PhotosynthesisPhotosynthesis

Photosynthesis is the is the process by which plants, process by which plants, use the energy from use the energy from sunlight to produce sunlight to produce sugar, converts into sugar, converts into ATP, , the "fuel" used by all the "fuel" used by all living things. The living things. The conversion of unusable conversion of unusable sunlight energy into sunlight energy into usable chemical energy, usable chemical energy, is associated with the is associated with the actions of the green actions of the green pigment pigment chlorophyll. .

The raw materials of photosynthesis, The raw materials of photosynthesis, water and carbon dioxide, enter the water and carbon dioxide, enter the cells of the leaf, and the products of cells of the leaf, and the products of photosynthesis, sugar and oxygen, photosynthesis, sugar and oxygen, leave the leaf.leave the leaf.

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Since only absorbed light can excite Since only absorbed light can excite molecules and thus deliver its energy, so molecules and thus deliver its energy, so

a photosynthetic pigment can act as a photosynthetic pigment can act as absorbers of visible light.absorbers of visible light.

The leaves of higher plants contain two The leaves of higher plants contain two kinds of chlorophyll which differ only kinds of chlorophyll which differ only

slightly in structure and absorption slightly in structure and absorption spectra. Chlorophyll a and chlorophyll b.spectra. Chlorophyll a and chlorophyll b.

Chlorophyll b has a CHO group instead of Chlorophyll b has a CHO group instead of the methyl group at the position shown.the methyl group at the position shown.

Many photosynthetic cells contain, in Many photosynthetic cells contain, in addition to chlorophyll, other light addition to chlorophyll, other light ––absorbing pigments, known as accessory absorbing pigments, known as accessory pigments.pigments.

e.g:e.g: Carotenes ( Carotenes (YellowYellow, , brownbrown or or redred).).

Phycocyanins (Phycocyanins (BlueBlue).).

Phycoerythrins (Phycoerythrins (RedRed))

Structure of chlorophyll a and Structure of chlorophyll a and bb

If a pigment absorbs light energy, one of three If a pigment absorbs light energy, one of three things will occur. things will occur.

1-1- Energy is dissipated as heat. Energy is dissipated as heat.

2-2- The energy may be emitted immediately as a longer The energy may be emitted immediately as a longer wavelength, a phenomenon known as fluorescence. wavelength, a phenomenon known as fluorescence.

3-3- Energy may trigger a chemical reaction, as in Energy may trigger a chemical reaction, as in photosynthesis. photosynthesis.

Chlorophyll only triggers a chemical reaction when Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a it is associated with proteins embedded in a membrane (as in a chloroplast)membrane (as in a chloroplast)

The structure of the chloroplast and The structure of the chloroplast and photosynthetic membranesphotosynthetic membranes

The thylakoid is the structural unit of photosynthesis.

Thylakoids are stacked like pancakes in stacks known collectively as grana. The areas between grana are referred to as stroma.

The chloroplast has three membranes, forming three compartments.

Stages of PhotosynthesisStages of Photosynthesis Photosynthesis is a two stage process. The first Photosynthesis is a two stage process. The first

process is the Light Dependent Process (process is the Light Dependent Process (Light Light reactionreaction), requires the direct energy of light to ), requires the direct energy of light to make energy carrier molecules that are used in the make energy carrier molecules that are used in the second process.second process.

The Light Independent Process (The Light Independent Process (Dark reactionDark reaction) ) occurs when the products of the Light Reaction are occurs when the products of the Light Reaction are used to form C-C covalent bonds of carbohydrates.used to form C-C covalent bonds of carbohydrates.

The Light Reactions occur in the The Light Reactions occur in the granagrana and the and the Dark Reactions take place in the Dark Reactions take place in the stromastroma of the of the chloroplasts.chloroplasts.

Light Light ReactionReaction

Dark Reaction

Energy transductionEnergy transduction The energy of excitation, in raising an electron to a The energy of excitation, in raising an electron to a

higher energy orbital, dramatically changes the higher energy orbital, dramatically changes the standard reduction potentialstandard reduction potential EEoo of the pigment such of the pigment such

that it becomes a much more effective electron that it becomes a much more effective electron donor.donor.

Reaction of this excited-state electron donor with an Reaction of this excited-state electron donor with an electron acceptor leads to the transformation, orelectron acceptor leads to the transformation, or

transductiontransduction,, of light energy (photons) to chemical of light energy (photons) to chemical energy (reducing power), the potential for electron-energy (reducing power), the potential for electron-

transfer reactions.transfer reactions.

Transduction of light energy into chemical energy, Transduction of light energy into chemical energy, the photochemical event, is the essence of the photochemical event, is the essence of

photosynthesisphotosynthesis ..

Role of chlorophyllRole of chlorophyll

Chlorophyll molecules are photochemically Chlorophyll molecules are photochemically reactive, and it led to the concept that reactive, and it led to the concept that

photosynthesis occurs in functionally discrete photosynthesis occurs in functionally discrete units.units.

Chlorophyll serves two roles in photosynthesis. Chlorophyll serves two roles in photosynthesis.

1-1-It is involved in light harvesting and the It is involved in light harvesting and the transfer of light energy to photo-reactive sites transfer of light energy to photo-reactive sites

by excitation transfer.by excitation transfer. 2-2- It participates directly in the It participates directly in the

photochemical events whereby light energy photochemical events whereby light energy becomes chemical energy. becomes chemical energy.

AA photosynthetic unitphotosynthetic unit can serve as an can serve as an antenna of several hundred light-antenna of several hundred light-

harvesting chlorophyll molecules plus a harvesting chlorophyll molecules plus a special pair of photochemically reactive special pair of photochemically reactive

chlorophyllchlorophyll aa molecules called themolecules called the reaction center.reaction center.

Chlorophyll in plants is excited by visible Chlorophyll in plants is excited by visible light, no flourescence or heat is light, no flourescence or heat is

observed.observed.

The high energy electron moves from the The high energy electron moves from the excited chlorophyll molecule to the first excited chlorophyll molecule to the first components of a chain electron carriers components of a chain electron carriers

leading to the generation of NADPH. H+ leading to the generation of NADPH. H+ which coupled to form ATP.which coupled to form ATP.

Photosystem I and IIPhotosystem I and II Two light reactions participate in oxygen-evolving Two light reactions participate in oxygen-evolving

photosynthetic cells, one using light of wavelength 700 photosynthetic cells, one using light of wavelength 700 nm and the other using light of wavelength 680 nm or nm and the other using light of wavelength 680 nm or

less.less.

The existence of two light reactions established the The existence of two light reactions established the presence of two photosystems I and IIpresence of two photosystems I and II. .

Photosystem IPhotosystem I)) PSI)PSI):: is defined as containing reaction is defined as containing reaction center chlorophylls with maximal red light absorption at center chlorophylls with maximal red light absorption at

700 nm; PSI is not involved in oxygen700 nm; PSI is not involved in oxygen evolution.evolution.

Photosystem IIPhotosystem II (PSII) (PSII):: functions in oxygenfunctions in oxygen evolution, evolution, using reaction centers that exhibit maximal red light using reaction centers that exhibit maximal red light

absorption at 680 nmabsorption at 680 nm..

Components of Photosystem IComponents of Photosystem I

An Oxygen-Evolving Complex An Oxygen-Evolving Complex in PSII Regenerates P680in PSII Regenerates P680

In a reaction center, two integralIn a reaction center, two integral proteins, , D1 and D2, bind D1 and D2, bind the special-pairthe special-pair chlorophyllschlorophylls 680 ,two other680 ,two other chlorophyllschlorophylls

(Chl), two pheophytins (Pheo), one Fe atom, and two (Chl), two pheophytins (Pheo), one Fe atom, and two quinones (QAquinones (QA and QB)and QB).. All of these are used forAll of these are used for electron electron

transporttransport following light absorption by an associated light following light absorption by an associated light harvesting complexharvesting complex

ThreeThree extrinsic proteinsextrinsic proteins)) 23, 3323, 33 and 17 kDa) comprise the and 17 kDa) comprise the

oxygen-evolving complex; they bind the four Mnoxygen-evolving complex; they bind the four Mn ions and ions and the Cathe Ca and Cland Cl- - ions that function in the splitting of Hions that function in the splitting of H22O, and O, and they maintain the environment essential for high rates of Othey maintain the environment essential for high rates of O22

evolution.evolution. Z is tyrosine residue 161 of the D1Z is tyrosine residue 161 of the D1 polypeptidepolypeptide; ; it conducts it conducts

electrons from the Mn atoms to the oxidized reaction-center electrons from the Mn atoms to the oxidized reaction-center chlorophyll P680chlorophyll P680++ reducing it to the ground statereducing it to the ground state P680. P680.

PhotophosphorylationPhotophosphorylation

Is the process of converting energy from a light-excited electron into the pyrophosphate bond of an ADP molecule. This occurs when the electrons from water are excited by the light in the presence of P680. Light energy causes the removal of an electron Light energy causes the removal of an electron from a molecule of P680 that is part of Photosystem from a molecule of P680 that is part of Photosystem II. II. The P680 requires an electron, which is taken from a water molecule, breaking the water into H+ ions and O-2 ions. These O-2 ions combine to form the diatomic O2 that is released. The electron is "boosted" to a higher energy state The electron is "boosted" to a higher energy state and attached to a primary electron acceptor, which and attached to a primary electron acceptor, which begins a series of redox reactions, passing the begins a series of redox reactions, passing the electron through a series of electron carriers, electron through a series of electron carriers, eventually attaching it to a molecule in eventually attaching it to a molecule in Photosystem I. Photosystem I.

Light acts on a molecule of P700 in Photosystem I, causing an electron to be "boosted" to a still higher potential. The electron is attached to a different primary electron acceptor (that is a different molecule from the one associated with Photosystem II). The electron is passed again through a series of redox reactions, eventually being attached to NADP+ and H+ to form NADPH, an energy carrier needed in the Light Independent Reaction.

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The electron from Photosystem II replaces the The electron from Photosystem II replaces the excited electron in the P700 molecule. There is excited electron in the P700 molecule. There is thus a continuous flow of electrons from water to thus a continuous flow of electrons from water to NADPH. NADPH.

Cyclic Electron Flow occurs in some eukaryotes Cyclic Electron Flow occurs in some eukaryotes and primitive photosynthetic bacteria. and primitive photosynthetic bacteria. No No NADPH NADPH is produced, is produced, only ATP. only ATP.

This occurs when cells may require additional This occurs when cells may require additional ATP, or when there is no NADPATP, or when there is no NADP++ to reduce to to reduce to NADPH.NADPH.

Role of photosystems I Role of photosystems I and IIand II

Photosystem I:Photosystem I: Provides reducing power in the Provides reducing power in the form of NADPH. form of NADPH.

Photosystem II:Photosystem II: Splits water, producing Splits water, producing oxygenoxygen, , and feeds the electrons released into and feeds the electrons released into an electron transport chain that couples PSII an electron transport chain that couples PSII to PSI. to PSI.

Electron transfer between PSII and PSI pumps Electron transfer between PSII and PSI pumps protons for chemiosmotic ATP synthesis. protons for chemiosmotic ATP synthesis.

Photosynthesis involves the reduction of Photosynthesis involves the reduction of NADPNADP++ using electrons derived from water using electrons derived from water and activated by lightand activated by light hvhv ..

ATP is generated in the ATP is generated in the processprocess . .

The standard reduction potential for the NADPThe standard reduction potential for the NADP//NADPH NADPH couple is -0.32 V. Thus, a strong reductant withcouple is -0.32 V. Thus, a strong reductant with EoEo' ' more more negative than -0.32 V is required to reduce NADPnegative than -0.32 V is required to reduce NADP+ + under under standard conditions.standard conditions.

By similar reasoning, a very strong oxidant will be required By similar reasoning, a very strong oxidant will be required

to oxidize water to oxygen because( Oto oxidize water to oxygen because( O22/H/H22o) is +0.82 V. o) is +0.82 V.

Separation of the oxidizing and reducing aspectsSeparation of the oxidizing and reducing aspects of of photosynthesis is accomplished in nature by devoting PSI to photosynthesis is accomplished in nature by devoting PSI to

NADPNADP++ reduction and PSII to water oxidation.reduction and PSII to water oxidation.

PSI and PSII are linked via an electron PSI and PSII are linked via an electron transport chain so that the weak reductant transport chain so that the weak reductant

generated by PSII can provide an electron to generated by PSII can provide an electron to reduce the weak oxidant side of P700.reduce the weak oxidant side of P700.

ThusThus, , electrons flow from Helectrons flow from H22O to NADPO to NADP++ driven driven by light energy absorbed at the reaction by light energy absorbed at the reaction

centers. Oxygen is a by-product of thecenters. Oxygen is a by-product of the photolysis (photolysis (light-splitting of waterlight-splitting of water((..

Accompanying electron flow is production of a Accompanying electron flow is production of a

proton gradient and ATP synthesis.proton gradient and ATP synthesis.

This light-driven phosphorylation is termedThis light-driven phosphorylation is termed photophosphorylationphotophosphorylation..