8885dc197063/1/041:59 PM Page706mac76mac76:385reb: Chapter 19 Oxidative Phosphorylation and Photophosphorylation BOX 19-1 THE WORLD OF BIOCHEMISTRY Hot, Stinking Plants and Alternative Respiratory Pathways Many flowering plants attract insect pollinators by re- leasing odorant molecules that mimic an insect,s nat ural food sources or potential egg-laying sites. Plants pollinated by flies or beetles that normally feed on or lay their eggs in dung or carrion sometimes use foul smelling compounds to attract these insects One family of stinking plants is the araceae, which includes philodendrons, arum lilies, and skunk cab- bages. These plants have tiny flowers densely packed on an erect structure, the spadix, surrounded by a modified leaf, the spathe. The spadix releases odors FIGURE 1 Eastern skunk cabbage of rotting flesh or dung. Before pollination the spadix also heats up, in some species to as much as 20 to 40C above the ambient temperature. Heat produc- ATP is instead released as heat. Plant mitochondria tion (thermogenesis) helps evaporate odorant mole- also have an alternative NADH dehydrogenase, insen cules for better dispersal, and because rotting flesh sitive to the Complex I inhibitor rotenone(see Table and dung are usually warm from the hyperactive me- 19-4), that transfers electrons from NADH in the ma- tabolism of scavenging microbes, the heat itself might trix directly to ubiquinone, bypassing Complex I and also attract insects. In the case of the eastern skunk its associated proton pumping And plant mitochon- cabbage(Fig. 1), which flowers in late winter or early dria have yet another NADH dehydrogenase, on the spring when snow still covers the ground, thermogen- external face of the inner membrane, that transfers sis allows the spadix to grow up through the snow. electrons from NADPh or NADH in the intermem How does a skunk cabbage heat its spadix? The brane space to ubiquinone, again bypassing Complex mitochondria of plants, fungi, and unicellular eukary- I. Thus when electrons enter the alternative respira- otes have electron-transfer systems that are essen- tory pathway through the rotenone-insensitive NADH tially the same as those in animals, but they also dehydrogenase, the external Nadh dehydrogenase have an alternative respiratory pathway. A cyanide- or succinate dehydrogenase(Complex D, and pass to resistant QH2 oxidase transfers electrons from the O2 via the cyanide-resistant alternative oxidase, en- ubiquinone pool directly to oxygen, bypassing the two ergy is not conserved as AtP but is released as heat proton-translocating steps of Complexes Ill and Iv A skunk cabbage can use the heat to melt snow, pro- (Fig. 2). Energy that might have been conserved as duce a foul stench, or attract beetles or flies External NAD(P)H Intermembrane NAD(P)*/dehydrogenase NAD(P)H Q NADnative NADH NAD+ Heat genase Matrix FIGURE 2 Electron carriers of the inner membrane of plant mitochondria. Electrons can flow through Complexes I, Ill, and IV, as in animal mitochondria, or through plant-specific alterna- ve carriers by the paths shown with blue arrows706 Chapter 19 Oxidative Phosphorylation and Photophosphorylation BOX 19–1 THE WORLD OF BIOCHEMISTRY Hot, Stinking Plants and Alternative Respiratory Pathways Many flowering plants attract insect pollinators by re￾leasing odorant molecules that mimic an insect’s nat￾ural food sources or potential egg-laying sites. Plants pollinated by flies or beetles that normally feed on or lay their eggs in dung or carrion sometimes use foul￾smelling compounds to attract these insects. One family of stinking plants is the Araceae, which includes philodendrons, arum lilies, and skunk cab￾bages. These plants have tiny flowers densely packed on an erect structure, the spadix, surrounded by a modified leaf, the spathe. The spadix releases odors of rotting flesh or dung. Before pollination the spadix also heats up, in some species to as much as 20 to 40
C above the ambient temperature. Heat produc￾tion (thermogenesis) helps evaporate odorant mole￾cules for better dispersal, and because rotting flesh and dung are usually warm from the hyperactive me￾tabolism of scavenging microbes, the heat itself might also attract insects. In the case of the eastern skunk cabbage (Fig. 1), which flowers in late winter or early spring when snow still covers the ground, thermogen￾esis allows the spadix to grow up through the snow. How does a skunk cabbage heat its spadix? The mitochondria of plants, fungi, and unicellular eukary￾otes have electron-transfer systems that are essen￾tially the same as those in animals, but they also have an alternative respiratory pathway. A cyanide￾resistant QH2 oxidase transfers electrons from the ubiquinone pool directly to oxygen, bypassing the two proton-translocating steps of Complexes III and IV (Fig. 2). Energy that might have been conserved as ATP is instead released as heat. Plant mitochondria also have an alternative NADH dehydrogenase, insen￾sitive to the Complex I inhibitor rotenone (see Table 19–4), that transfers electrons from NADH in the ma￾trix directly to ubiquinone, bypassing Complex I and its associated proton pumping. And plant mitochon￾dria have yet another NADH dehydrogenase, on the external face of the inner membrane, that transfers electrons from NADPH or NADH in the intermem￾brane space to ubiquinone, again bypassing Complex I. Thus when electrons enter the alternative respira￾tory pathway through the rotenone-insensitive NADH dehydrogenase, the external NADH dehydrogenase, or succinate dehydrogenase (Complex II), and pass to O2 via the cyanide-resistant alternative oxidase, en￾ergy is not conserved as ATP but is released as heat. A skunk cabbage can use the heat to melt snow, pro￾duce a foul stench, or attract beetles or flies. FIGURE 1 Eastern skunk cabbage. FIGURE 2 Electron carriers of the inner membrane of plant mitochondria. Electrons can flow through Complexes I, III, and IV, as in animal mitochondria, or through plant-specific alterna￾tive carriers by the paths shown with blue arrows. Heat I IV Intermembrane space Q Matrix Cyt c NAD+ NAD(P)+ External NAD(P)H dehydrogenase Alternative oxidase Alternative NADH dehydrogenase III NADH H2O NAD(P)H 1 –2 O2 8885d_c19_706 3/1/04 1:59 PM Page 706 mac76 mac76:385_reb: