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HIGHER PLANTS ARE AUTOTROPHIC ORGANISMS that can synthesize their organic molecular components out of inorganic nutrients obtained from their surroundings. For many mineral nutrients, this process involves absorption from the soil by the roots (see Chapter 5) and incorporation into the organic compounds that are essential for growth and development. This incorporation of mineral nutrients into organic substances such as pigments, enzyme cofactors, lipids, nucleic acids, and amino acids is termed nutrient assimilation | Chapter 2 Assimilation of Mineral Nutrients HIGHER PLANTS ARE AUTOTROPHIC ORGANISMS that can synthesize their organic molecular components out of inorganic nutrients obtained from their surroundings. For many mineral nutrients this process involves absorption from the soil by the roots see Chapter 5 and incorporation into the organic compounds that are essential for growth and development. This incorporation of mineral nutrients into organic substances such as pigments enzyme cofactors lipids nucleic acids and amino acids is termed nutrient assimilation. Assimilation of some nutrients particularly nitrogen and sulfur requires a complex series of biochemical reactions that are among the most energy-requiring reactions in living organisms In nitrate NO3 assimilation the nitrogen in NO3 is converted to a higher-energy form in nitrite NO3 then to a yet higher-energy form in ammonium NH4 and finally into the amide nitrogen of glutamine. This process consumes the equivalent of 12 ATPs per nitrogen Bloom et al. 1992 . Plants such as legumes form symbiotic relationships with nitrogen-fixing bacteria to convert molecular nitrogen N2 into ammonia NH3 . Ammonia NH3 is the first stable product of natural fixation at physiological pH however ammonia is protonated to form the ammonium ion NH4 . The process of biological nitrogen fixation together with the subsequent assimilation of NH3 into an amino acid consumes about 16 ATPs per nitrogen Pate and Layzell 1990 Vande Broek and Vanderleyden 1995 . The assimilation of sulfate SO42- into the amino acid cysteine via the two pathways found in plants consumes about 14 ATPs Hell 1997 . For some perspective on the enormous energies involved consider that if these reactions run rapidly in reverse say from NH4NO3 ammonium nitrate to N2 they become explosive liberating vast amounts of energy as motion heat and light. Nearly all explosives are based on the rapid oxidation of nitrogen or sulfur compounds. 260 Chapter 12 Assimilation of other
