PHOTOSYNTHESIS

Dr. Riddhi Datta

Assistant Professor

PG Department of Botany

Barasat Government College

For UG students

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Dr. Riddhi Datta

• The term photosynthesis means literally “synthesis using light.”

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• Photosynthetic organisms use solar energy to synthesize complex carbon compounds (carbohydrates) and generation of oxygen from carbon dioxide and water.

• The thylakoid reactions or light reactions of photosynthesis take place in the thylakoids of the chloroplast.

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• The end products of thylakoid reactions are the high-energy compounds ATP and NADPH.

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• This APT and NADPH are used for the synthesis of sugars in the carbon fixation reactions.

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• These synthetic processes take place in the stroma of the chloroplast.

PHOTOSYNTHESIS

Dr. Riddhi Datta

• Light has characteristics of both a particle and a wave

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• A wave is characterized by a wavelength (λ) which is the distance between successive wave crests.

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• Frequency (ν) is the number of wave crests that pass an observer in a given time.

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c= λν

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where c is the speed of the wave.

For light it is 3.0 × 10⁸ m s^–1.

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• The light wave is a transverse electromagnetic wave.

LIGHT – PARTICLE & WAVE NATURE

Dr. Riddhi Datta

• Light is also a particle called photon.

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• Each photon contains an amount of energy that is called a quantum.

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• The energy content of light is delivered in discrete packets, the quanta.

• The energy (E) of a photon depends on the frequency of the light according to a relation known as Planck’s law:

E = hν

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where h is Planck’s constant (6.626 × 10^–34 J s)

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• Our eyes are sensitive to only a small range of frequencies—the visible-light region of the electromagnetic spectrum.

LIGHT – PARTICLE & WAVE NATURE

Dr. Riddhi Datta

• The absorption spectrum of chlorophyll a indicates the approximate portion of the solar output that is used by plants.

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• An absorption spectrum provides information about the amount of light energy taken up or absorbed by a molecule or substance as a function of the wavelength of the light.

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• Chlorophyll appears green to our eyes because it absorbs light mainly in the red and blue parts of the spectrum and reflects the light enriched in green wavelengths (about 550 nm).

ABSORPTION SPECTRUM

Dr. Riddhi Datta

• Action spectrum depicts the magnitude of a response of a biological system to light as a function of wavelength.

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• An action spectrum for photosynthesis can be constructed from measurements of oxygen evolution at different wavelengths.

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• Often an action spectrum can identify the chromophore (pigment) responsible for a particular light-induced phenomenon.

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• Action spectra were very important for the discovery of two distinct photosystems operating in O₂-evolving photosynthetic organisms.

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• If the pigment used to obtain the absorption spectrum is the same as that which causes the response, the absorption and action spectra will match.

Discrepancies are found in the region of carotenoid absorption, from 450 to 550 nm, indicating that energy transfer from carotenoids to chlorophylls is not as effective as energy transfer between chlorophylls.

ACTION SPECTRUM

Dr. Riddhi Datta

• Emerson measured the quantum yield of photosynthesis as a function of wavelength and revealed an effect known as the red drop.

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• Quantum yield for the wavelengths at which chlorophyll absorbs light are fairly constant.

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• This indicates that any photon absorbed by chlorophyll is as effective as any other photon in driving photosynthesis.

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• The yield drops dramatically in the far-red region of chlorophyll absorption (greater than 680 nm).

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• This indicates that light with a wavelength greater than 680 nm is much less efficient than light of shorter wavelengths.

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• The slight dip near 500 nm reflects the somewhat lower efficiency of photosynthesis using light absorbed by accessory pigments, carotenoids.

RED DROP EFFECT

Dr. Riddhi Datta

ENHANCEMENT EFFECT

Dr. Riddhi Datta

• Absorption of blue light excites the chlorophyll to a higher energy state than absorption of red light.

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• Chlorophyll (Chl) in its lowest energy, or ground, state absorbs a photon (hν) and makes a transition to a higher energy, or excited, state (Chl*):

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Chl + hν → Chl*

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• In the higher excited state, chlorophyll is extremely unstable; it rapidly gives up some of its energy to the surroundings as heat, and enters the lowest excited state.

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CHLOROPHYLL

Dr. Riddhi Datta

In the lowest excited state, the excited chlorophyll has four alternative pathways for disposing of its available energy:

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1. Fluorescence:
• Excited chlorophyll can re-emit a photon and thereby return to its ground state.
• The wavelength of fluorescence is slightly longer (and of lower energy) than the wavelength of absorption.
• A portion of the excitation energy is converted into heat before the fluorescent photon is emitted.
• Chlorophylls fluoresce in the red region of the spectrum.

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CHLOROPHYLL

Dr. Riddhi Datta

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2. The excited chlorophyll can return to its ground state by directly converting its excitation energy into heat, with no emission of a photon.

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• Energy transfer: Chlorophyll may participate in energy transfer, during which an excited chlorophyll transfers its energy to another molecule.

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• Photochemistry: The energy of the excited state causes chemical reactions to occur. The photochemical reactions of photosynthesis are among the fastest known chemical reactions.

CHLOROPHYLL

Dr. Riddhi Datta

• The chlorophylls have a porphyrin-like ring structure with a magnesium ion (Mg) coordinated in the center and a long hydrophobic hydrocarbon tail that anchors them in the photosynthetic membrane.

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• Porphyrin is a cyclic tetrapyrrole. There is a 5^th isocyclic ring.

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• The porphyrin-like ring is the site of the electron rearrangements that occur when the chlorophyll is excited, and of the unpaired electrons when it is either oxidized or reduced.

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• Chlorophylls a differs from one another by substituents around the rings and the pattern of double bonds. Thus, chlorophyll a has a –CH3 group while chlorophyll b has –CHO group on C-3 of ring II.

• Chlorophylls a and b: most green plants
• Chlorophylls c, d, and f: some protists and cyanobacteria
• Bacteriochlorophyll: photosynthetic bacteria

CHLOROPHYLL

Dr. Riddhi Datta

• Carotenoids found in photosynthetic organisms are all linear molecules with multiple conjugated double bonds.

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• Absorption bands in the 400 to 500 nm region give carotenoids their characteristic orange color.

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• Carotenoids are integral constituents of the thylakoid membrane and are usually associated intimately with many of the proteins that make up the photosynthetic apparatus.

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• The light energy absorbed by the carotenoids is transferred to chlorophyll for photosynthesis; because of this role they are called accessory pigments.

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• Carotenoids also help protect the organism from damage caused by light.

CAROTENOIDS

Dr. Riddhi Datta

• The photosynthetic membrane is damaged if the large amount of energy absorbed cannot be stored by photochemistry.

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• The photoprotection mechanism is a safety valve, venting excess energy before it can cause damage.

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• The excited state is quenched when the energy of the excited chlorophyll molecules is rapidly dissipated by excitation transfer or photochemistry.

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• If the excited state of chlorophyll is not rapidly quenched, it can react with molecular oxygen to form an excited state of oxygen known as singlet oxygen (¹O₂ *). This extremely reactive singlet oxygen damage many cellular components, especially lipids.

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• Carotenoids rapidly quench the excited state of chlorophyll.

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• The excited state of carotenoids does not have sufficient energy to form singlet oxygen, so it decays back to its ground state while losing its energy as heat.

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PHOTOPROTECTION BY CAROTENOIDS

Dr. Riddhi Datta

• Nonphotochemical quenching is the quenching of chlorophyll fluorescence by processes other than photochemistry.

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• A large fraction of the excitations in the antenna system caused by intense illumination are quenched by conversion into heat. This protects the photosynthetic machinery against overexcitation and subsequent damage.

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• Three carotenoids (called xanthophylls) involved in nonphotochemical quenching are:
• Violaxanthin
• Antheraxanthin
• Zeaxanthin

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violaxanthin antheraxanthin zeaxanthin

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• Binding of protons and zeaxanthin to light-harvesting antenna proteins is thought to cause conformational changes that lead to quenching and heat dissipation.

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• Associated with a peripheral antenna complex of PSII, the PsbS protein.

NON-PHOTOCHEMICAL QUENCHING

High light

High light

Low light

Low light

Enzyme: violaxanthin de-epoxidase.

Dr. Riddhi Datta

• Chloroplast is the site of photosynthesis.

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• A wide variety of proteins essential to photosynthesis are embedded in the thylakoid membranes.

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• The reaction centers, the antenna pigment–protein complexes, and most of the electron carrier proteins are all integral membrane proteins.

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• Photosystems I and II are spatially separated in the thylakoid membrane.

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ORGANIZATION OF PHOTOSYNTHETIC APPARATUS

• The PSII reaction center, along with its antenna chlorophylls and associated electron transport proteins, is located predominantly in the grana lamellae.

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• The PSI reaction center and its associated antenna pigments, electron transfer proteins, and the ATP synthase are found almost exclusively in the stroma lamellae and at the edges of the grana lamellae.

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• The cytochrome b₆f complex of the electron transport chain that connects the two photosystems is evenly distributed between stroma and grana lamellae.

Dr. Riddhi Datta

• A portion of the light energy absorbed by chlorophylls and carotenoids is stored as chemical energy.

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• This complex process depends on cooperation between many pigment molecules and a group of electron transfer proteins.

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• The majority of the pigments serve as an antenna complex, collecting light and transferring the energy to the reaction center complex.

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• The chemical oxidation and reduction reactions occur in the reaction center leading to long-term energy storage.

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• So there is a division of labor between antenna and reaction center pigments.

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• The reaction centers and most of the antenna complexes are integral components of the photosynthetic membrane: chloroplast and thylakoid membranes in eukaryotes and plasma membrane or membranes derived from it in prokaryotes.

ANTENNA COMPLEX

Dr. Riddhi Datta

• The size of the antenna system varies considerably in different organisms:
• 20 to 30 bacteriochlorophylls per reaction center in photosynthetic bacteria
• 200 to 300 chlorophylls per reaction center in higher plants
• a few thousand pigments per reaction center in some algae

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• The antenna pigments are associated with proteins to form pigment–protein complexes.

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• Excitation energy is conveyed from the chlorophyll that absorbs the light to the reaction center through a non-radiative process called fluorescence resonance energy transfer (FRET).

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• Energy transfer in antenna complexes is very efficient. 95 to 99% of the photons absorbed by the antenna pigments have their energy transferred to the reaction center.

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• Whereas energy transfer is a purely physical phenomenon, electron transfer involves chemical (redox) reactions.

ANTENNA COMPLEX

Dr. Riddhi Datta

• The sequence of pigments within the antenna that funnel absorbed energy toward the reaction center has absorption maxima that are progressively shifted toward longer red wavelengths.

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• So the energy of the excited state is somewhat lower nearer the reaction center than in the more peripheral portions of the antenna system.

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• The difference in energy between two consecutive excited chlorophylls is lost to the environment as heat.

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• This effect gives the energy-trapping process a degree of directionality or irreversibility and makes the delivery of excitation to the reaction center more efficient.

ANTENNA COMPLEX

Dr. Riddhi Datta

• The antenna proteins associated with PSII and are called light-harvesting complex II (LHCII) proteins and those associated with PSI and are called LHCI proteins.

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• These antenna complexes are also known as chlorophyll a/b antenna proteins.

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• Light absorbed by carotenoids or chlorophyll b in the LHC proteins is rapidly transferred to chlorophyll a and then to other antenna pigments that are intimately associated with the reaction center.

ANTENNA COMPLEX

Dr. Riddhi Datta

• Photosynthetic unit or quantasome is the smallest group of collaborating pigment molecules necessary for a photochemical act.

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• A photochemical act involves absorption and migration of a light quantum to the reaction center where it promotes release of electron.

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• Evolution of 1 molecule of O₂ requires:
• 2500 chlorophyll molecules
• 8 quanta light

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• Therefore, 300 chlorophyll molecules (2500/8) are required for processing of each quantum of light.

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• These group of 300 chlorophyll molecules is termed as photosynthetic unit or quantasome.

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• Out of these 300, only one specialized chlorophyll molecule participates in photochemical reaction and others absorb and transfer light to that special molecule.

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PHOTOSYNTHETIC UNIT

Dr. Riddhi Datta

• The quantum yield of photochemistry (ϕ) is defined as is the rate at which that defined event occurs relative to the rate of photon absorption by the system.

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• The quantum efficiency is a measure of the fraction of absorbed photons that engage in photochemistry.

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• The energy efficiency is a measure of how much energy in the absorbed photons is stored as chemical products.

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• The solar energy storage efficiency is a measure of how much of the energy in the entire solar spectrum is converted to usable form.

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• If red light of wavelength 680 nm is absorbed, the total energy input is 1760 kJ per mole of oxygen formed.

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• The standard state free-energy change of generation of one mole of oxygen is +467 kJ mol–1.

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• The efficiency of conversion of light energy at the optimal wavelength into chemical energy is therefore about 27% (467/1760*100).

Dr. Riddhi Datta

• The reaction center consists of 4-6 molecules of chlorophyll a, called the reaction center chlorophyll, and associated proteins and cofactors.

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• The center chlorophyll is an energy sink with longest wavelength and lowest energy absorbing chlorophyll in the complex.

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• The reaction center is the site of primary photochemical redox reaction where light energy is converted to chemical energy.

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• The reaction centers of PSI and PSII are designated as P700 and P680 as the corresponding pigment has absorption maxima at 700 nm and 680 nm respectively.

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• The reaction center remains associated with the antenna complex which increases efficiency of absorption and utilization of light.

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• Electron transfer occurs at reaction center while energy transfer occurs in the antenna complex.

REACTION CENTER

Dr. Riddhi Datta

PSI REACTION CENTER

• The PSI reaction center complex is a large multi-subunit complex.

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• It consists of a heterodimer of 2 intrinsic membrane proteins, PsaA and PsaB each binding one reaction center chlorophyll molecule, P700.

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• A pair of chlorophyll a, designated as A0 remain linked to the heterodimer that accept electron immediately from P700.

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• In addition, there are minor proteins PsaC to PsaN.

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• A core antenna consisting of about 100 chlorophylls is an integral part of the PSI reaction center. The PSI reaction center complex in higher plants also contains LHCI complexes in addition to the core structure.

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• There are two molecules of vitamin K1, called phylloquinone bound to the heterodimer, one per subunit.

• A series of 3 membrane associated Fe–S centers - FeS_X , FeS_A , and FeS_B are also present. A soluble iron– sulfur protein ferredoxin (Fd) is also a part of PSI.

Dr. Riddhi Datta

PSII REACTION CENTER

• In higher plants, PSII is contained in a multi-subunit protein supercomplex that has two complete reaction centers and some antenna complexes.

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• The core of the reaction center consists of two membrane proteins known as D1 and D2, as well as other proteins.

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• A special pair of photon-absorbing chlorophyll-a, named P680, remains attached to D1 and D2 proteins.

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• Additional chlorophylls, carotenoids, pheophytins (colourless chlorophyll lacking Mg), and plastoquinones are also bound to the membrane proteins D1 and D2.

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• Other proteins serve as antenna complexes or are involved in oxygen evolution.

Dr. Riddhi Datta

• PSI produces a strong reductant, capable of reducing NADP⁺, and a weak oxidant.

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• PSII produces a very strong oxidant, capable of oxidizing water, and a weaker reductant than the one produced by PSI.

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• The reductant produced by PSII re-reduces the oxidant produced by PSI.

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• The two photosystems are linked by an electron transport chain.

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P680 and P700 refer to the wavelengths of maximum absorption of the reaction center chlorophylls in PSII and PSI, respectively.

Z-scheme of photosynthesis

Oxygen-evolving organisms have two photosystems that operate in series

Dr. Riddhi Datta

MECHANISMS OF NON CYCLIC ELECTRON TRANSPORT

• Almost all the chemical processes that make up the light reactions of photosynthesis are carried out by four major protein complexes:
• PSII
• Cytochrome b₆f complex
• PSI
• ATP synthase

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• These four integral membrane complexes are vectorially oriented in the thylakoid membrane.

Dr. Riddhi Datta

MECHANISMS OF NON CYCLIC ELECTRON TRANSPORT

• PSII oxidizes water to O₂ in the thylakoid lumen and in the process releases protons into the lumen. The reduced product of photosystem II is plastohydroquinone (PQH₂).

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• Cytochrome b₆f oxidizes PQH₂ molecules that were reduced by PSII and delivers electrons to PSI via the soluble copper protein plastocyanin. The oxidation of PQH₂ is coupled to proton transfer into the lumen from the stroma, generating a proton motive force.

Dr. Riddhi Datta

MECHANISMS OF NON CYCLIC ELECTRON TRANSPORT

• PSI reduces NADP⁺ to NADPH in the stroma by the action of ferredoxin (Fd) and the flavoprotein ferredoxin–NADP⁺ reductase (FNR).

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• ATP synthase produces ATP as protons diffuse back through it from the lumen into the stroma.

Dr. Riddhi Datta

MECHANISMS OF NON CYCLIC ELECTRON TRANSPORT

• The first reaction that converts electron energy into chemical energy (primary photochemical event) is the transfer of an electron from the excited state of a chlorophyll in the reaction center to an acceptor molecule.

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• Immediately after the photochemical event, the reaction center chlorophyll is in an oxidized state (electron deficient, or positively charged), and the nearby electron acceptor molecule is reduced (electron rich, or negatively charged).

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• The acceptor transfers its extra electron to a secondary acceptor and so on down the electron transport chain.

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• The oxidized reaction center of the chlorophyll that had donated an electron is re-reduced by a secondary donor, which in turn is reduced by a tertiary donor.

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• In plants, the ultimate electron donor is H₂O, and the ultimate electron acceptor is NADP⁺.

Dr. Riddhi Datta

THE Z-SCHEME

• Electrons from chlorophyll travel through the carriers organized in the Z scheme.

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• Photons excite the specialized chlorophyll of the reaction centers (P680 for PSII; P700 for PSI), and an electron is ejected.

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• The excited PSII reaction center chlorophyll, P680*, transfers an electron to pheophytin (Pheo).

Dr. Riddhi Datta

• On the oxidizing side of PSII, P680 oxidized by light is re-reduced by Y_z which has received electrons from oxidation of water.

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• On the reducing side of PSII, pheophytin transfers electrons to the acceptors PQ_A and PQ_B, which are plastoquinones.

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• The cytochrome b₆f complex transfers electrons to plastocyanin (PC), a soluble protein, which in turn reduces P700⁺ (oxidized P700).

THE Z-SCHEME

Dr. Riddhi Datta

• The acceptor of electrons from P700* is A₀, a chlorophyll, and the next acceptor (A₁) is a quinone.

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• A series of membrane-bound iron–sulfur proteins (FeS_X, FeS_A, and FeS) transfers electrons to soluble ferredoxin (Fd).

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• The soluble flavoprotein ferredoxin–NADP⁺ reductase (FNR) reduces NADP⁺ to NADPH, which is used in the Calvin–Benson cycle to reduce CO₂.

THE Z-SCHEME

Dr. Riddhi Datta

WATER IS OXIDIZED TO OXYGEN BY PSII

• Water is oxidized as follows:

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• Four electrons are removed from two water molecules, generating an oxygen molecule and four hydrogen ions.

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• Manganese (Mn) is an essential cofactor in the water-oxidizing process.

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• The 4 electrons removed from water do not pass directly to P680⁺ that can accept only one electron at a time. Instead, an oxygen evolving complex (Mn-protein) passes 4 electrons, one at a time to P680⁺.

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• The intermediate electron donor to P680+ is a tyrosine residue (Y_Z) in the D1 protein of PSII reaction center.

Dr. Riddhi Datta

WATER IS OXIDIZED TO OXYGEN BY PSII

• This tyrosine residue regains its missing electron by oxidizing a cluster of 4 Mn ions in the oxygen evolving complex.

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• In this state, the Mn complex can take 4 electrons from a pair of water molecules and release 4 H⁺ ions and oxygen.

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• The Mn ions undergo a series of oxidations—known as S states that are linked to this oxygen evolving complex: S₀, S₁, S₂, S₃, S_4.

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• Each transition between S₀ to S₄ is a photon driven redox reaction, while transition from S₄ to S₀ results in evolution of oxygen.

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• These 5 steps release 4 water-driven protons into the inner thylakoid space.

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• Cl^– and Ca²⁺ ions are also essential for O₂ evolution.

Dr. Riddhi Datta

Q CYCLE

• The electrons and protons flow through the cytochrome b₆f complex through a mechanism known as the Q cycle.

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• This complex contains two b-type cytochromes (Cyt b), a c-type cytochrome (Cyt c or cytochrome f ), a Rieske Fe–S protein (FeSR), and two quinone oxidation–reduction sites.

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The noncyclic or linear processes:

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• A plastohydroquinone (PQH₂) molecule produced by the action of PSII is oxidized near the lumenal side of the complex, transferring its two electrons to the Rieske Fe–S protein and one of the b-type

• The electron transferred to FeSR is passed to cytochrome f (Cyt f) and then to plastocyanin (PC), which reduces P700 of PSI.

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• The reduced b-type cytochrome transfers an electron to the other b-type cytochrome, which reduces a plastoquinone (PQ) to the plastosemiquinone (PQ•–) state.

cytochromes and simultaneously expelling two protons to the lumen.

Dr. Riddhi Datta

Q CYCLE

The cyclic processes:

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• A second PQH₂ is oxidized, with one electron going from FeSR to PC and finally to P700.

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• The second electron goes through the two b-type cytochromes and reduces the plastosemiquinone to the plastohydroquinone, picking up two protons from the stroma.

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• Overall, four protons are transported across the membrane for every two electrons delivered to P700.

Dr. Riddhi Datta

CYCLIC ELECTRON TRANSPORT

• A second mode of electron transport involves excitation of PSI but there is no oxidation of water or reduction of NADP⁺. This is called cyclic electron transport system.

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• The phenomenon involves cytochrome b₆ that is not involved in non-cyclic electron transport.

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• It occurs when NADP⁺ is not available for reduction.

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• Electrons ejected from P700 are shunted back to the central electron transport chain via cytochrome b₆ and pass back to the electron hole of PSI.

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• Coupled to this electron flow is the phosphorylation of ADP to ATP.

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• Cyclic electron flow is especially important as an ATP source in the bundle sheath chloroplasts of some plants that carry out C4 carbon fixation.

Dr. Riddhi Datta

HERBICIDES BLOCKERS OF PHOTOSYNTHETIC ELECTRON FLOW

• Herbicides, dichloro phenyl di methyl urea (DCMU or diuron) and paraquat, block photosynthetic electron flow.

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• DCMU blocks electron flow at the quinone acceptors of PSII, by competing for the binding site of plastoquinone that is normally occupied by PQ_B .

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• Paraquat accepts electrons from the early acceptors of PSI and then reacts with oxygen to form superoxide.

Dr. Riddhi Datta

PHOTOPHOSPHORYLATION

• A fraction of the captured light energy is used for light-dependent ATP synthesis, which is known as photophosphorylation.

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• Under normal cellular conditions, photophosphorylation requires electron flow. Electron flow without accompanying phosphorylation is said to be uncoupled.

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Chemiosmotic theory:

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• Photophosphorylation works via the chemiosmotic mechanism proposed by Peter Mitchell.

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• The basic principle of chemiosmosis is that ion concentration differences and electrical potential differences across membranes are sources of free energy that can be used by the cell.

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• According to second law of thermodynamics, any non uniform distribution of matter or energy represents a source of energy.

Dr. Riddhi Datta

PHOTOPHOSPHORYLATION

• The photosynthetic membrane is asymmetric in nature and proton flow from one side of the membrane to the other accompanied by electron flow.

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• The direction of proton translocation is such that the stroma becomes more alkaline (fewer H+ ions) and the lumen becomes more acidic (more H+ ions) as a result of electron transport.

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• Mitchell proposed that the total energy available for ATP synthesis, called the proton motive force (∆p), is the sum of a proton chemical potential and a transmembrane electrical potential.

∆p = ∆E – 59(pH_i – pH_o )

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where ∆E: transmembrane electrical potential

pH_i – pH_o (or ∆pH): pH difference across the membrane

Proportionality constant (at 25°C) is 59 mV per pH unit

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• Therefore, a transmembrane pH difference of one pH unit is equivalent to a membrane potential of 59 mV.

Dr. Riddhi Datta

PHOTOPHOSPHORYLATION

• The ATP is synthesized by an enzyme complex (mass ~400 kDa) known as ATP synthase or ATPase or Cf_o-CF₁.

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• This enzyme consists of two parts:
• a hydrophobic membrane-bound portion called CF_o
• a portion that sticks out into the stroma called CF₁

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• CF_o forms a channel across the membrane through which protons can pass.

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• CF₁ is made up of several peptides, including three copies of each of the α and β peptides arranged alternately.

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• The catalytic sites are located largely on the β polypeptide. Other peptides have regulatory functions.

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• CF₁ is the portion of the complex that synthesizes ATP.

Dr. Riddhi Datta

PHOTOPHOSPHORYLATION

• The internal stalk and probably much of the CF_o portion of the enzyme rotate during catalysis. The enzyme is actually a tiny molecular motor.

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• Three ATP molecules are synthesized per rotation.

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• CF_o part of the chloroplast ATP synthase indicates that it contains 14 copies of the integral membrane subunit.

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• Each subunit can translocate one proton across the membrane each time the complex rotates.

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• This suggests that the stoichiometry of protons translocated to ATP formed is 14/3, or 4.67.

Dr. Riddhi Datta

SIMILARITIES OF PHOTOSYNTHETIC AND RESPIRATORY ELECTRON FLOW

Dr. Riddhi Datta

PHOTOINHIBITION

• Photoinhibition is a complex set of molecular processes defined as the inhibition of photosynthesis by excess light.

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• Protection against photodamage is a multilevel process.

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• The first line of defense is suppression of damage by quenching of excess excitation as heat.

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• If this defense is not sufficient and toxic photoproducts form, a variety of scavenging systems eliminate the reactive photoproducts.

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• If this second line of defense also fails, the photoproducts can damage the D1 protein of PSII. This damage leads to photoinhibition.

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• The D1 protein is then excised from the PSII reaction center and degraded.

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• A newly synthesized D1 is reinserted into the PSII reaction center to form a functional unit.