Module 4-2
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Manchester West High School**We aren't endorsed by this school
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SCIENCE 213
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Biology
Date
Dec 16, 2024
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8
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e LS 116 18 (a) Identify the condition of nutrient level i g nt levels found in ®) pescribe one consequence of nutrient levels remain- ing as they are in Lake A. (1 pt.) (ii) Describe a way that cycle. (1 pt.) (iii) Describe an ecolog problem from part g (i (h) Explain which nutrient is ecosystems. (1 pt.) B - [ The Carbon and Nitrogen Cycles Thus far in Unit 1, we have examined how the biotic and abiotic conditions help to determine the distribution of plant and animal species in terrestrial and aquatic biomes. Given that all species are comprised of matter— including carbon, nitrogen, phosphorus, and water— these atoms and molecules are critical for the functioning of species in ecosystems. Other elements are also required, although in much smaller amounts, such as calcium, magnesium, potassium, and sulfur. An interesting insight is that when it comes to matter, all ecosystems combined represent a closed system, meaning these atoms and molecules are not lost or gained but instead they cycle within and among ecosystems in different forms. The specific chemical forms that elements take determine how they cycle within the biosphere. Because the movement of matter within and between eco- systems involves cycles of biological, geological, and chem- ical processes, these cycles are known as biogeochemical cycles. To keep track of the movement of matter in bio- geochemical cycles, we refer to the components of the biogeochemical cycle that contain the matter—including Learning Goals After reading this module yor 4-1 explain how carbon cy ecosystems. 4-2 describe how nitrogen ecosystems. air, water, and organisms—as re can serve as a “source” of the e molecules leave a reservoir and Biogeochemical cycle The move between ecosystems involving cyc and chemical processes. Reservoirs The components of th contain the matter, including air, wi MODULE 4 = The Carbon ai Scanned with CamScanner
; nt and molecules get stored in the rescrvoir. The mosx;erl:l; e of the atoms and molecules betyveen dlfferelnt tr;is e occurs through biotic and abiotic pmcesses(.i nitrogen o ule, we examine the cycles of carbon and n i8¢ Foser= several different forms with a focus on the maj i e voirs where carbon and nitrogen are stf)red ocnesses ot well as the important biotic and abiotic pro tatie In cause these elements to cycle among the ;65;08 i Module 5, we will examine the cycling of phosp and water. 4-1 How does carbon cycle within ecosystems? The carbon cycle moves carbon between air, water, and land Carbon is the most important element in living organisms; it makes up about 20 percent of their total body weight. Carbon is the basis of the long chains of organic molecules that form the membranes and walls of cells, constitute the backbones of proteins, and store energy for later use. Other than water, there are few molecules in the bodies of organ- isms that lack carbon. To understand the carbon cycle, which is the movement of carbon around the biosphere among reservoir sources and sinks, we need to eXamine the flows that move carbon and the major reservoirs that con- tain carbon. The Carhon Cycle FIGURE 4.1 illustrates the seven processes that drive the car- bon cycle: photosynthesis, respiration, exchange, sedimen- tation, burial, extraction, and combustion, These processes can be categorized as either fast or slow. The fast part of the cycle involves processes that are associated with living organisms that hold carbon for a relatively short period of time. It also includes the exchange of CO,; between Carbon cycle .The movement of carbon around the biosphere among reservoir sources and sinks. Aerobic respiration The process by which cells convert glucose and oxygen into energy, carbon dioxide Steady state When a system’s inputs the system is not changing over time. 52 UNIT1 n The Living World: Ecosystems Photosynthesis and Respl_ratlon ' Let’s take a closer look at Figure 4.1, which depicgg the of the carbon cycle. When plants and algae processes 8 latid St ol h Use | photosynthesis, whether on lan or in er, they uge | solar energy to convert carbon dioxide (CO,) and Watg, | (H,0) into glucose (C¢H;,O4) and oxygen (O,). A Portj, of this energy is consumed by herbivores and Sl‘bsequem]y by predators of those herbivores. : These organisms return a portion of their carbon C when they use aerobic respiration, whereby cellg con, vert glucose and oxygen into energy, carbon dioxide, i water. Additional carbon is returned after organisms g When organisms die, carbon that was part of the liye big. mass reservoir becomes part of the dead biomass Teservy, Decomposers break down the dead material, which Tetury CO; to the water or air via respiration and thus Continye, the cycle. Exchange, Sedimentation, and Burial As you can see on the left side of Figure 4.1, carbop jg exchanged between the atmosphere and the ocean. The amount of carbon released from the ocean into the atmg_ sphere roughly equals the amount of atmospheric CQ, thy diffuses into ocean water. Some of the CO, dissolved in the ocean enters the food web via photosynthesis by algae. Another portion of the CO, dissolved in the ocean com- bines with calcium ions in the water to form calcium carbop. ate (CaCO3),a compound that can precipitate out of the wate; and form limestone and dolomite rock via sedimentation and amounts of calcium carbonate s accumulated over millions of years to produce the largest car- bon reservoir in the slow part of the carbon cycle, A small fraction of the organic carbon in the dead bio- incorporated into ocean sedi- Scanned with CamScanner
Atmospheric Excr_range Respiration Combustion CO, in the atmosphere Sugars are converted " Fossil fuels and and CO, dissolved in back into CO,,. " plant matter are water are constantly g converted int exchanged. Photosynthesis CO, is converted into sugars. ‘Extraction Human extraction of fossil fuels = i - brings carbon to . Earth's surface, Dissolved CO, Sedimentation Calcium carbonate precipitates out of the water as seciments. Sedimentary % rocks § where it can be combusted. gt FIGURE 4.1 The carbon cycle. Producers take up carbon from the atmosphere via photosyn- thesis and pass it on to consumers and decomposers. Some inorganic carbon sediments pre- cipitate out of the water to form sedimentary rock while some organic carbon may be buried and become fossil fuels. Respiration by organisms returns carbon to the atmosphere and water. Com- bustion of fossil fuels and other organic matter returns carbon to the atmosphere. carbon by fires or volcanoes release carbon into the atmo- sphere as CO, or into the soil as ash. As you can see, combustion, respiration, and decomposi- tion operate in very similar ways: All three processes cause organic molecules to be broken down to produce CO,, water, and energy. However, respiration and decomposition are biotic processes, whereas combustion is an abiotic process. Human Impacts on the Carbon Cycle In the absence of human disturbance, the exchange of car- bon between Earth’s surface and atmosphere is in a steady state. Carbon taken up by photosynthesis eventually ends up in the soil. Decomposers in the soil gradually release that carbon at roughly the same rate it is added. Similarly, the gradual movement of carbon into the buried or fossil fuel reservoirs is offset by the slow processes that release it. AP® Exam Tip A very important part of learning about cycles is to % know how humans have impacted each cycle. sl e ] MODULE 4 = The Carbon and Nitrogen Cycles 53 Scanned with CamScanner
Solar energy 1\ Heat-trapping (greenhouse) gases FIGURE 4.2 The Earth-surface energy balance. As Earth’s surface is warmed by the Sun, it radiates heat outward. Heat-trapping gases absorb the outgoing heat and reradiate some of it back to Earth. Without these greenhouse gases, Earth would be much cooler. A major current concern is that the combustion of CO, is leading to the warming of our planet. As FIGURE 4.2 shows, our thick planetary atmosphere contains many gases. When light energy from the Sun hits Earth’s surface, the energy is absorbed and a portion is emitted as heat. Some of the gases in the Earth’s atmosphere, known as greenhonse gases absorb this heat. The gases then re-emit this absorbed heat ; all directions, including back to Earth’s surface, which causes the planet to become warmer. As a result, greenhouse gases help keep Earth’s surface within the range of temperatures at which life can flourish. The gas that contributes most to warming of the atmosphere is carbon dioxide (CO,). During most of the history of life on Earth, greenhouse gases have been present in the atmosphere at fairly constant con- centrations for long periods of time. For example, before the Industrial Revolution, the atmospheric concentration of car- bon dioxide had changed very little for hundreds of years, as You can see in FIGURE 4.3. As we have discussed, during this time carbon entering any of the various reservoirs was bal- anced by carbon leaving these reservoirs. Since the Industrial Revolution, human activities have had a major influence on carbon cycling. Largely due to the combustion, or burning, of fossil fuels, humans have been moving fossilized carbon into the atmosphere at a much faster rate than carbon leaves the atmosphere through the processes of sedimentation and burial, As a result, atmo- spheric carbon concentrations have increased and this has upset the balance between Earth’s carbon reservoirs and the > Greenhouse gases Gases in Earth’s atmosphere that trap heat near the surface. 54 UNIT1 = The Living World: Ecosystems 425 i I L 15.0 === Carbon dioxide 148 400 - (parts per million) —— Global temperature (°C) 148 T a § 375 F144 9 £ F142 § £ ~ g 350 § g ~14.0 g £ 8 a5 138 $ o ® 8 ~136 § 300 0] // 134 754 N~ 132 0 :%L T 1600 1700 1800 1900 2000 Year FIGURE 4.3 Changes in average global surface temper,. ture and in atmospheric CO, concentrations. Earth’s averg, o global surface temperature has increased steadily for at least the past 100 years. Carbon dioxide concentrations in the atmo- sphere have varied over geologic time, but have risen steadily since 1960. (Data from htrp://da!a.g/ss.nasa,gongstemp/graphs_v&/and hito:/wwy .esrl,naaa,gov/gmd/ccgg/{rends/#m/o_fu/l) atmospheric reservoir. The excess CO;, in the atmosphere acts to increase the retention of heat energy in the biosphere, Tree harvesting is another human activity that can affect the carbon cycle. Trees store a large amount of carbon in planted to recapture the carbon, the destruction of forests will upset the balance of CO,. To date, large areas of for- est, including tropical forests as well as North American and European temperate forests, have been converted into pastures, grasslands, and croplands. In addition to destroy- ing a great deal of biodiversity, this destruction of forests have been partly offset by an increase in carbon absorption by the ocean. While some regions of the world have expe- rienced increased reforestation, such as the northeastern United States, the loss of forest remains a global concern. As a result of combusting fossil fuels and tree harvesting, atmospheric CO, concentrations have dramatically increased during the past 200 years. From 1600 to 1800, CO; con- centrations were steady at approximately 280 parts per mil- lion (ppm). Since 1800, CO, concentrations have rapidly increased to approximately 420 ppm today, as you can see in Figure 4.3. In this figure, you can also see that during roughly the same period, global temperatures have displayed an overall increase, as we would expect given that CO, is a greenhouse gas. (Note that this graph has two y axes. See the appendix Scanned with CamScanner |
SN “Reading Graphs” at the end of the book to learn more about reading a graph like this one.) The current scientific evidence is that the increase in atmospheric CO, during the Jast two centuries is caused by human activities. The increase in global temperatures due to humans producing more green- house gases is known as global warming. Global warming is a major concern among environmental scientists and policy makers. We will discuss this in much greater detail in Unit 9. 4-2 How does nitrogen cycle within ecosystems? The nitrogen cycle includes many chemical transformations Nitrogen is used to form amino acids, the building blocks of proteins, and nucleic acids, the building blocks of DNA and RNA. Because so much of it is required, nitrogen is often a limiting nutrient for plants and algae. A limiting nutrient is a nutrient required for the growth of an organism but available in a lower quantity than other nutrients. The pres- ence of a limiting nutrient, such as nitrogen, constrains the growth of plants and algae. Adding other nutrients, such as water or phosphorus, will not improve their growth. The Nitrogen Cycle The nitrogen cycle is the movement of nitrogen around the biosphere among reservoir sources and sinks. As nitrogen moves through an ecosystem, it experiences many chemical transfor- mations. FIGURE 4.4 shows the five major transformations in the nitrogen cycle: nitrogen fixation, nitrification, assimilation, min- eralization, and denitrification. Unlike the carbon cycle, most reservoirs hold nitrogen for relatively short periods of time. Nitregen Fixation While Farth’s atmosphere is 78 percent nitrogen by volume, makin it the largest nitrogen reservoir, the vast majority of that nitrogen is in the form of nitrogen gas, which most plants and algae cannot use. However, nitrogen fixation is a process that converts nitrogen gas (N,) in the atmosphere into forms of nitro- gen that plants and algae can use. As you can see in Figure 4.4, nitrogen fixation can occur through biotic or abiotic processes. In the biotic process, a few species of bacteria can con- vert N, gas directly into ammonia (NH3), which is rapidly converted to ammonium (NH4*), a form that is readily used by plants and algae. Nitrogen-fixing organisms include cyanobacteria (also known as blue-green algae) and cer- tain bacteria that live within the roots of legumes, which include plants such as peas, beans, and a few species of trees. Nitrogen-fixing organisms use the fixed nitrogen to synthe- size their own tissues, then excrete any excess. Cyanobac- teria, which are primarily aquatic organisms, excrete excess ammonium ions into the water, where they can be taken up by aquatic plants and algae. Nitrogen-fixing bacteria that live within plant roots excrete excess ammonium ions into the plant’s root system; the plant, in turn, supplies the bacte- ria with sugars it produces via photosynthesis. o Nitrogen fixation can also occur through two ablqtlc pathways. N, can be fixed in the atmosphere by lightning or during combustion processes such as fires and the k?um- ing of fossil fuels. These processes convert N intf) nitrate (NOj3"), which is carried to Earth’s surface in precipitation, where it is then usable by plants. Humans have developed techniques for nitrogen fixation into ammonia or nitrate to be used in plant fertilizers. Although these processes require a great deal of energy, humans now fix more nitrogen than is fixed in nature. The development of synthetic nitrogen fertilizers has led to increases in crop yields, particularly for crops such as corn that require large amounts of nitrogen. Nitrification Another step in the nitrogen cycle is nitrification, which is the conversion of ammonium (NH,*) into nitrite (NO,") and then into nitrate (NO3~). These conversions are con- ducted by specialized species of bacteria. Although nitrite is not used by most plants and algae, nitrate is readily used. Assimilation Once plants or algae take up nitrogen in the form of ammo- nia, ammonium, nitrite, or nitrate, they incorporate the element into their tissues in a process called assimilation. When herbivores feed on the plants and algae, some of the nitrogen is assimilated into the tissues of the herbivores while the rest is eliminated as waste products. Mineralization Eventually, organisms die and their tissues decompose. In a pro- cess called mineralization, fungal and bacterial decomposers break down the organic matter found in dead bodies and waste products and convert these organic compounds back into inorganic compounds such as ammonium (NH,*). Because this process produces ammonium, the process of mineraliza- tion is sometimes called ammonification. The ammonium Global warming The increase in global temperatures due to humans producing more greenhouse gases. Limiti_ng nutrient A nutrient required for the growth of an organism but available in a lower quantity than other nutrients. N.itrogen cycle The movement of nitrogen around the biosphere among reservoir sources and sinks. Nitrogen fixation The process that converts nitrogen gas in the atmosphere (N,) into forms of nitrogen that plants and algae can use. Nitrification The conversion of ammonia (NH4*) into nitrite (NO;") and then into nitrate NO3"). Assimilation A process by which plants and algae incorporate nitrogen into their tissues. Mineralization The process by which fungal and bacterial decomposers break down the organic matter found in dead bodies and waste products and convert these organic compounds back into inorganic compounds, such as inorganic ammonium (NH4*). Also known as ammonification. MODULE 4 = The Carbon and Nitrogen Cycles 55 Scanned with CamScanner
Nitrogen fixation Atmospheric nitroge producers can use. ammonia, is carried out by iated wi 4 some bacteria assocu; o nitrates, Is carried out by fixation, which produc lightning, combustion, and Mineralization ter Decomposers in sgll and wal break down biological nitrogen compounds into ammonium. ':"'g‘g Decomposers Nitrification Oz Nitrifying bacteria Herbivores convert ammonium & Predators. into nitrite and then into nitrate. T 8 Y & Plants&m Y g " \.\"' o Qi f > R = . | Denitrification In a series of steps, denitrifying bacteria in oxygen-poor soil and stagnant water convert nitrate into nitrite, nitrous oxide, and nitrogen gas. it is converted to forms thal giolic fixation, which produces cyanobacteria and th plant roots. Abiotic fertilizer prod‘llc‘lif?"- Atmospheric nitrogen (mostly N,) Decomposers ¢ Plants'&JAIg: Herbivores & |/ Predators o & Leaching FIGURE 4.4 The nitrogen cycle. The nitrogen cycle moves nitrogen from the atmosphere and into soils through several fixation pathways, including the production of fertilizers by humans. In the soil, nitrogen can exist in severai forms. Denitrifying bacteria release nitrogen gas back into the atmosphere. produced by this process can either be taken up by plants and algae in the ecosystem or be converted into nitrite (NO,”) and nitrate (NOj") through the process of nitrification. Denitrification The conversion of nitrate (NO5") in a series of steps into the gases nitrous oxide (N,0) and, eventually, nitrogen gas (N,), which is emitted into the atmosphere. Anaerobic An environment that lacks oxygen. 56 UNIT1 = The Living World: Ecosystems Denitrification The final step that completes the nitrogen cycle is denitrification, which is the conversion of nitrate (NOy) in a series of steps into the gases nitrous oxide (N,0) and, eventually, nitrogen gas (N,), which is emitted into the atmosphere. Denitrification is conducted by special- ized bacteria that live under anaerobic conditions, which means the environment lacks oxygen. Such environments include waterlogged soils or the bottom sediments of Scanned with CamScanner
P—_ A summary of the five transformations of nitrogen that TABLE 4.1 Tl occur in the nitrogen cycle Nitrogen fixation | Nitrogen fixation converts N, from the atmosphere. Biotic processes convert N, to ammonia (NHg), whereas abiotic processes convert N, to nitrate (NO3”). Nitrifying bacteria convert ammonium (NH,4*") into nitrite (NO,™) and then into nitrate NO3"). Plants and algae take up either ammonium (NH,4*) or nitrate (NO3"). Herbivores assimilate nitrogen by eating plants and algae. Decomposers in soil and water break Nitrification Assimilation Mineralization/ Ammonification | down biological nitrogen compounds into ammonium (NHg*). Denitrification In a series of steps, denitrifying bacteria in oxygen-poor soil and stagnant water convert nitrate (NO3") into nitrous oxide (N,0) and eventually nitrogen gas (No). oceans, lakes, and swamps. These bacteria do not live under aerobic conditions, which are environments with abun- dant oxygen. As you can see, the nitrogen cycle is a fairly complicated cycle because of the many transformations of nitrogen that take place. A summary of these processes can be found in TABLE 4.1. Human Impacts on the Nitrogen Cycle Nitrogen is a limiting nutrient in most terrestrial ecosys- tems, so excess inputs of various forms of nitrogen can have consequences in these ecosystems. For example, nitrate is readily transported through the soil with water through leaching, a process in which dissolved molecules are transported through the soil via groundwater. In addition, adding nitrogen to soils in fertilizers ultimately increases atmospheric concentrations of nitrogen in regions where the fertilizer is applied. This nitrogen can be transported through the atmosphere and deposited by rainfall in natural ecosystems that have adapted over time to a par- ticular level of nitrogen availability. The added nitrogen can alter the distribution or abundance of species in those ecosystems. In one study of nine different terrestrial ecosystems across the United States, scientists added nitrogen fertilizer to some plots and left other plots unfertilized as controls. They found that adding nitrogen reduced the number of species by up to 48 percent because some species that could survive under low-nitrogen conditions could no longer compete against larger plants that thrived under high-nitrogen conditions. Other studies have documented cases in which plant communities that have grown on low-nitrogen soils for millennia are now experiencing changes in their species composition as 2 result of nitrogen being deposited from the atmosphere. An influx of fixed nitrogen due to human activities has favored colonization by new species that are better adapted to soils with higher fertility. In this module, we have examined the reservoirs and pro- cesses that cause carbon and nitrogen to cycle around the planet. We have also considered the many ways in which human activities can impact these reservoirs and processes. In the next module, we build on this knowledge by exam- ining the cycling of phosphorus and water, which also play important roles in aquatic and terrestrial biomes. * Aerobic An environment with abundant oxygen. Leaching A process in which dissolved molecules are transported through the soil via groundwater. Module 4 AP® Review Preparing for the AP*’ Exam Learning Goals Revisited 4-1 How does carbon cycle within ecosystems? Carbon cycles through ecosystems as a result of seven pro- cesses: photosynthesis, respiration, exchange, sedimenta- tion, burial, extraction, and combustion. 4-2 How does nitrogen cycle within ecosystems? Nitrogen cycles through ecosystems as a result of five pro- cesses: nitrogen fixation, nitrification, assimilation, mineral- ization, and denitrification. MODULE 4 = The Carbon and Nitrogen Cycles 57 Scanned with CamScanner
58 AP® Practice Questions Multiple-Choice Questions 1: Which process in the carbon cycle is considered a slow part of the cycle? (a) sedimentation (b) respiration (c) photosynthesis (d) combustion . The largest carbon reservoir is found in (a) oceans. (b) the atmosphere. () living organisms. (d) fossil fuels. . Which process is causing a major human impact on the carbon cycle? (a) sedimentation (b) combustion (c) photosynthesis (d) respiration . Which process in the nitrogen cycle involves converting nitrogen gas (N,) into ammonia (NH3)? (a) fixation (b) nitrification (c) assimilation (d) mineralization . Which of the following processes is also known as ammonification? (a) nitrogen fixation (b) nitrification (c) mineralization (d) denitrification UNIT1 = The Living World: Ecosystems ) Preparing for the [:\-L] Exap, 6. Which organism and process shows the correct Pfliring; (a) photosynthesis: animals (b) nitrification: viruses (c) denitrification: bacteria (d) photosynthesis: bacteria Free-Response Question Nitrogen is crucial for sustaining life in both terrestrig] ang aquatic ecosystems. See the steps of the nitrogen cycle op the diagram below: Nitrogen Gas Ny oenimfican/ \Ni!mgen fixation Nitrite, Nitrate o NO,~ NOg~ Organic Nitrogen Nitrificali\ Ammonification Ammonium NH* (2) Identify which organisms fix nitrogen. (2 pts.) (b) Identify which organisms do nitrification. (2 pts.) (c) Describe the following steps in the nitrogen cycle: (i) nitrogen fixation (1 pt.) (ii) ammonification (1 pt.) (d) Identify an anthropogenic effect on the nitrogen cycle. (2 pts.) () Human sources of nitrogen can lead to acid precipi- tation. Propose a solution to reduce sources of acid precipitation. (2 pts.) Scanned with CamScanner