THE LIGHT DEPENDENT REACTION. OXIDATION AND REDUCTION Oxidation Is a Loss of electrons (OIL)...

THE LIGHT DEPENDENT REACTION

OXIDATION AND REDUCTION

Oxidation 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 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

Fe3+ + e- Fe2+

NB The iron atom in the haem group of haemoglobin does not go through a redox reactionHaemoglobin is oxygenated or deoxygenated

Reduction

Oxidation

Reduction

Oxidation


CLASSIFYING ORGANISMS ACCORDING TO THEIR CARBON SOURCE AND ENERGY SUPPLIES

Type of Organism CarbonSource

EnergySource

Electron Donors Examples

Photolithotrophs CO2 LightInorganic compounds

(H2O, H2S, S)

Green plants,photosynthetic protoctists,blue-greens,photosynthetic bacteria

Photoorganotrophs Organiccompounds Light Organic compounds

Non-sulphur purplebacteria

Chemolithotrophs CO2

Redoxreactions

Inorganic compounds(H2, S, H2S, Fe2+, NH3)

Hydrogen, sulphur, ironand denitrifying bacteria

ChemoorganotrophsOrganic

compoundsRedox

reactions

Organic compounds(e.g. Glucose)

Animals, fungi, non-photosynthetic protoctists,saprophytic and parasiticbacteria


The origin of oxygen in photosynthesis CO2 or H2O?

Van Neil 1932 Comparing the biochemistry of autotrophs

Photosynthetic sulphur bacteria use H2S as their source of hydrogenCO2 + 2H2S (CH2O) + H2O + 2S

This suggested that in green plants the oxygen originates from the water molecule

CO2 + 2H2O (CH2O) + O2 + H2ORuben 1941 Confirmed this hypothesis using

the heavy isotope 18O and mass spectrometry© 2010 Paul Billiet ODWS

Using chloroplasts in vitro

Hill 1937 Studying redox reactions in photosynthesis using artificial electron acceptors

Oxidised electron acceptor

Reduced electron acceptor

H2O

CO2 absent

No (CH2O) produced

O2 produced

LIGHT

CHLOROPLAST


Oxidised electron acceptor

No reduction of electron acceptor

DARK

CHLOROPLASTH2O

Using chloroplasts in vitro


The Hill reaction using natural electron

acceptors Arnon 1954

ADP +PiNADP

ATPNADPH + H+

H2O

CO2 absent No (CH2O) produced

O2 produced

CHLOROPLAST

LIGHT


Then ……

Arnon had effectively separated the light dependent reaction, which produces ATP, NADPH + H+ and oxygen, from the light independent reaction, which produces sugars

ATPNADPH+H+

ADP + PiNADP

(CH2O) produced

CHLOROPLAST

DARK

Add CO2


CHLOROPHYLL AND PHOTOSYNTHESIS

Pigments in the leaves of green plants and algae

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 underwaterLight received from

the sunSpace 200 to 4000 nm

Atmosphere

Ground 300 to 1000 nm

Light used by greenplants

Photosynthesis 400 to 700 nm

Underwater blue light penetrates thedeepest as it has most energy. Greenlight next finally red light penetrates least.The distribution of algae with differentphotosynthetic pigments is related to this.

Redalgae

Brownalgae

Greenalgae


The fluorescence of chlorophyll

Pure chlorophyll + light Red fluorescence

Chlorophyll in chloroplasts + light Splits water, synthesises ATP and NADPH + H+


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


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)


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 CO2 CH2O is water and the source of energy is light


The chloroplast

outer membrane

inner membrane

Chloroplast envelope

Starch grains

Grana

Frets

Thylakoid membrane

Stroma


X 22 000 Open University S Hurry (1965) Murray X 80 000 Open University S Hurry (1965) Murray

X 33 300 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)


Photolysis

The electrons that are lost are replaced by splitting water

2H2O 4H+ + 4e- + O2

So 1 molecule of oxygen released requires 4 photons of light


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


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)


REACTION PATHWAY

Mo

re +ve

RE

DO

X P

OT

EN

TIA

L

Mo

re -ve

NADPH + H+

ATPADP

e-

e-

NADP

Ferredoxin

NADPH reductase

Plastoquinone

Plastocyanin

Cytochrome b6 – f complex

PHOTOSYSTEM IPHOTOSYSTEM II

H2O O2 + 4H+

Non-cyclic photophosphorylation

Cyclic photophosphorylation


THE LIGHT DEPENDENT REACTION. OXIDATION AND REDUCTION Oxidation Is a Loss of electrons (OIL)...

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