lecture 12 - Oxidative Stress Plant Tissues Section II...

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Oxidative Stress Plant Tissues
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Section II Diversity of vascular plants, photosynthesis and respiration Session 7 Bryophytes, Vascular Plants Session 8 Flowering Plants Session 9 Photosynthesis Session 10 C4 photosynthesis and Respiration Session 11 Mineral nutrition Session 12 Plant tissues
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Oxidative Stress and Vitamins C and E Oxygen is potentially toxic for living organisms. While it is great as an electron sink, in the presence of free radicals, it can form highly reactive, super-oxidizing molecules that destroy proteins, nucleic acids and lipids Plants have evolved systems to detoxify superoxides, using ascorbic acid (Vitamin C) and α -tocopherol (vitamin E) as key components. Humans cannot synthesize ascorbic acid and α -tocopherol and thus must obtain them in their diet in order to be able to detoxify reactive oxygen molecules. Hence they are called Vitamins. Oxidative stress leads to accelerated aging in plants and animals. In humans, it is associated with degenerative diseases
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Sources of Oxidative Stress Oxidizing pollutants (ozone, sulfur dioxide) Heavy metals (aluminum, nickel, selenium) Electron transport pathways Bright light in Photosystem II and I Low temperature Disease Drought and salt stress
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Reactive Oxygen Species = partially reduced forms of atmospheric O 2 Superoxide (O 2 - ) – from the addition of one electron; mostly from electron transport reactions Hydrogen peroxide (H 2 O 2 ) – from the addition of two electrons to O 2 Hydroxyl radical (OH - ) – from the addition of three electrons Singlet oxygen ( 1 O 2 ) – mostly from excited chlorophyll transferring energy to O 2 and exciting an electron in O 2 Ozone (O 3 ) – from air pollution
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The Path of Electron and Proton Flow NADP reductase H 2 O ⅟₂ O 2 + 2H + H + + NADP + NADPH 2 e - 4 photons 4 photons 2 H + CYT b 6 /f PC PSI PSII Fd in Photosynthetic Electron Transport PQ Normal Photosynthetic Electron Transport
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The Path of Electron and Proton Flow NADP reductase H 2 O ⅟₂ O 2 + 2H + H + + NADP + NADPH 2 e - 4 photons 4 photons 2 H + CYT b 6 /f PC PSI PSII Fd in Photosynthetic Electron Transport PQ Abnormal Photosynthetic Electron Transport Æ leading to production of reactive oxygen species O 2 O 2 - Zap
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The Water – Water – Cycle O 2 - Superoxide dismutase H 2 O 2 H 2 O SOD Ascorbic acid Ascorbate peroxidase Mono- Dehydro- ascorbate Reduced Ferredoxin Oxidized Ferredoxin from PSI One example of how Vitamin C detoxifies superoxides (in chloroplasts) From PSII
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The Anti-Oxidation Mechanism of Vitamin E OH - Ascorbic acid Mono- Dehydro- ascorbate NADP + (in Non-Photosynthetic Cells) Hydroxyl radical Lipid radical NADPH α -tocopherol (Vitamin E) tocopherol radical Normal Lipid Fatty acid (Vitamin C) Lipid breakdown and regeneration Lipid detoxification
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Some Other Important Antioxidants Carotenoids (e.g. beta carotene) – Convert the excited electron in P680 chlorophylls to a ground-state chlorophyll
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This note was uploaded on 06/21/2011 for the course BIO BIO251 taught by Professor Busch during the Fall '11 term at University of Toronto- Toronto.

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lecture 12 - Oxidative Stress Plant Tissues Section II...

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