and Energy in the Environment Reading Essentials
The Greenhouse Effect Carbon dioxide and other gases in the atmosphere absorb thermal energy from the Sun. This energy keeps Earth warm. This process is called the greenhouse effect. How does the greenhouse effect help Earth? Life on Earth could not exist without the greenhouse effect. As you can see in the figure below, some of this energy is reflected back into space, and some passes through Earth s atmosphere. Greenhouse gases in the atmosphere absorb thermal energy that reflects off Earth s surface. The greenhouse effect helps keep Earth from becoming too hot or too cold. How does the greenhouse effect harm Earth? While the greenhouse effect is necessary for life, a steady increase in greenhouse gases can harm ecosystems. For example, carbon is stored in fossil fuels, such as coal, oil, and natural gas. When people burn fossil fuels to heat homes, to power cars, or to provide electricity, carbon dioxide gas is released into the atmosphere. The amount of carbon dioxide in the air has increased because of both natural and human activities. The more greenhouse gases released, the greater the gas layer becomes and the more heat is absorbed. Reading Check 10. Define What is the greenhouse effect? 11. Describe Explain how the greenhouse effect benefits Earth. 4 Some heat is absorbed by greenhouse gases, such as carbon dioxide, in the atmosphere. Earth 1 Atmosphere The Sun gives off solar radiation. 3 Some of the radiation passes through the atmosphere and is absorbed by Earth's surface. Sun Some solar 2 radiation is reflected by Earth's atmosphere. 12. Explain What might happen if heat were not absorbed by greenhouse gases? Reading Essentials Matter and Energy in the Environment 341
Mini Glossary condensation (kahn den SAY shun): the process during which water vapor changes into liquid water evaporation (ih va puh RAY shun): the process during which liquid water changes into a gas called water vapor nitrogen fixation (NI truh jun fihk SAY shun): the process that changes atmospheric nitrogen into nitrogen compounds that are usable to living things precipitation (prih sih puh TAY shun): water that falls from clouds to Earth s surface 1. Review the terms and their definitions in the Mini Glossary. Choose a term and write at least two sentences explaining how the term is linked to the nitrogen cycle. 2. On the lines in the graphic below, identify the processes that make up the water cycle. 3. How did highlighting the main point of each paragraph, along with a detail or example to explain that point, help you understand what you read? What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON 342 Matter and Energy in the Environment Reading Essentials