Photosynthesis PDF

Photosynthesis Dr. R. Ravindhran Photosynthesis Photosynthesis is the process used by plants, algae and certain bacteria to harness energy from sunlight into chemical energy. Photosynthesis Oxygenic Anoxygenic Oxygenic photosynthesis Most common and is...

Photosynthesis Dr. R. Ravindhran Photosynthesis Photosynthesis is the process used by plants, algae and certain bacteria to harness energy from sunlight into chemical energy. Photosynthesis Oxygenic Anoxygenic Oxygenic photosynthesis Most common and is seen in plants, algae and cyanobacteria. Light energy transfers electrons from water (H2O) to carbon dioxide (CO2), which produces carbohydrates. In this transfer, the CO2 is "reduced," or receives electrons, and the water becomes "oxidized," or loses electrons. Ultimately, oxygen is produced along with carbohydrates. Anoxygenic photosynthesis The process typically occurs in bacteria such as purple bacteria and green sulfur bacteria. Anoxygenic photosynthesis uses electron donors other than water. Produces sulphur when hydrogen sulfide is used as donor Significance i. Helps in conversion of solar energy into organic matter. ii. Consumes atmospheric carbon dioxide and yields carbohydrates and molecular oxygen. iii. Evolves molecular oxygen for use by other living organisms and maintains the level of atmospheric oxygen which is continuously consumed by plants and animals during respiration. iv. Produces carbohydrates and used by plants and animals to synthesize organic acids, proteins, fats, nucleic acids, pigments, hormones, vitamins, alkaloids and other metabolites. v. Provides energy for carrying out metabolic activities and different types of movements Photosynthetic requirements Photosynthetic eukaryotic organisms contain organelles called plastids in their cytoplasm. Plastids are surrounded by double layered membrane and contain pigments or can store nutrients. Chloroplast Contain chlorophyll Chromoplast Contain carotenoids Leucoplast Non pigmented and store starch and fat Chloroplast Chloroplasts are double membrane bound organelles that perform photosynthesis. The grana is the innermost portion of the organelle; a collection of disc- shaped membranes, stacked into columns like plates. The individual discs are called thylakoids. The thylakoids contain the light-harvesting complex where the transfer of electrons takes place. The empty spaces between columns of grana constitute the stroma. Thylakoids T h e t hy l a ko i d i s t h e structural unit of photosynthesis. The photosynthetic membrane is vesicular, defining a closed space with an outer water space (stromal phase) and an inner water space (lumen). Photosynthetic membrane containing most of the pigments and proteins re q u i re d fo r t h e l i g ht reactions Pigments Pigments are molecules that bestow color on plants, algae and bacteria, but they are also responsible for effectively trapping sunlight. Pigments of different colors absorb different wavelengths of light. Chlorophyll Carotenoids Phycobilins Pigments Chlorophylls are green-colored pigments capable of trapping blue and red light. Chlorophylls have three sub-types, namely chlorophyll a, chlorophyll b and chlorophyll c. Carotenoids are red, orange, or yellow-colored pigments that absorb bluish-green light. Phycobilins are red or blue pigments which absorb wavelengths of light that are not as well absorbed by chlorophylls and carotenoids. Phycobilins are seen in cyanobacteria and red algae. Absorption of light energy Plant photosynthesis is driven primarily by visible light (wavelengths from 400 to 700 nm) that is absorbed by pigment molecules (mainly chlorophyll a and b and carotenoids). Light is collected by 200-300 pigment molecules that are bound to light- harvesting protein complexes located in the photosynthetic membrane. The light-harvesting complexes surround the reaction centers that serve as an antenna. Absorption of light energy Photosynthesis is initiated by the absorption of a photon by an antenna molecule, which occurs in about a femtosecond (10-15 s) and causes a transition from the electronic ground state to an excited state. Within 10-13 s the excited state decays by vibrational relaxation to the first excited singlet state. Absorption of light energy Because of the proximity of other antenna molecules with the same or similar energy states, the excited state energy has a high probability of being transferred by resonance energy transfer to a near neighbor Over 90% of the absorbed quanta are transferred within a few hundred picoseconds from the antenna system to the reaction center which acts as a trap for the exciton. Chlorophyll fluorescence Some excitons are converted back into photons and emitted as fluorescence. These photons are emitted immediately as a longer wavelength Used as indicator of photosynthetic energy conversion in higher plants, algae and bacteria. Fluorescence can give insights into the ability of a plant to tolerate environmental stresses and into the extent to which those stresses have damaged the photosynthetic apparatus. Chlorophyll phosphorescence Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. Phosphorescence corresponds to light emissions accompanying radiative electronic transitions from triplet to ground states of pigment molecules. Photosynthesis Light Dark reaction reaction Light Dependent Process (Light Reactions), requires the direct energy of light to make energy carrier molecules that are used in the second process Light Independent Process (or Dark Reactions) 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.  Light reaction In the Light Dependent Processes (Light Reactions) light strikes chlorophyll a in such a way as to excite electrons to a higher energy state. In a series of reactions the energy is converted (along an electron transport process) into ATP and NADPH. Water is split in the process, releasing oxygen as a by-product of the reaction. The ATP and NADPH are used to make C-C bonds in the Light Independent Process (Dark Reactions). Light reaction Light-dependent reactions take place on the thylakoid membranes contains four major integral membrane protein complexes that catalyze the light reactions Photosystem II (PSII), Cytochrome b6f complex, Photosystem I (PSI), and ATP synthase. Light reaction The two photosystems absorb light energy through pigments - primarily the chlorophylls The light-dependent reactions begin in photosystem II. When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, an electron in this molecule attains a higher energy level. As the electron is very unstable, the electron is transferred from one to another molecule creating a chain of redox reactions, called an electron transport chain (ETC). Light reaction The electron flow goes from PSII to cytochrome b6f to PSI In PSI, the electron gets the energy from another photon. The final electron acceptor is NADP. Cytochrome b6f and ATP synthase work together to create ATP. This process is called photophosphorylation, which occurs in two different ways. Non-cyclic Cyclic Photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis Within the photosystem, enzymes capture photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce plastoquinone to plastoquinol. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen. The hydrogen ions (protons) generated by the oxidation of water help to create a proton gradient that is used by ATP synthase to generate ATP. Cytochrome b6f complex The cytochrome b6f complex (plastoquinol—plastocyanin reductase) is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae. During photosynthesis, the cytochrome b6f complex transfers electrons from Photosystem II to Photosystem I, whereby pumping protons into the thylakoid space and creating an electrochemical (energy) gradient where it is later used to create adenosine triphosphate (ATP). Photosystem I Photosystem I (PS I, or plastocyanin: ferredoxin oxidoreductase) is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. The electron transfer components of the reaction center of PSI are a primary electron donor P-700 (chlorophyll dimer) and five electron acceptors: A0 (chlorophyll), A1 (a phylloquinone) and three 4Fe-4S iron-sulphur centres: Fx, Fa, and Fb Mediates electron transfer from plastocyanin to ferredoxin. Ferredoxin (Fd) is a soluble protein that facilitates reduction of NADP+ to NADPH through Ferredoxin-NADP+ reductase (FNR) ATP synthase ATP synthase is an important enzyme that creates the energy storage molecule adenosine triphosphate (ATP). ATP synthase consists of two regions the FO portion embedded within the membrane. the F1 portion of the ATP synthase is outside the membrane The proton gradient across the thylakoid membrane creates a proton- motive force, used by ATP synthase to form ATP Cyclic photophosphorylation In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from not only PSII but also PSI to create more ATP and to stop the production of NADPH. Cyclic phosphorylation is important to create ATP and maintain NADPH in the right proportion for the light- independent reactions. Calvin cycle The Calvin cycle is a process that plants and algae use to turn carbon dioxide from the air into sugar. The Calvin cycle has also been called carbon fixation reaction or dark reaction. The Calvin cycle occurs in the stroma and has four main steps: 1. carbon fixation, 2. reduction phase, 3. carbohydrate formation, and 4. regeneration phase. Energy to fuel chemical reactions in this sugar-generating process is provided by ATP and NADPH produced in light reactions. Carbon fixation The enzyme RuBisCO catalyses the carboxylation of ribulose-1,5-bisphosphate, RuBP, a 5-carbon compound, by carbon dioxide (a total of 6 carbons) in a two-step reaction. The product of the first step is enediol-enzyme complex that can capture CO2 or O2. Thus, enediol-enzyme complex is the real carboxylase/oxygenase. The CO2 that is captured by enediol in second step produces a six-carbon intermediate initially that immediately splits in half, forming two molecules of 3-phosphoglycerate, or 3-PGA, a 3- carbon compound (also: 3-phosphoglyceric acid, PGA, 3PGA). Carbon reduction The enzyme phosphoglycerate kinase catalyses the phosphorylation of 3-PGA by ATP. 1,3-Bisphosphoglycerate (1,3BPGA, glycerate-1,3-bisphosphate) and ADP are the products. The enzyme glyceraldehyde 3-phosphate dehydrogenase catalyses the reduction of 1,3BPGA by NADPH. Glyceraldehyde 3-phosphate is produced, and the NADPH itself is oxidized and becomes NADP+. Regeneration phase 1. Tr i o s e p h o s p h a t e isomerase converts all of the G3P reversibly into dihydroxyacetone phosphate (DHAP), also a 3-carbon molecule. 2. A l d o l a s e and fructose-1,6- bisphosphatase convert a G3P and a DHAP into fructose 6- phosphate (6C). A phosphate ion is lost into solution. Regeneration phase 3. F6P has two carbons removed by transketolase, giving erythrose-4- phosphate. The two carbons on t r a n s ke t o l a s e a r e added to a G3P, giving the ketose xylulose-5- phosphate (Xu5P). 4. E4P and a DHAP are converted into sedoheptulose-1,7- bisphosphate (7C) by aldolase enzyme. Regeneration phase 5. S e d o h e p t u l o s e - 1 , 7 - bisphosphatase cleaves sedoheptulose-1,7- bisphosphate into sedoheptulose-7- phosphate, releasing an inorganic phosphate ion into solution. 6. The ketose S7P has two carbons removed by transketolase, giving ribose-5-phosphate ( R 5 P ) , a n d t h e t wo carbons remaining on transketolase are transferred to one of the G3P, giving another Xu5P. Regeneration phase 7. R5P is converted into ribulose-5-phosphate ( R u 5 P, R u P ) b y phosphopentose isomerase. Xu5P is converted into RuP by phosphopentose epimerase. 8. F i n a l l y , phosphoribulokinase (another plant-unique enzyme of the p a t h w a y ) phosphorylates RuP into R u B P, ribulose-1,5- bisphosphate, completing the Calvin cycle. This requires the input of one ATP. Regeneration phase Thus, of six G3P produced, five are used to make three RuBP (5C) molecules (totaling 15 carbons), with only one G3P available for subsequent conversion to hexose. This requires nine ATP molecules and six NADPH molecules per three CO2 molecules. Photorespiration RuBisCO also reacts competitively with O2 instead of CO2 in photorespiration. The rate of photorespiration is higher at high temperatures. Photorespiration turns RuBP into 3- PGA and 2-phosphoglycolate, a 2-carbon molecule that can be converted via glycolate and glyoxalate to glycine. Via the glycine cleavage system and tetrahydrofolate, two glycines are converted into serine+CO2. Serine can be converted back to 3-phosphoglycerate. Photorespiration has very negative consequences for the plant, because, rather than fixing CO2, this process leads to loss of CO2 C4 carbon fixation C4 carbon fixation is named for the 4-carbon molecule oxaloacetate present in the first product of carbon fixation C4 carbon fixation evolved more recently to circumvent photorespiration, but occurs only in certain plants native to very warm or tropical climates – Eg. corn This is achieved in a more efficient environment for RubisCo by shuttling CO2 via malate or aspartate from mesophyll cells to bundle-sheath cells. In these bundle-sheath cells, RuBisCO is isolated from atmospheric oxygen and saturated with the CO2 released by decarboxylation of the malate. C4 leaf anatomy The C4 plants often possess a characteristic leaf anatomy called kranz anatomy Their vascular bundles are surrounded by two rings of cells; the inner ring, called bundle sheath cells, contains starch-rich chloroplasts lacking grana, which differ from those in mesophyll cells present as the outer ring. Hence, t h e c h l o ro p l a s t s a re c a l l e d dimorphic. C4 leaf anatomy The primary function of kranz anatomy is to provide a site in which CO2 can be concentrated around RuBisCO, thereby avoiding photorespiration. In order to maintain a significantly higher CO2 concentration in the bundle sheath compared to the mesophyll, the boundary layer of the kranz has a low conductance to CO 2 , a property that may be enhanced by the presence of suberin. C4 pathway C4 carbon fixation was elucidated by Marshall Davidson Hatch and C. R. Slack, in Australia, in 1966; it is sometimes called the Hatch-Slack pathway. The first step in the pathway is the conversion of pyruvate to phosphoenolpyruvate (PEP), by the enzyme pyruvate orthophosphate dikinase. The next step is the fixation of CO2 into oxaloacetate by the enzyme PEP carboxylase. Both of these steps occur in the mesophyll cells. C4 pathway O2 is a very poor substrate for PEP carboxylase. Thus, at relatively low concentrations of CO2, most CO2 will be converted into bicarbonate and fixed by this pathway. The product is usually converted to malate, a simple organic compound, which is transported to the bundle-sheath cells surrounding a nearby vein. Here, it is decarboxylated by malic enzyme to produce CO2 and pyruvate. The CO2 now enters the Calvin cycle and the pyruvate is transported back to the mesophyll cell. In the mesophyll chloroplasts, the enzyme pyruvate orthophosphate dikinase (PPDK) uses ATP and Pi to convert pyruvate back to PEP, completing the C4 cycle. C4 pathway Since every CO2 molecule has to be fixed twice, first by 4- carbon organic acid and second by RuBisCO, the C4 pathway uses more energy than the C3 pathway. C4 pathway requires 30 molecules of ATP. There are several variants of this pathway: The 4-carbon acid transported from mesophyll cells may be malate, as above, or aspartate The 3-carbon acid transported back from bundle-sheath cells may be pyruvate, as above, or alanine The enzyme that catalyses decarboxylation in bundle-sheath cells differs. In maize and sugarcane, the enzyme is NADP-malic enzyme; in millet, it is NAD-malic enzyme; and, in Panicum maximum, it is PEP carboxykinase. NADP-malic enzyme pathway NAD-malic enzyme pathway PEP carboxykinase pathway Crassulacean acid metabolism (CAM) This pathway is named after the Crassulaceae, a family in which many species display this type of metabolism, but it also occurs commonly in other families, such as the Cactaceae, the Euphorbiaceae, the Orchidaceae, and the Bromeliaceae. CAM plants are known for their capacity to fix carbon dioxide at night, using PEP carboxylase as the primary carboxylating enzyme and the accumulation of malate (which is made by the enzyme malate dehydrogenase) in the large vacuoles of their cells. Deacidification occurs during the day, when carbon dioxide is released from malate and fixed in the Calvin-Benson cycle, using Rubisco. Crassulacean acid metabolism (CAM) During daylight hours, the stomata are closed to prevent water loss. The stomata are open at night when the air is cooler and more humid, and this setting allows the leaves of the plant to assimilate carbon dioxide. Since their stomata are closed during the day, CAM plants require considerably less water than both C3 and C4 plants that fix the same amount of carbon dioxide in photosynthesis. The unusual capacity of CAM plants to fix carbon dioxide into organic acids in the dark, causing nocturnal acidification, with deacidification occurring during the day, Crassulacean acid metabolism (CAM) During the night, CAM plant has its stomata open The carbon dioxide is fixed in the cytoplasm of mesophyll cells by a PEP reaction similar to that of C4 pathway. But, unlike the C4 mechanism, the resulting organic acids are stored in vacuoles for later use; that is, they are not immediately passed on to the Calvin cycle. Crassulacean acid metabolism (CAM) During the day, the CO2- storing organic acids are released from the vacuoles of the mesophyll cells and enter the stroma of the chloroplasts where an enzyme releases the CO2, which then enters into the Calvin cycle.