Energy Flow and Photosynthesis in Living Systems Whereas materials are recycled through ecosystems, the flow of useful energy is essentially a one-way process Incoming solar energy can be regarded as high-grade energy because it can cause useful reactions to occur, such as production of electricity in photovoltaic cells or photosynthesis in plants. As shown in Figure 1.2, solar energy captured by green plants energizes chlorophyll, which in turn powers metabolic processes that produce carbohydrates from water and carbon dioxide These carbohydrates are repositories of stored chemical energy that can be converted to heat and work by metabolic reactions with oxygen in organisms. Ultimately, most of the energy is converted to low-grade heat, which is eventually reradiated away from Earth by infrared radiation Energy Utilization During the last two centuries, the growing, enormous human impact on energy utilization has resulted in many of the environmental problems now facing humankind. This time period has seen a transition from the almost exclusive use of energy captured by photosynthesis and utilized as biomass(food to provide muscle power, wood for heat)to the use of fossil fuel petroleum, natural gas, and coal for about 90 percent, and nuclear energy for about 5 percent, of all energy employed commercially. Although fossil sources of energy have greatly exceeded the pessimistic estimates made during the energy crisis"of the 1970s, they are limited and their pollution potential is high. Of particular importance is the fact that all fossil fuels produce carbon dioxide, a greenhouse gas. Therefore, it will be necessary to move toward the utilization of alternate renewable energy sources, including solar energy and biomass. The study of energy utilization is crucial in the environmental sciences, and it is discussed in greater detail in Chapter 18, Industrial Ecology Resources, and Energy 1.6. MATTER AND CYCLES OF MATTER Cycles of matter(Figure 1.3), often based on elemental importance in the environment. b These cycles are summarized here and are the viewpoint of various reservoirs, such as oceans, sedin cles can be regarded from discussed further in later chapters. Global geochemical cy nents, and the atmosphere connected by conduits through which matter moves continuously. The movement of a specific kind of matter between two particular reservoirs may be reversible or irre- versible. The fluxes of movement for specific kinds of matter vary greatly as do the contents of such matter in a specified reservoir. Cycles of matter would occur even in the absence of life on Earth but are strongly influenced by life for ms, particularly plants and microorganisms. Organisms participate in biogeochemical cycles, which describe the circulation of matter, particularly plant and animal nutrients, through ecosystems. As part of the carbon cycle, atmospheric carbon in CO, is fixed as biomass, as part of the nitrogen cycle, atmospheric N, is fixed in organic matter. The reverse of these kinds of processes is mineralization, in which biologically bound elements are returned to inorganic states. Biogeochemical cycles are ultimately C 2000 CRC Press llcEnergy Flow and Photosynthesis in Living Systems Whereas materials are recycled through ecosystems, the flow of useful energy is essentially a one-way process. Incoming solar energy can be regarded as high-grade energy because it can cause useful reactions to occur, such as production of electricity in photovoltaic cells or photosynthesis in plants. As shown in Figure 1.2, solar energy captured by green plants energizes chlorophyll, which in turn powers metabolic processes that produce carbohydrates from water and carbon dioxide. These carbohydrates are repositories of stored chemical energy that can be converted to heat and work by metabolic reactions with oxygen in organisms. Ultimately, most of the energy is converted to low-grade heat, which is eventually reradiated away from Earth by infrared radiation. Energy Utilization During the last two centuries, the growing, enormous human impact on energy utilization has resulted in many of the environmental problems now facing humankind. This time period has seen a transition from the almost exclusive use of energy captured by photosynthesis and utilized as biomass (food to provide muscle power, wood for heat) to the use of fossil fuel petroleum, natural gas, and coal for about 90 percent, and nuclear energy for about 5 percent, of all energy employed commercially. Although fossil sources of energy have greatly exceeded the pessimistic estimates made during the “energy crisis” of the 1970s, they are limited and their pollution potential is high. Of particular importance is the fact that all fossil fuels produce carbon dioxide, a greenhouse gas. Therefore, it will be necessary to move toward the utilization of alternate renewable energy sources, including solar energy and biomass. The study of energy utilization is crucial in the environmental sciences, and it is discussed in greater detail in Chapter 18, “Industrial Ecology, Resources, and Energy.” 1.6. MATTER AND CYCLES OF MATTER Cycles of matter (Figure 1.3), often based on elemental cycles, are of utmost importance in the environment. 6 These cycles are summarized here and are discussed further in later chapters. Global geochemical cycles can be regarded from the viewpoint of various reservoirs, such as oceans, sediments, and the atmosphere, connected by conduits through which matter moves continuously. The movement of a specific kind of matter between two particular reservoirs may be reversible or irre￾versible. The fluxes of movement for specific kinds of matter vary greatly as do the contents of such matter in a specified reservoir. Cycles of matter would occur even in the absence of life on Earth but are strongly influenced by life forms, particularly plants and microorganisms. Organisms participate in biogeochemical cycles, which describe the circulation of matter, particularly plant and animal nutrients, through ecosystems. As part of the carbon cycle, atmospheric carbon in CO2 is fixed as biomass; as part of the nitrogen cycle, atmospheric N2 is fixed in organic matter. The reverse of these kinds of processes is mineralization, in which biologically bound elements are returned to inorganic states. Biogeochemical cycles are ultimately © 2000 CRC Press LLC