Chapter 12: Respiration and Lipid Metabolism

The chapter explores how transamination reactions subsequently distribute nitrogen throughout the plant to synthesize diverse amino acids essential for protein synthesis and cellular function. Sulfur assimilation follows parallel pathways where sulfate is activated through adenosine phosphosulfate formation, reduced stepwise to sulfide, and incorporated into cysteine, serving as the precursor for methionine and other sulfur-containing compounds critical for enzyme function and plant defense mechanisms. Phosphorus metabolism focuses on the uptake of inorganic phosphate and its incorporation into high-energy compounds like adenosine triphosphate, nucleic acids for genetic material, and phospholipids for membrane structure. The chapter emphasizes the sophisticated regulatory mechanisms governing these assimilation pathways, including light-dependent regulation, feedback inhibition by end products, and transcriptional control that coordinates nutrient assimilation with photosynthetic carbon fixation and overall plant energy status. Special attention is given to symbiotic nitrogen fixation in leguminous plants, where rhizobial bacteria within specialized root nodules use the nitrogenase enzyme complex to convert atmospheric nitrogen gas into ammonia under carefully maintained anaerobic conditions facilitated by leghemoglobin, demonstrating the remarkable evolutionary adaptations that allow plants to access atmospheric nitrogen reserves.