Part l bic solar energy as the driving force for this global process Atmospheric All living organisms also require a source of nitro- gen,which is necessary for the synthesis of amino acids nucleotides, and other compounds. Plants can generall Ise either ammonia or nitrate as their sole source of ni- trogen, but vertebrates must obtain nitrogen in the form ids or other organic compounds. Only organisms-the cyanobacteria and many species of soil cteria that live symbiotica the roots of so plants-are capable of converting (fixing)atmos- Ammonia pheric nitrogen(N2) into ammonia. Other bacteria(the nitrifying bacteria) oxidize ammonia to nitrites and ni- trates; yet others convert nitrate to N2. Thus, in addi Animals tion to the global carbon and oxygen cycle, a nitrogen bacteria cycle operates in the biosphere, turning over huge amounts of nitrogen(Fig. 2). The cycling of carbon, oxy- gen,and nitrogen, which ultimately involves all species depends on a proper balance between the activities of Amino Nitrates the producers (autotrophs) and consumers(het erotrophs) in our biosphere These cycles of matter are driven by an enormous flow of energy into and through the biosphere, begin- Plants ning with the capture of solar energy by photosynthetic organisms and use of this energy to generate energy rich carbohydrates and other organic nutrients; these FIGURE 2 Cycling of nitrogen in the biosphere. Gaseous nitrogen nutrients are then used as energy sources by het- (N2) makes up 80% of the earths atmosphere erotrophic organisms. In metabolic processes, and in all energy transformations, there is a loss of useful energy (free energy) and an inevitable increase in the amount through the biosphere: organisms cannot regenerate of unusable energy(heat and entropy). In contrast useful energy from energy dissipated as heat and to the cycling of matter, therefore, energy flows one way entropy. Carbon, oxygen, and nitrogen recycle continu- ously, but energy is constantly transformed into unus. Metabolism, the sum of all the chemical transfo ations taking place in a cell or organism, occurs through a series of enzyme-catalyzed reactions that co stitute metabo lic pathways. Each of the consecutive steps in a metabolic pathway brings about a specific, small chemical change, usually the removal, transfer, ion of a particular atom or functional group precursor is converted into a product through a series of metabolic intermediates called metabolites. The term intermediary metabolism is often applied to the combined activities of all the metabolic pathways that interconvert precursors, metabolites, and products of Photosynthetic Heterotrophs low molecular weight(generally, M<1,000 autotrophs Catabolism is the degradative phase of metabolism in which organic nutrient molecules(carbohydrates fats, and proteins) are converted into smaller, simpler end products(such as lactic acid, CO2, NH3). Catabolic pathways release energy, of which is conserved in FIGURE 1 Cycling of carbon dioxide and oxygen between the auto. the formation of ATP and reduced electron carriers trophic (photosynthetic)and heterotrophic domains in the biosphere. (NADH, NADPH, and FADH2); the rest is lost as heat The flow of mass through this cycle is enormous about 4 x 10 In anabolism, also called biosynthesis, small, simple ric tons of carbon are tumed over in the biosphere annually. precursors are built up into larger and more cosolar energy as the driving force for this global process (Fig. 1). All living organisms also require a source of nitro￾gen, which is necessary for the synthesis of amino acids, nucleotides, and other compounds. Plants can generally use either ammonia or nitrate as their sole source of ni￾trogen, but vertebrates must obtain nitrogen in the form of amino acids or other organic compounds. Only a few organisms—the cyanobacteria and many species of soil bacteria that live symbiotically on the roots of some plants—are capable of converting (“fixing”) atmos￾pheric nitrogen (N2) into ammonia. Other bacteria (the nitrifying bacteria) oxidize ammonia to nitrites and ni￾trates; yet others convert nitrate to N2. Thus, in addi￾tion to the global carbon and oxygen cycle, a nitrogen cycle operates in the biosphere, turning over huge amounts of nitrogen (Fig. 2). The cycling of carbon, oxy￾gen, and nitrogen, which ultimately involves all species, depends on a proper balance between the activities of the producers (autotrophs) and consumers (het￾erotrophs) in our biosphere. These cycles of matter are driven by an enormous flow of energy into and through the biosphere, begin￾ning with the capture of solar energy by photosynthetic organisms and use of this energy to generate energy￾rich carbohydrates and other organic nutrients; these nutrients are then used as energy sources by het￾erotrophic organisms. In metabolic processes, and in all energy transformations, there is a loss of useful energy (free energy) and an inevitable increase in the amount of unusable energy (heat and entropy). In contrast to the cycling of matter, therefore, energy flows one way through the biosphere; organisms cannot regenerate useful energy from energy dissipated as heat and entropy. Carbon, oxygen, and nitrogen recycle continu￾ously, but energy is constantly transformed into unus￾able forms such as heat. Metabolism, the sum of all the chemical transfor￾mations taking place in a cell or organism, occurs through a series of enzyme-catalyzed reactions that con￾stitute metabolic pathways. Each of the consecutive steps in a metabolic pathway brings about a specific, small chemical change, usually the removal, transfer, or addition of a particular atom or functional group. The precursor is converted into a product through a series of metabolic intermediates called metabolites. The term intermediary metabolism is often applied to the combined activities of all the metabolic pathways that interconvert precursors, metabolites, and products of low molecular weight (generally, Mr
1,000). Catabolism is the degradative phase of metabolism in which organic nutrient molecules (carbohydrates, fats, and proteins) are converted into smaller, simpler end products (such as lactic acid, CO2, NH3). Catabolic pathways release energy, some of which is conserved in the formation of ATP and reduced electron carriers (NADH, NADPH, and FADH2); the rest is lost as heat. In anabolism, also called biosynthesis, small, simple precursors are built up into larger and more complex 482 Part II Bioenergetics and Metabolism Heterotrophs O2 H2O Photosynthetic autotrophs Organic products CO2 FIGURE 1 Cycling of carbon dioxide and oxygen between the auto￾trophic (photosynthetic) and heterotrophic domains in the biosphere. The flow of mass through this cycle is enormous; about 4 1011 met￾ric tons of carbon are turned over in the biosphere annually. Plants Nitrates, nitrites Nitrifying bacteria Denitrifying bacteria Animals Amino acids Ammonia Nitrogen￾fixing bacteria Atmospheric N2 FIGURE 2 Cycling of nitrogen in the biosphere. Gaseous nitrogen (N2) makes up 80% of the earth’s atmosphere.