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Chapter 14

Respiration and
Photorespiration

 

Aerobic respiration involves the controlled oxidation of reduced organic compounds to CO2 and H2O. Much of the free energy released during respiration is conserved in the form of ATP. In glycolysis, the first stage of respiration, carbohydrates are oxidized to organic acids in the cytosol. The organic acids produced during glycolysis are completely oxidized to CO2 in the mitochondrial matrix via the citric acid cycle. The electrons released during the operation of the citric acid cycle are transferred through a series of multiprotein complexes located in the inner mitochondrial membrane, ultimately reducing O2 to H2O. The free energy released during mitochondrial electron transfer is used to generate a proton electrochemical gradient across the inner membrane. The energy available in the proton gradient is subsequently used by another protein complex, ATP synthase, to synthesize ATP from ADP and Pi. Mitochondrial respiration is regulated by the availability of ADP and Pi and by the presence of additional electron transfer complexes that allow respiration to proceed without forming a proton gradient. Plant mitochondria participate in several metabolic processes besides respiration, including providing reducing equivalents to other cellular compartments and carbon skeletons for amino acid biosynthesis.
      Plant mitochondria also participate in the biosynthesis of sugars from lipids in some germinating seeds and in the decarboxylation reactions associated with photosynthesis in some plants having C4 and CAM pathways. The movement of metabolites into and out of mitochondria requires specific transporters in the inner mitochondrial membrane, some of which are regulated by the proton gradient.
      Photorespiration involves the light-dependent uptake of O2 and evolution of CO2 during photosynthesis in green plant tissues. The first step in photorespiration is associated with the oxygenase activity of the photosynthetic enzyme Rubisco. Phospho-glycolate formed during the oxygenase reaction is metabolized through the photorespiratory carbon cycle to save 75% of the carbon in the form of phosphoglycerate; the remaining 25% is lost as CO2. The reactions of the photorespiratory carbon cycle occur in three organelles: chloroplasts, peroxisomes, and mitochondria. The loss of CO2 during photorespiration can represent an appreciable percentage of the carbon fixed during photosynthesis, decreasing the overall efficiency of photosynthesis. Photorespiration reflects the evolutionary origin of Rubisco in an anaerobic environment and may influence the competitiveness of some plants in response to future changes in atmospheric CO2 concentrations.


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