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PHOTOSYNTHESIS

PHOTOSYNTHESIS. An anabolic process in which carbohydrates are synthesized from CO 2 and H 2 O by green plants using radiant energy of the Sun, oxygen being a byproduct. 6CO 2 +12H 2 O → C 6 H 12 O 6 +6O 2 +6H 2 O Autotrophs, Heterotrophs Raw materials Products

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PHOTOSYNTHESIS

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  1. PHOTOSYNTHESIS An anabolic process in which carbohydrates are synthesized from CO2 and H2O by green plants using radiant energy of the Sun, oxygen being a byproduct. 6CO2+12H2O → C6H12O6 +6O2 +6H2O • Autotrophs, Heterotrophs • Raw materials • Products • Energy source • Site of photosynthesis - Chloroplast

  2. Each mesophyll cell has 20 – 100 chloroplasts. 3 – 10 micro meter in length, 1 – 5 micro meter diameter. Each chloroplast – 40 – 60 grana Each granum – 50 or more thylakoids.

  3. Outer surface of thylakoid membrane is in contact with stroma .Inner surface is in contact with thylakoid lumen. - photolysis of water, electron carrier involved in light reactions, light absorbing pigments, enzymes, etc. • Stroma contains molecules involved in dark reaction.

  4. Photosynthetic pigments: • Primary pigments- Chlorophyll a C55H72O5N4Mg – red and blue light. has methyl group – CH3 • Accessory pigments- Chlorophyll b C55H70O6N4Mg – other wave lengths has aldehyde group- CHO Carotenoids – β carotene, Xanthophylls

  5. Photosystems - R Emerson in 1957 • Consists of around 200 mol. of chlorophylls and about 50 mol. of Carotenoids. • Consists of photochemical reaction center having chlorophyll a with specific protein. • Other pigment molecules are light harvesting mol. or antenna mol. • A single thylakoid possess hundreds of photosystems.

  6. Photosystem I [P700 ] made of chlorophyll a, b - absorbs red light of 700nm • Photosystem II [P680 ] responds better to slightly shorter wavelength. • Only reaction centers give up energized electrons. • Water is the source of H to reduce carbon and molecular O2 is released during photosynthesis.

  7. LIGHT DEPENDENT REACTIONS • Photo excitation of chlorophyll chlo. → chlo+ (oxidized) + e- • Photolysis or Photo oxidation of water 2H2O → 4H+ + 4e- + O2 ↑ • Photophosphorylation and Electron transport.

  8. The Stroma/ Carbon/ dark reactions: • Melvin Calvin (Nobel laureate 1961), J A Bassham, A A Benson • Independent of light • Requires 18 ATPs, 12 NADPH from light reactions, CO2 from surroundings, Ribulose1,5-biphosphate, enzymes found in stroma.

  9. Fixation of CO2 by Ru 1,5-BP: • Ru 1,5- BP acts as an acceptor molecule for CO2 from air. • Carboxylation is catalyzed by RuBP Carboxylase Rubisco- most abundant protein on earth. • The 5 C sugar Ru1,5BP becomes an unstable 6 C compound 2-Carboxy-3-Ketoarabinitol-1,5 bisphosphate.

  10. Splitting • Unstable 6 C compound hydrolyzed into 2 molecules of 3-phosphoglycerate-first stable product of dark rxn, hence called C3 cycle and the plants, C3 plants.

  11. Second Phosphorylation : • 3PGA receives a phosphate group from ATP and becomes 1,3- Bisphosphoglycerate. Reduction : • 1,3- Bisphosphoglycerate accepts Hydrogen from NADPH and gets reduced into energy rich 3C sugar Glyceraldehyde-3- Phosphate or Phosphoglyceraldehyde (PGAL). * Reduction of 6CO2 forms12 PGAL in the stroma.

  12. Regeneration of Ru1,5BP : • Out of 12 PGAL , 10 molecules are used in regenerating Ru1,5BP. Uses 6ATPs and formation of intermediate compounds in the form of 3,4,5,6 and 7 C sugars. * 6 revolutions of the cycle are required to produce the 6C carbohydrate.

  13. Carbohydrate Synthesis • It occurs in the cytosol • Two molecules of PGAL is transported from stroma to the cytosol by a phosphate translocator – an antiport protein. • It exchanges two PGAL for two molecules of Pi across the chloroplast membrane. • Most Glucose is converted into transport sugar sucrose or storage sugar starch.

  14. C4 Photosynthesis ( Hatch – Slack Pathway) • MD Hatch and CR Slack – 1966, Mainly in monocots with the first product 4C molecule oxaloacetate. • C4 plants exhibit a unique leaf anatomy called Kranz (= ahalo), where there are two rings of cells around a vascular bundle, the inner bundle sheath cells and outer mesophyll cells. • The bundle sheath cells have rudimentary chloroplasts and mesophyll cells well developed chloroplasts – dimorphic.

  15. C4 pathway starts in the mesophyll cells where carbon dioxide condenses with 3C compound phosphoenol pyruvate (PEP). To produce 4C oxaloacetate, which is reduced to malate. • Malate enters the bundle sheath cells and is decarboxylated into pyruvate and CO2. • The pyruvate returns to mesophyll cells to form PEP again and repeat the process.

  16. Advantages • C4 plants have higher affinity for CO2 and hence the plants can carry out photosynthesis even at low CO2 concentrations. • Carry out rapid photosynthesis at higher temperatures. • Avoid photorespiration in which CO2 is released without ATP synthesis.

  17. Crassulacean Acid Metabolism (CAM) • Adaptation seen in succulents • First discovered in the family Crassulaceae. • In these plants the stomata is scotactive – closed during day and open at night. • Water loss is prevented and photorespiration cannot occur. • In the night when stomata opens at cooler temperatures CO2 is fixed by C4 pathway.

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