THE LIGHT DEPENDENT REACTION

OXIDATION AND REDUCTIONOxidation Is a Loss of electrons (OIL)Reduction Is a Gain of electrons (RIG) 2010 Paul Billiet ODWS

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

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

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

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

Oxidised electron acceptorNo reduction of electron acceptor DARK H2OUsing chloroplasts in vitro 2010 Paul Billiet ODWS

The Hill reaction using natural electron acceptors Arnon 1954 ADP +PiNADPATPNADPH + H+H2OCO2 absentNo (CH2O) producedO2 producedLIGHT 2010 Paul Billiet ODWS

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

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

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

The fluorescence of chlorophyllPure chlorophyll + light Red fluorescenceChlorophyll in chloroplasts + light Splits water, synthesises ATP and NADPH + H+ 2010 Paul Billiet ODWS

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

ChlorophyllsAbsorption spectra of the main photosynthetic pigments

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

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

The chloroplastouter membraneinner membraneChloroplast envelopeStarch grainsGranaFretsThylakoid membraneStroma 2010 Paul Billiet ODWS

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

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

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

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

NADPH + H+ATPADPe-e-NADPNon-cyclic photophosphorylationCyclic photophosphorylation 2010 Paul Billiet ODWS