Mitochondrial respiration of the photosynthesizing cell

Current notions on respiration of photosynthesizing cells are reviewed. Over the past three decades, the modern methods based on isotope techniques and reverse and molecular genetics provided convincing evidence that mitochondrial respiration is functional in the light and contributes to the creation of optimal conditions for photosynthesis and for protection of cells from photodegradation. Novel data are presented on the substrates that are used for respiration in the light. Individual respiration steps are considered in the context of their possible role in photosynthesizing cells. The mechanisms and carriers mediating the export of reducing equivalents from chloroplasts for their subsequent oxidation in the mitochondrial electron-transport chain are discussed. The regulation of nonphosphorylating (unrelated to energy generation) electron transport pathways mediated by alternative oxidase and alternative type II NADPH-dehydrogenases, as well as the role of uncoupling proteins in plant mitochondria, are analyzed. These components were shown to play a significant role in NAD(P)H oxidation for maintaining the redox balance in mitochondria and whole green cells. A generalized scheme of biochemical interactions between organelles-chloroplasts, mitochondria, and peroxisomes-is presented. The directions for future research are outlined.