James N. Siedow
David A. Day

Chapters 12 and 13 have described how plants use light energy to assimilate carbon into sugars and starch and how these molecules are subsequently broken down and converted into organic acids and other compounds. In this chapter we examine aerobic respiration— the further oxidation of these compounds to CO₂ and H₂O in the mitochondrion—and review the mechanisms by which the energy released during respiration is conserved as ATP, a process called oxidative phosphorylation. We examine how plants can minimize ATP production while maintaining respiration rates, a mitochondrial attribute that may affect plant responses to environmental stress. We also describe how mitochondria and other cellular compartments interact by means of substrate shuttles across the inner mitochondrial membrane.
      In addition to aerobic respiration, another respiratory process takes place in the leaves of many plants. This novel process, photorespiration, is the light-dependent release of CO₂ and uptake of O₂. The O₂ uptake occurs in the chloroplast as a result of the oxygenase reaction of Rubisco, which leads to production of phosphoglycolate. Metabolism of this compound involves a complex interaction among chloroplasts, peroxisomes, and mitochondria and leads to release of CO₂. Although photorespiration in effect drains carbon from plants and adversely affects growth, the process seems to be an unavoidable side reaction of CO₂ fixation in most plants. Some plants, however, have evolved special anatomical and biochemical features that minimize the oxygenase reaction. These plants, known as C₄ plants and Crassulacean acid metabolism (CAM) plants (see Chapter 12), demonstrate very low rates of photorespiration.