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Plant Physiology and Plant Development

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Plant Physiology and Plant Development Plant Physiology and Plant Development PTS 351 B.Sc. B.Ed. Semester – VI Course Instructor Dr. Gautam Kumar Dr. Gautam Kr. Dept. of Life Sc. 1 Carbon Assimilation Light absorption and energy conversion Carbon dioxide uptake and assimilation Calvin Cycle (C3 Pathway) Hatch-Slack pathway (C4 Pathway) Photorespiration (C2 Pathway)/Glycolate metabolism Dr.Gautam Kr. Dept. of Life Sc. 2 • Visible light is electromagnetic radiation of wavelengths 400 to 700 nm, a small part of the electromagnetic spectrum ranging from violet to red. • The energy of a single photon (a quantum of light) is greater at the violet end of the spectrum than at the red end; shorter wavelength (and higher frequency) corresponds to higher energy. Dr.Gautam Kr. Dept. of Life Sc. 3 Solar energy as the ultimate source of all biological energy Photosynthetic organisms use the energy of sunlight to manufacture glucose and other organic products, which heterotrophic cells use as energy and carbon sources. The light reactions of photosynthesis generate energy rich NADPH and ATP at the expense of solar energy. These products are used in the carbon assimilation reactions, which occur in light or darkness, to reduce CO2 to form trioses and more complex compounds (such as glucose) derived from trioses. Dr.Gautam Kr. Dept. of Life Sc. 4 Absorption of visible light by Photo-pigments Plants are green because their pigments absorb light from the red and blue regions of the spectrum, leaving primarily green light to be reflected or transmitted Absorption spectra of the pigments with the spectrum of sunlight reaching the earth’s surface Dr.Gautam Kr. Dept. of Life Sc. 5 The most important light-absorbing pigments in the thylakoid membranes are the chlorophylls, green pigments with polycyclic, planar structures resembling the protoporphyrin of hemoglobin except that Mg+2, not Fe+2, occupies the central position “E” five-membered ring not present in heme 4- Porphyrin Ring with a Mg+2 Ester Bond atom & Long Phytol Chain (Tetrapyrroles) Dr.Gautam Kr. Dept. of Life Sc. 6 Chlorophyll a • The wavelength of light that is most effective in supporting photosynthesis. • Slide bacteria known to migrate toward • which pigments can channel energy into Dr.Gautam Kr. Dept. of Life Sc. 7 regions of high O2 concentration photosynthesis Stroma Grana Organization of proteins on thylakoid membrane Dr.Gautam Kr. Dept. of Life Sc. 8 • Antenna molecules: An antenna molecule is a light absorbing accessory pigment molecule. They are called antenna molecules because they absorb light energy • Absorption of a photon excites chlorophyll molecules and other (accessory) pigments, which funnel the energy into reaction centers in the thylakoid membranes. Dr.Gautam Kr. Dept. of Life Sc. 9 Light Dependent Reactions • The light dependent reactions take place in and around the thylakoid membrane of the granna. • In Chloroplasts the photosynthetic pigments (chlorophyll-a, chlorophyll-b and caratinoids) are clustered in membrane bound structures called the photosystem I (PSI) and photosystem II (PSII). • Electrons are transported from PSII to PSI via the electron transport chain. • The electron transport chain also translocates protons from the stroma into the intrathylakoid space. The ATP synthase is responsible for the synthesis of ATP via a mechanism called "chemiosmosis.“ • Light stimulates chlorophyll to release electrons, which results in the production of ATP • Light energy also splits water molecules (photolysis), producing oxygen and hydrogen • The hydrogen is taken up by a hydrogen carrier (NADP+) to form NADPH • The splitting of water also releases electrons, which replace those lost by the chlorophyll • The ATP and hydrogen (NADPH) are taken to the site of the light independent reactions Dr.Gautam Kr. Dept. of Life Sc. 10 Z - scheme Pheophytin: (chlorophyll lacking the central Mg+2 ion) Photosystem II (PSII): is a pheophytin-quinone type of system containing roughly equal amounts of chlorophylls a and b. Photosystem I (PSI): is reaction center designated P700 and a DCMU high ratio of chlorophyll a to chlorophyll b. • DCMU herbicide block flow of electron from PSII to Cyt-b6f • Excitation of its reaction center P680 drives electrons through the cytochrome b6f complex with concomitant movement of protons across the thylakoid membrane. • Excited P700 passes electrons to the Fe-S protein ferredoxin, then to NADP, producing Dr.Gautam Kr. Dept. of Life Sc. 11 NADPH. Light independent reaction The Calvin-Benson cycle is used to fix Carbon dioxide into organic compounds A component of the Calvin-Benson cycle (Glyceraldehyde phosphate) is removed from the cycle and used to synthesize glucose The reactions of the Calvin-Benson cycle require ATP and NADPH which are synthesized in the light dependent reactions The high energy electrons in NADPH are transfered to the intermediates of the CalvinDr.Gautam -Kr.Benson Dept. of Life Sc. cycle 12 Three stages of CO2 assimilation in photosynthetic organisms Dr.Gautam Kr. Dept. of Life Sc. 13 First Step: Carboxylation Dr.Gautam Kr. Dept. of Life Sc. 14 Second step: Transfer of a phosphoryl group from ATP to 3-phosphoglycerate (cofactor) * glyceraldehyde 3-phosphate dehydrogenase present in chloroplasts can use NADP but isozymes in cytoplasm use NAD Stromal and Cytosolic enzymes are isozymes; both sets of enzymes catalyze the same reactions, but they are the products of different genes The chloroplast Stroma contains all the glycolytic enzymes except Phosphoglycerate mutase Triose phosphate isomerase then interconverts glyceraldehyde 3-phosphate and dihydroxyacetone phosphate Dr.Gautam Kr. Dept. of Life Sc. 15 Third Step: Regeneration of Ribulose 1,5-Bisphosphate from Triose Phosphates Most of the triose phosphate thus produced is used to regenerate ribulose 1,5- bisphosphate Intermediates in this pathway include three-, four-, five-, six-, and seven-carbon sugars The rest is either converted to starch in the chloroplast and stored for later use or immediately exported to the cytosol and converted to sucrose for transport to growing regions of the plant Reversible Condensation (Aldolase) 16 Dr.Gautam Kr. Dept. of Life Sc. /(FBPase-1) Irreversible General reaction catalyzed by transketolase: the transfer of a two-carbon group, carried temporarily on enzyme- bound TPP, from a ketose donor to an aldose acceptor Transketolase Catalyzed reactions of the Calvin cycle. * Transketolase requires the cofactor thiamine pyrophosphate 17 Dr.Gautam Kr. Dept. of Life Sc. (Thiamine pyrophosphate (TPP) Unique enzyme to plastids TPP as a cofactor for transketolase Transketolase transfers a two-carbon group from sedoheptulose 7-phosphate to glyceraldehyde 3-phosphate, producing two pentose phosphates 18 Dr.Gautam Kr. Dept. of Life Sc. Rubisco can incorporate O2 rather than CO2 into ribulose 1,5-bisphosphate The unstable intermediate thus formed splits into 2-phosphoglycolate and 3- phosphoglycerate, which can re-enter the Calvin cycle The glycolate pathway converts two molecules of 2-phosphoglycolate to a molecule of serine (three carbons) and a molecule of CO2 In the chloroplast, a phosphatase converts 2-phosphoglycolate to glycolate, which is exported to the peroxisome In Peroxisome glycolate is oxidized by molecular oxygen, and the resulting aldehyde (glyoxylate) undergoes transamination to glycine Dr.Gautam Kr. Dept. of Life Sc. 19 Oxygenase activity of Rubisco Glycolate pathway (C2 cycle) • The unstable intermediate thus formed splits into 2- phosphoglycolate and 3-phosphoglycerate, which can re enter the Calvin cycle. • Formation of 3PGA from Glycolic acid, Glyoxylate, Glycine and Serine 20 Dr.Gautam Kr. Dept. of Life Sc. Glycine passes from the peroxisome to the mitochondrial matrix, where it undergoes oxidative decarboxylation by the glycine decarboxylase complex The glycine decarboxylase complex oxidizes glycine to CO2 and NH3, with the concomitant reduction of NAD+ to NADH and transfer of the remaining carbon from glycine to the cofactor tetrahydrofolate * Glycine decarboxylase in plant mitochondria is a complex of four types of subunits, with the stoichiometry P4H27T9L2 Glycine decarboxylase complex Dr.Gautampyridoxal Kr. Dept. of phosphate Life Sc. (PLP) 21 The serine is converted to hydroxypyruvate, to glycerate, and finally to 3- phosphoglycerate, which is used to regenerate ribulose 1,5-bisphosphate, completing the long, expensive cycle In bright sunlight, the flux through the glycolate salvage pathway can be very high, producing about five times more CO2 than is typically produced by all the oxidations of the citric acid cycle. In non-photosynthetic parts of a plant, such as potato tubers, mitochondria have very low concentrations of the glycine decarboxylase complex. The combined activity of the rubisco oxygenase and the glycolate salvage pathway consumes O2 and produces CO2—hence the name photorespiration This pathway is perhaps better called the oxidative photo-synthetic carbon cycle / C2 cycle, names that do not invite comparison with respiration in mitochondria Unlike mitochondrial respiration, “photorespiration” does not conserve energy and may actually inhibit net biomass formation as much as 50% Dr.Gautam Kr. Dept. of Life Sc. 22 Glycolate pathway (C2 cycle) • Formation of 3PGA from Glycolic acid, Glyoxylate, Glycine and Serine • The combined activity of the rubisco oxygenase and the glycolate salvage pathway consumes O2 and produces CO2 hence the name photorespiration. • This pathway is perhaps better called the oxidative photosynthetic
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