Photosynthesis Since only absorbed light can excite molecules and thus deliver its energy, so a photosynthetic pigment can act as absorbers of visible light. Since only absorbed light can excite molecules and thus deliver its energy, so a photosynthetic pigment can act as absorbers of visible light. The leaves of higher plants contain two kinds of chlorophyll which differ only slightly in structure and absorption spectra. Chlorophyll a and chlorophyll b. The leaves of higher plants contain two kinds of chlorophyll which differ only slightly in structure and absorption spectra. Chlorophyll a and chlorophyll b. Chlorophyll b has a CHO group instead of the methyl group at the position shown. Chlorophyll b has a CHO group instead of the methyl group at the position shown. Many photosynthetic cells contain, in addition to chlorophyl, other light absorbing pigments, known as accessory pigments. Many photosynthetic cells contain, in addition to chlorophyl, other light absorbing pigments, known as accessory pigments. e.g: Carotenes (Yellow, brown or red). e.g: Carotenes (Yellow, brown or red). Phycocyanins (Blue). Phycocyanins (Blue). Phycoerythrins (Red) Phycoerythrins (Red) Structure of chlorophyll a and b Energy transduction The energy of excitation, in raising an electron to a higher energy orbital, dramatically changes the standard reduction potential, Eo', of the pigment such that it becomes a much more effective electron donor. The energy of excitation, in raising an electron to a higher energy orbital, dramatically changes the standard reduction potential, Eo', of the pigment such that it becomes a much more effective electron donor. Reaction of this excited-state electron donor with an electron acceptor leads to the transformation, or transduction, of light energy (photons) to chemical energy (reducing power, the potential for electron-transfer reactions. Reaction of this excited-state electron donor with an electron acceptor leads to the transformation, or transduction, of light energy (photons) to chemical energy (reducing power, the potential for electron-transfer reactions. Transduction of light energy into chemical energy, the photochemical event, is the essence of photosynthesis. Transduction of light energy into chemical energy, the photochemical event, is the essence of photosynthesis. Role of chlorophyll Chlorophyll molecules are photochemically reactive, and it led to the concept that photosynthesis occurs in functionally discrete units. Chlorophyll molecules are photochemically reactive, and it led to the concept that photosynthesis occurs in functionally discrete units. Chlorophyll serves two roles in photosynthesis. Chlorophyll serves two roles in photosynthesis. 1-It is involved in light harvesting and the transfer of light energy to photoreactive sites by exciton transfer. 1-It is involved in light harvesting and the transfer of light energy to photoreactive sites by exciton transfer. 2- It participates directly in the photochemical events whereby light energy becomes chemical energy. 2- It participates directly in the photochemical events whereby light energy becomes chemical energy. A photosynthetic unit can serve as an antenna of several hundred light-harvesting chlorophyll molecules plus a special pair of photochemically reactive chlorophyll a molecules called the reaction center. A photosynthetic unit can serve as an antenna of several hundred light-harvesting chlorophyll molecules plus a special pair of photochemically reactive chlorophyll a molecules called the reaction center. Chlorophyll in plants is excited by visible light, no flourescence or heat is observed. Chlorophyll in plants is excited by visible light, no flourescence or heat is observed. The high energy electron moves from the excited chlorophyll molecule to the first components of a chain electron carriers leading to the generation of NADPH. H+ which coupled to form ATP. The high energy electron moves from the excited chlorophyll molecule to the first components of a chain electron carriers leading to the generation of NADPH. H+ which coupled to form ATP. Photosystem I and II Two light reactions participate in oxygen-evolving photosynthetic cells, one using light of wavelength 700 nm and the other using light of wavelength 680 nm or less. Two light reactions participate in oxygen-evolving photosynthetic cells, one using light of wavelength 700 nm and the other using light of wavelength 680 nm or less. The existence of two light reactions established the presence of two photosystems, I and II. The existence of two light reactions established the presence of two photosystems, I and II. Photosystem I) PSI): is defined as containing reaction center chlorophylls with maximal red light absorption at 700 nm; PSI is not involved in oxygen evolution. Photosystem I) PSI): is defined as containing reaction center chlorophylls with maximal red light absorption at 700 nm; PSI is not involved in oxygen evolution. Photosystem II (PSII): functions in oxygen evolution, using reaction centers that exhibit maximal red light absorption at 680 nm. Photosystem II (PSII): functions in oxygen evolution, using reaction centers that exhibit maximal red light absorption at 680 nm. Components of Photosystem I An Oxygen-Evolving Complex in PSII Regenerates P680 In a reaction center, two integral proteins, D1 and D2, bind the special-pair chlorophylls 680,two other chlorophylls (Chl), two pheophytins (Pheo), one Fe atom, and two quinones (QA and QB). All of these are used for electron transport following light absorption by an associated light harvesting complex In a reaction center, two integral proteins, D1 and D2, bind the special-pair chlorophylls 680,two other chlorophylls (Chl), two pheophytins (Pheo), one Fe atom, and two quinones (QA and QB). All of these are used for electron transport following light absorption by an associated light harvesting complexproteinschlorophylls electron transportproteinschlorophylls electron transport Three extrinsic proteins) 23, 33 and 17 kDa) comprise the oxygen-evolving complex; they bind the four Mn ions and the Ca and Cl ions that function in the splitting of H 2 O, and they maintain the environment essential for high rates of O2 evolution. Three extrinsic proteins) 23, 33 and 17 kDa) comprise the oxygen-evolving complex; they bind the four Mn ions and the Ca and Cl ions that function in the splitting of H 2 O, and they maintain the environment essential for high rates of O2 evolution.extrinsic proteinsextrinsic proteins Z is tyrosine residue 161 of the D1 polypeptide; it conducts electrons from the Mn atoms to the oxidized reaction-center chlorophyll P680+ reducing it to the ground state P680. Z is tyrosine residue 161 of the D1 polypeptide; it conducts electrons from the Mn atoms to the oxidized reaction-center chlorophyll P680+ reducing it to the ground state P680.polypeptide Role of photosystems I and II Photosystem I: Provides reducing power in the form of NADPH. Photosystem I: Provides reducing power in the form of NADPH. Photosystem II: Splits water, producing oxygen, and feeds the electrons released into an electron transport chain that couples PSII to PSI. Photosystem II: Splits water, producing oxygen, and feeds the electrons released into an electron transport chain that couples PSII to PSI. Electron transfer between PSII and PSI pumps protons for chemiosmotic ATP synthesis. Electron transfer between PSII and PSI pumps protons for chemiosmotic ATP synthesis. Photosynthesis involves the reduction of NADP + using electrons derived from water and activated by light hv. Photosynthesis involves the reduction of NADP + using electrons derived from water and activated by light hv. ATP is generated in the process. The standard reduction potential for the NADP/NADPH couple is V. Thus, a strong reductant with Eo' more negative than V is required to reduce NADP+ under standard conditions. The standard reduction potential for the NADP/NADPH couple is V. Thus, a strong reductant with Eo' more negative than V is required to reduce NADP+ under standard conditions. By similar reasoning, a very strong oxidant will be required to oxidize water to oxygen because( O 2 /H 2 o) is V. By similar reasoning, a very strong oxidant will be required to oxidize water to oxygen because( O 2 /H 2 o) is V. Separation of the oxidizing and reducing aspects of photosynthesis is accomplished in nature by devoting PSI to NADP + reduction and PSII to water oxidation. Separation of the oxidizing and reducing aspects of photosynthesis is accomplished in nature by devoting PSI to NADP + reduction and PSII to water oxidation. PSI and PSII are linked via an electron transport chain so that the weak reductant generated by PSII can provide an electron to reduce the weak oxidant side of P700. PSI and PSII are linked via an electron transport chain so that the weak reductant generated by PSII can provide an electron to reduce the weak oxidant side of P700. Thus, electrons flow from H 2 O to NADP + driven by light energy absorbed at the reaction centers. Oxygen is a by-product of the photolysis (light-splitting of water(. Thus, electrons flow from H 2 O to NADP + driven by light energy absorbed at the reaction centers. Oxygen is a by-product of the photolysis (light-splitting of water(. Accompanying electron flow is production of a proton gradient and ATP synthesis. Accompanying electron flow is production of a proton gradient and ATP synthesis. This light-driven phosphorylation is termed photophosphorylation. This light-driven phosphorylation is termed photophosphorylation.