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BCH_311 Chapter 23

BCH_311 Chapter 23 - Chapter 23 The Metabolism of Nitrogen...

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Chapter 23 – The Metabolism of Nitrogen Background Information Nitrogen is the most abundant gas in the atmosphere. It occupies about 78% of the atmospheric gases. However, gaseous nitrogen cannot be used by most organisms. Instead, bacteria living in root nodules of leguminous plants must first convert the gaseous form of nitrogen (N2) to ammonia (NH3). Ammonia is the incorporated into the amino acid, glutamate. Glutamate can be converted to praline, arginine, and other amino acids, via long and complex pathways. Amino acids then can contribute nitrogen to other nitrogenous compounds, such as bases of nucleic acids, coenzymes such as NAD+ and NADP+, hormones, and neurotransmitters. Toxicitiy of Nitrogen Excess ammonia is toxic. In mammals, nitrogen in the form of glutamate, glutamine, and alanine are transported to the liver. In the liver, amino groups enter the urea cycle pathway. The urea that forms is carried by the blood to the kidneys. The urea is excreted into the urine. In fish, excess nitrogen is excreted as ammonia. In birds, excess nitrogen is excreted as uric acid. What Processes Constitute Nitrogen Metabolism? 1. Anabolism and catabolism of amino acids, porphyrins, and nucleotides. 2. Nitrogen fixation : capture of atmospheric nitrogen, which is the ultimate source of this element in biomolecules. Nitrogen gas (N2) is converted to ammonia, then incorporated into organic compounds. 3. Nitrification : reduction of nitrate to ammonia 4. Denitrification : return of nitrogen to the atmosphere See figure 23.1 on page 630 Role of Bacteria in Nitrogen Flow in the Biosphere Nitrogenase-containing bacteria convert nitrogen gas to ammonia Nitrifying bacteria convert nitrite to nitrate Denitrifying bacteria convert nitrate to nitrogen gas Decomposing bacteria convert organic nitrogen (from death/decay) to nitrate
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Forms of Nitrogen Nitrogen gas (N2) Nitrate ion (NO3-) Nitrite ion (NO2-) Ammonia (NH3) Ammonium ion (NH4+) Amino group (NH3+) Nitrogen and Agriculture Crop production is enhanced with fertilizers, or by rotating crops with a leguminous crop, alfalfa. Alfalfa has nitrogen fixing bacteria in root nodules.
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