LIGHTDEPREACTN.ppt
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Transcript of LIGHTDEPREACTN.ppt
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THE LIGHT DEPENDENT REACTION
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OXIDATION AND REDUCTIONOxidation Is a Loss of electrons (OIL)Reduction Is a Gain of electrons (RIG) 2010 Paul Billiet ODWS
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Natural Electron ACCEPTORSNicotinamide Adenine Dinucleotide Phosphate (NADP) used in photosynthesis in chloroplastsNADP+ + 2H+ + 2e- NADPH + H+Ferredoxin the most difficult to reduce (and most easily oxidised)Cytochromes Conjugate proteins which contain a haem group.The iron atom undergoes redox reactions Fe3+ + e- Fe2+ NB The iron atom in the haem group of haemoglobin does not go through a redox reaction Haemoglobin is oxygenated or deoxygenated
2010 Paul Billiet ODWS
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CLASSIFYING ORGANISMS ACCORDING TO THEIR CARBON SOURCE AND ENERGY SUPPLIES 2010 Paul Billiet ODWS
Type of Organism
Carbon Source
Energy Source
Electron Donors
Examples
Photolithotrophs
CO2
Light
Inorganic compounds
(H2O, H2S, S)
Green plants, photosynthetic protoctists, blue-greens, photosynthetic bacteria
Photoorganotrophs
Organic compounds
Light
Organic compounds
Non-sulphur purple bacteria
Chemolithotrophs
CO2
Redox reactions
Inorganic compounds
(H2, S, H2S, Fe2+, NH3)
Hydrogen, sulphur, iron and denitrifying bacteria
Chemoorganotrophs
Organic compounds
Redox reactions
Organic compounds
(e.g. Glucose)
Animals, fungi, non-photosynthetic protoctists, saprophytic and parasitic bacteria
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The origin of oxygen in photosynthesis CO2 or H2O?Van Neil 1932 Comparing the biochemistry of autotrophsPhotosynthetic sulphur bacteria use H2S as their source of hydrogen CO2 + 2H2S (CH2O) + H2O + 2SThis suggested that in green plants the oxygen originates from the water moleculeCO2 + 2H2O (CH2O) + O2 + H2ORuben 1941 Confirmed this hypothesis using the heavy isotope 18O and mass spectrometry 2010 Paul Billiet ODWS
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Using chloroplasts in vitroHill 1937 Studying redox reactions in photosynthesis using artificial electron acceptorsOxidised electron acceptorReduced electron acceptorH2OCO2 absentNo (CH2O) producedO2 producedLIGHT 2010 Paul Billiet ODWS
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Oxidised electron acceptorNo reduction of electron acceptor DARK H2OUsing chloroplasts in vitro 2010 Paul Billiet ODWS
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The Hill reaction using natural electron acceptors Arnon 1954 ADP +PiNADPATPNADPH + H+H2OCO2 absentNo (CH2O) producedO2 producedLIGHT 2010 Paul Billiet ODWS
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Then Arnon had effectively separated the light dependent reaction, which produces ATP, NADPH + H+ and oxygen, from the light independent reaction, which produces sugarsATPNADPH+H+ADP + PiNADP(CH2O) produced DARK Add CO2 2010 Paul Billiet ODWS
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CHLOROPHYLL AND PHOTOSYNTHESISPigments in the leaves of green plants and algae 2010 Paul Billiet ODWS
PIGMENT
COLOUR
ABSORPTION PEAK / nm
Chlorophyll a
Blue-green
430 and 660
Chlorophyll b
Yellow-Green
455 and 640
Phycocyanins
Blue-Grey
560 to 660
Phycoerythrins
Red
550 to 570
Carotenoids
Yellow- Orange
430 to 570
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Pigments underwater 2010 Paul Billiet ODWS
Light received from the sun
Space
200 to 4000 nm
Atmosphere
Ground
300 to 1000 nm
Light used by green plants
Photosynthesis
400 to 700 nm
Underwater blue light penetrates the deepest as it has most energy. Green light next finally red light penetrates least. The distribution of algae with different photosynthetic pigments is related to this.
Red algae
Brown algae
Green algae
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The fluorescence of chlorophyllPure chlorophyll + light Red fluorescenceChlorophyll in chloroplasts + light Splits water, synthesises ATP and NADPH + H+ 2010 Paul Billiet ODWS
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FluorescenceThe excitement of an electron to a high energy level by the action of light energyFollowed by the release of that energy as light again as the electron falls back to its former low energy level 2010 Paul Billiet ODWS
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ChlorophyllsAbsorption spectra of the main photosynthetic pigments
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OXIDATION AND REDUCTIONSomething must be happening in the chloroplast to capture these electrons and use their energyFree electrons can lead to OXIDATION AND REDUCTION reactionsRememberOxidation Is a Loss of electrons (OIL)Reduction Is a Gain of electrons (RIG) 2010 Paul Billiet ODWS
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Oxidation & reduction in photosynthesisWhen compounds are oxidised energy is releasedIf this release of energy is COUPLED to biological reactions then WORK can be doneSimilarly when compounds are reduced energy has to be put into the systemIn photosynthesis the source of electrons for reducing CO2 CH2O is water and the source of energy is light 2010 Paul Billiet ODWS
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The chloroplastouter membraneinner membraneChloroplast envelopeStarch grainsGranaFretsThylakoid membraneStroma 2010 Paul Billiet ODWS
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CHLOROPHYLL IN THE CHLOROPLASTPigment molecules are located on the thylakoid membranesThe pigment molecules are arranged in an antenna complexLight strikes the antenna complex and it is channelled towards the reaction centreThe electrons are excited by the light energy in the reaction centreThe electrons are picked up by electron acceptors (1 photon of light = 1 electron released) 2010 Paul Billiet ODWS
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PhotolysisThe electrons that are lost are replaced by splitting water2H2O 4H+ + 4e- + O2So 1 molecule of oxygen released requires 4 photons of light 2010 Paul Billiet ODWS
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The photosystemsTwo types of pigment systems have been foundPHOTOSYSTEM I Mainly chlorophyll aPHOTOSYSTEM II Chlorophyll b, some chlorophyll a plus other pigments 2010 Paul Billiet ODWS
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The photosystemsThese photosystems bring about three reactions:Photolysis of water to provide electrons (e-) and protons (H+)Photophosphorylation to produce ATP from coupled redox reactions in an electron transport chainReduction of NADP to NADPH + H+ (NADP is therefore the final electron acceptor) 2010 Paul Billiet ODWS
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NADPH + H+ATPADPe-e-NADPNon-cyclic photophosphorylationCyclic photophosphorylation 2010 Paul Billiet ODWS