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

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

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

If a pigment absorbs light energy, one of three things will occur. If a pigment absorbs light energy, one of three things will occur. 1- Energy is dissipated as heat. 2- The energy may be emitted immediately as a longer wavelength, a phenomenon known as fluorescence. 3- Energy may trigger a chemical reaction, as in photosynthesis. Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane (as in a chloroplast) Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane (as in a chloroplast)

The structure of the chloroplast and photosynthetic membranes The thylakoid is the structural unit of photosynthesis.thylakoid Thylakoids are stacked like pancakes in stacks known collectively as grana. The areas between grana are referred to as stroma.granastroma The chloroplast has three membranes, forming three compartments.

Stages of Photosynthesis Photosynthesis is a two stage process. The first process is the Light Dependent Process (Light reaction), requires the direct energy of light to make energy carrier molecules that are used in the second process. Photosynthesis is a two stage process. The first process is the Light Dependent Process (Light reaction), requires the direct energy of light to make energy carrier molecules that are used in the second process. The Light Independent Process (Dark reaction) occurs when the products of the Light Reaction are used to form C-C covalent bonds of carbohydrates. The Light Independent Process (Dark reaction) occurs when the products of the Light Reaction are used to form C-C covalent bonds of carbohydrates. The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts. The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts.

Light Reaction Dark Reaction

Energy transduction The energy of excitation, in raising an electron to a higher energy orbital, dramatically changes the standard reduction potential E o 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 E o 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 photo-reactive sites by excitation transfer. 1-It is involved in light harvesting and the transfer of light energy to photo-reactive sites by excitation 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 O 2 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 O 2 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

Photophosphorylation 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 from a molecule of P680 that is part of Photosystem II. Light energy causes the removal of an electron from a molecule of P680 that is part of Photosystem 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 O 2 that is released. The electron is "boosted" to a higher energy state and attached to a primary electron acceptor, which begins a series of redox reactions, passing the electron through a series of electron carriers, eventually attaching it to a molecule in Photosystem I. The electron is "boosted" to a higher energy state and attached to a primary electron acceptor, which begins a series of redox reactions, passing the electron through a series of electron carriers, eventually attaching it to a molecule in 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.

Continue.. The electron from Photosystem II replaces the excited electron in the P700 molecule. There is thus a continuous flow of electrons from water to NADPH. The electron from Photosystem II replaces the excited electron in the P700 molecule. There is thus a continuous flow of electrons from water to NADPH. Cyclic Electron Flow occurs in some eukaryotes and primitive photosynthetic bacteria. No NADPH is produced, only ATP. Cyclic Electron Flow occurs in some eukaryotes and primitive photosynthetic bacteria. No NADPH is produced, only ATP. This occurs when cells may require additional ATP, or when there is no NADP + to reduce to NADPH. This occurs when cells may require additional ATP, or when there is no NADP + to reduce to NADPH.

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.