Chloroplasts and mitochondria are traditionally considered to be autonomous organelles but they are not as independent as they were once thought to be. Mitochondrial metabolism, particularly the bioenergetic reactions of oxidative electron transport and phosphorylation, continue to be active in the light and are essential for sustaining photosynthetic carbon assimilation. The marked and mutually beneficial interaction between mitochondria and chloroplasts is intriguing. The key compartments within plant cells, including not only mitochondria and chloroplasts but also the peroxisomes and cytosol, appear to be in a delicate metabolic equilibrium. Disturbance of any of these compartments perturbs the metabolism of whole cell. Nevertheless, mitochondria appear to be the key players because they function during both photorespiration and dark respiration.