THE LIGHT DEPENDENT REACTION

OXIDATION AND REDUCTION Oxidation Is a Loss of electrons (OIL) Reduction Is a Gain of electrons (RIG) © 2010 Paul Billiet ODWSODWS

Natural Electron ACCEPTORS Nicotinamide Adenine Dinucleotide Phosphate (NADP) used in photosynthesis in chloroplasts NADP + + 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 Fe 3+ + e - Fe 2+ NB The iron atom in the haem group of haemoglobin does not go through a redox reaction Haemoglobin is oxygenated or deoxygenated Reduction Oxidation Reduction Oxidation © 2010 Paul Billiet ODWSODWS

CLASSIFYING ORGANISMS ACCORDING TO THEIR CARBON SOURCE AND ENERGY SUPPLIES © 2010 Paul Billiet ODWSODWS

The origin of oxygen in photosynthesis CO 2 or H 2 O? Van Neil 1932 Comparing the biochemistry of autotrophs Photosynthetic sulphur bacteria use H 2 S as their source of hydrogen CO 2 + 2H 2 S  (CH 2 O) + H 2 O + 2S  This suggested that in green plants the oxygen originates from the water molecule CO 2 + 2H 2 O  (CH 2 O) + O 2  + H 2 O Ruben 1941 Confirmed this hypothesis using the heavy isotope 18 O and mass spectrometry © 2010 Paul Billiet ODWSODWS

Using chloroplasts in vitro Hill 1937 Studying redox reactions in photosynthesis using artificial electron acceptors Oxidised electron acceptor Reduced electron acceptor H2OH2O CO 2 absent No (CH 2 O) produced O 2 produced LIGHT CHLOROPLAST © 2010 Paul Billiet ODWSODWS

Oxidised electron acceptor No reduction of electron acceptor DARK CHLOROPLAST H2OH2O Using chloroplasts in vitro © 2010 Paul Billiet ODWSODWS

The Hill reaction using natural electron acceptors Arnon 1954 ADP +Pi NADP ATP NADPH + H + H2OH2O CO 2 absent No (CH 2 O) produced O 2 produced CHLOROPLAST LIGHT © 2010 Paul Billiet ODWSODWS

Then …… Arnon had effectively separated the light dependent reaction, which produces ATP, NADPH + H + and oxygen, from the light independent reaction, which produces sugars ATP NADPH+H + ADP + Pi NADP (CH 2 O) produced CHLOROPLAST DARK Add CO 2 © 2010 Paul Billiet ODWSODWS

CHLOROPHYLL AND PHOTOSYNTHESIS Pigments in the leaves of green plants and algae © 2010 Paul Billiet ODWSODWS

Pigments underwater © 2010 Paul Billiet ODWSODWS

The fluorescence of chlorophyll Pure chlorophyll + light  Red fluorescence Chlorophyll in chloroplasts + light  Splits water, synthesises ATP and NADPH + H + © 2010 Paul Billiet ODWSODWS

Fluorescence The excitement of an electron to a high energy level by the action of light energy Followed by the release of that energy as light again as the electron falls back to its former low energy level © 2010 Paul Billiet ODWSODWS

Chlorophylls Absorption spectra of the main photosynthetic pigments Chlorophyll a molecule

OXIDATION AND REDUCTION Something must be happening in the chloroplast to capture these electrons and use their energy Free electrons can lead to OXIDATION AND REDUCTION reactions Remember Oxidation Is a Loss of electrons (OIL) Reduction Is a Gain of electrons (RIG) © 2010 Paul Billiet ODWSODWS

Oxidation & reduction in photosynthesis When compounds are oxidised energy is released If this release of energy is COUPLED to biological reactions then WORK can be done Similarly when compounds are reduced energy has to be put into the system In photosynthesis the source of electrons for reducing CO 2  CH 2 O is water and the source of energy is light © 2010 Paul Billiet ODWSODWS

The chloroplast outer membrane inner membrane Chloroplast envelope Starch grains Grana Frets Thylakoid membrane Stroma © 2010 Paul Billiet ODWSODWS

X Open University S Hurry (1965) Murray X Open University S Hurry (1965) Murray X Open University S Hurry (1965) Murray

CHLOROPHYLL IN THE CHLOROPLAST Pigment molecules are located on the thylakoid membranes The pigment molecules are arranged in an antenna complex Light strikes the antenna complex and it is channelled towards the reaction centre The electrons are excited by the light energy in the reaction centre The electrons are picked up by electron acceptors (1 photon of light = 1 electron released) © 2010 Paul Billiet ODWSODWS

Photolysis The electrons that are lost are replaced by splitting water 2H 2 O  4H + + 4e - + O 2  So 1 molecule of oxygen released requires 4 photons of light © 2010 Paul Billiet ODWSODWS

The photosystems Two types of pigment systems have been found PHOTOSYSTEM I Mainly chlorophyll a PHOTOSYSTEM II Chlorophyll b, some chlorophyll a plus other pigments © 2010 Paul Billiet ODWSODWS

The photosystems These 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 chain Reduction of NADP to NADPH + H + (NADP is therefore the final electron acceptor) © 2010 Paul Billiet ODWSODWS

REACTION PATHWAY More +ve REDOX POTENTIAL More -ve NADPH + H + ATPADP e-e- e-e- NADP Ferredoxin NADPH reductase Plastoquinone Plastocyanin Cytochrome b 6 – f complex PHOTOSYSTEM I PHOTOSYSTEM II H 2 O O 2 + 4H + Non-cyclic photophosphorylation Cyclic photophosphorylation © 2010 Paul Billiet ODWSODWS