photosynthesis Presentation on Agriculture

photosynthesis, the process by which green plants and certain other organisms transform light energy into chemical energy. During photosynthesis in green plants, light energy is captured and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds.

1. Photosynthesis Dr Saboohi Raza Associate Professor

2. Photosynthesis Life on earth depends on the sun Plant and Cyanobacteria chemically sequester light energy through photosynthesis Photosynthesis is a light derived process CO 2 is fixed to yield carbohydrate sunlight absorbed by chlorophyll 6 CO 2 + 6 H 2 O + energy  C 6 H 12 O 6 + 6 O 2

3. PLASTIDS Plastid are the characteristic feature of almost all plants Plastids are common name given to a group of cytoplasmic organelles which possess naturally occurring coloured substances (pigments) or contain oils, proteins and carbohydrates as storage material.

4. Plastid Classification

5. Choloroplast 1-1000 per cell Typically ~5 µm long ellipsoid It resembles with mitochondria in many ways Highly permeable outer membrane Nearly impermeable inner membrane encloses Stroma , a concentrated solution of enzyme, DNA, RNA and ribosome involved in synthesis of several chloroplast protein The membranous compartment in stroma is Thylakoid Thylakoid consist of stacks of disc like sacs named Grana

6. Photosynthesis and chloroplast Two phases of photosynthesis are The Light reaction : which use light energy to convert into chemical energy (NADPH & ATP) The dark reaction : which use chemical energy (NADPH & ATP) for the synthesis carbohydrate. The light reaction occur in the thylakoid membrane which process electron transport chain like mitochondria and oxidative phosphorylation NADP+: Nicotinamide adenine dinucleotide phosphate ATP: Adenosine tri phosphate

7. Light Reaction It was demonstrated by Robert Hill (1941), therfore called as Hills reaction The light reaction can be studied as Light Absorption Two pigment system Photo-Oxidation of Water Production of assimilatory powers (ATP & NADPH)

8. Light Absorption CHLOROPHYLL: The Principal photoreceptor in photosynthesis is cholorophyll The tetrapyrrole ring system is a hydrophilic portion. It chelates a magnesium ion which, like most metal ions, has a large cloud of electrons to which it can add to or lose electrons without much problem...just a charge change. The tetrapyrrole ring system that chelates this magnesium shows a system of conjugated double bonds that expands the electron cloud by resonance of the conjugated bonds. These bonds also provide the light absorption features of the chlorophyll molecule that give it the green colour. The difference in one ring distinguishes chlorophyll a (a reaction centre pigment) from chlorophyll b

9. Chlorophyll is not the only photosynthetic pigment While chlorophyll a plays a very important role in photosynthesis, plants have additional pigments that participate in photosynthesis. These are indeed called antenna pigments. For true plants, which taxonomists are generally defining as green algae, bryophytes, ferns, and seed plants, the pigments for photosynthesis are chlorophylls a and b, carotenoids , and xanthophylls.

10. Visible light is only a small part of the electromagnetic spectrum (all forms of light).

11. LIGHT behaves as if it were composed of "units" or "packets" of energy that travel in waves. These packets are photons . The wavelength of light determines its color.

12. Mechanism of absorbance of light Absorption of light by molecules usually occur through the promotion of an electron from its ground (lower energy) state molecular orbital to one higher energy. Energy difference between the two state must be equal to energy of absorbed photons

13. Two Pigment System Two pigment or photostem (PSI and PSII) are found within thylakoids Each pigment system is made up of: Core Complex (CC) : It is polypeptied containing 40-60 chlorophyll molecule, A reaction centre, electron donor and electron acceptor Light Harvesting Complex(LHC) or Antenna Complex : chlorophyll molecules and accessory pigments

14. Pigment system I or Photostem I PSI PS I is made up of about 200-400 chlorophyll molecules; 50 carotenoid molecules; one molecule of reaction centre P 700 (chl. A 700 ) Usually number of chl. b molecules are less Reaction centre is P 700 It is located on the outer surface of thylakoid In this system O 2 is not evolve NADPH is formed during this system

15. Pigment system II or Photostem II PSII PS II is made up about 200 chlorophyll molecule; 50 carotenoid molecules; one molecule of reaction centre P 680 (chl. a 680 ) Usually the number of Chl. b molecule is more Its reaction centre is P 680 . It is located on the inner surface of thylokoid In this system molecular O 2 is evolved as a result of breakdown of water NADPH is not formed during this system It participate only in photophosphorylation . It gives electron to PSI for the reduction of NADP

16. Photo- Oxidation of water Reaction centre P 680 , PSII . H 2 0 [H] + [OH] [H] H + + e - e - +[OH] OH - Light Chlorophyll

17. 4H 2 O 4(OH - ) + 4 H + 4 (OH - ) 2H 2 O + O2 +4 e-

18. Production of assimilatory Power (ATP & NADPH) Photo Phosphorylation ADP+ Pi ATP ATP synthase Electron transport system: 12NADP +24H + +24e - 12NADPH +H +

19. How ATP and NADPH is produced

20. Photosynthesis - overview – thylakoid membrane – thylakoid space – stroma • light reactions: • dark reactions: Light H 2 O Chloroplast Light Reactions NADP + P ADP + ATP NADPH O 2 Calvin Cycle CO 2 [CH 2 O] (sugar)

21. Dark Reaction The metabolic pathway by which plants incoporate CO 2 into carbohydrate is known as Calvin Cycle

22. Glycerdehyde 3 phosphate

23. Ribulose Bisphosphate Carboxylase (RuBP Carboxylase) catalyzes CO 2 fixation: ribulose-1,5-bisphosphate + CO 2 2 copies of 3-phosphoglycerate The enzyme is also called RuBP Carboxylase/ Oxygenase ( RuBisCO ). It is the most abundant enzyme on earth

24. Alternative mechanisms of carbon fixation have evolved in hot, arid climates Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis On hot, dry days, plants close stomata, which conserves H 2 O but also limits photosynthesis The closing of stomata reduces access to CO 2 and causes O 2 to build up These conditions favor an apparently wasteful process called photorespiration

25. Photorespiration In most plants ( C 3 plants ), initial fixation of CO 2 , via rubisco , forms a three-carbon compound (3-phosphoglycerate) In photorespiration , rubisco adds O 2 instead of CO 2 in the Calvin cycle, producing a two-carbon compound Photorespiration consumes O 2 and organic fuel and releases CO 2 without producing ATP or sugar

26. Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O 2 and more CO 2 Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

27. C 4 Plants C 4 plants minimize the cost of photorespiration by incorporating CO 2 into four-carbon compounds There are two distinct types of cells in the leaves of C 4 plants: Bundle-sheath cells are arranged in tightly packed sheaths around the veins of the leaf Mesophyll cells are loosely packed between the bundle sheath and the leaf surface

28. Sugar production in C 4 plants occurs in a three-step process: The production of the four carbon precursors is catalyzed by the enzyme PEP ( Phospho enol pyruvate ) carboxylase in the mesophyll cells PEP carboxylase has a higher affinity for CO 2 than rubisco does; it can fix CO 2 even when CO 2 concentrations are low

29. These four-carbon compounds are exported to bundle-sheath cells Within the bundle-sheath cells, they release CO 2 that is then used in the Calvin cycle

30. Mesophyll cell Bundle- sheath cell Photo- synthetic cells of C 4 plant leaf Vein (vascular tissue) C 4 leaf anatomy Stoma The C 4 pathway Mesophyll cell PEP carboxylase Oxaloacetate (4C) Malate (4C) Pyruvate (3C) CO 2 ADP PEP (3C) ATP CO 2 Calvin Cycle Bundle- sheath cell Sugar Vascular tissue

31. Mesophyll cell Bundle- sheath cell Photo- synthetic cells of C 4 plant leaf Vein (vascular tissue) C 4 leaf anatomy Stoma

32. The C 4 pathway Mesophyll cell PEP carboxylase Oxaloacetate (4C) Malate (4C) Pyruvate (3C) CO 2 ADP PEP (3C) ATP CO 2 Calvin Cycle Bundle- sheath cell Sugar Vascular tissue

33. CAM Plants Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon CAM plants open their stomata at night, incorporating CO 2 into organic acids Stomata close during the day, and CO 2 is released from organic acids and used in the Calvin cycle

34. Sugarcane Pineapple C 4 CO 2 CO 2 CAM Organic acid Organic acid Night Day CO 2 CO 2 Calvin Cycle Calvin Cycle Sugar Sugar Bundle- sheath cell (a) Spatial separation of steps (b) Temporal separation of steps Mesophyll cell 2 1 1 2

35. The Importance of Photosynthesis: A Review The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits In addition to food production, photosynthesis produces the O 2 in our atmosphere