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  1. 1. PHOTOSYNTHESIS group members Maryam wahab Hina zamir Amna javed Maleeha inayat Saleha qazi Yusra shair 7th may,2015
  2. 2. OUTLINE :  Overview of mechanism of photosynthesis  Pigments  Process of photosynthesis  Phosphorylation  Photosystems  Dark reaction  C4 and CAM pathway
  3. 3. OVERVIEW OF PHOTOSYNTHESIS  A process by which autotrophic organisms use light energy to make sugar & oxygen gas from carbon dioxide & water.  Occurs in chloroplast, an organelle in mesophyll
  4. 4. MECHANISM OF PHOTOSYNTHESIS Divided into 2 phases :  Light reaction  a) photolysis of water  b)phosphorylation  Dark reaction (carbon fixation or calvin cycle)  Light reaction is light dependent & takes place in grana of chloroplast & its product is NADPH & ATP.  Dark reaction is light independent & occurs in stroma of chloroplast.
  5. 5. FOUR PHASES OF PHOTOSYNTHESIS 1) light absorption & energy delivery by antenna systems  2) primary electron transfer in reaction centers  3) energy stabilization by secondary processes  4)synthesis & export of stable products  First 3 phases makeup the light reaction & fourth encompasses the dark reaction.
  6. 6. PIGMENTS Substances that have ability to absorb specific wavelengths of light & reflect all others.  Pigments are colored.  Easily excited by light energy.  The color we see is the net effect of all the light reflecting back at us…!  photosynthetic pigments are of 3 types  1)Chlorohylls (chlorophyll a & b)  2)Accessory photosynthetic pigment or carotenoids (carotene & xanthophyll)  3) phycobilins
  7. 7. CHLOROPHYLL A  Most important pigment in photosynthesis.  Absorbs blue,red & violet wavelengths in the visible spectrum.  Formula is C55H72O5N4Mg  Complex ring structure having 2 parts i.e. head & tail  Head(porphyrin ring) : 4 complex pyrole rings of carbon & nitrogen.  In the centre of porphyrin ring a single magnesium atom is attached to the nitrogen of each pyrrole ring.  Maximum absorption by chlorophyll a occurs in blue & red regions.  Tail is attached to one of the pyrrole rings.  It is a long hydrocarbon phytol (C20H39) & anchors chlorophyll molecule in thylakoid membrane.
  9. 9. CHLOROPHYLL B  Its structure is similar to chlorophyll a but CH3 is replaced by CHO.  So molecular formula is C55H70O6N4Mg  It primarily absorbs blue light .  It is used to complement absorption spectrum of a by extending the range of light wavelengths a photosynthetic organism is able to absorb.
  10. 10. ACCESSORY PIGMENTS  They are not directly involved in LDR .  includes carotenes & xanthophylls.  carotenes are hydrocrbons with general molecular formula C40H56  They absorb wavelengths that are not efficiently absorbed by chloropyhlls .  Carotenoid is yellow to orange in color  Xanthophyll is yellow in color.  Carotenoids have 2 important roles in plants  1)transfer the light energy they capture to chlorophyll to use in the LDR.  2)Protect chlorophyll a from photo-oxidation
  12. 12. PROCESS :  Photosynthesis occur in two phases :  Phase 1 : Light reaction ( granna -thalakoid membrane )  Phase 2 : Dark reaction( stroma)  Reaction 6 CO2 + 6 H2O sunlight C6 H12 O6 + 6 O2 carbon dioxide + water = glucose(sugar ) + oxygen
  13. 13. PHASE 1.LIGHT REACTION  Also called Light Dependent Reaction which contain Photosystem I and Photosystem II.  Occurance: chloroplast (granna - thylakoid)  Chlorophyll (thylakoid) traps energy from light  Requirement light ,water NADP + ADP + Pi  Products O2 ATP & NADPH  Two steps  Energy is Capture from Sunlight.  light energy, trapped by chlorophyll, is used  Step 1 photolysis of water  Water is Split into Hydrogen Ions ,electron and Oxygen (O2). The O2 Diffuses out of the Chloroplasts (Byproduct).  Formation of reduced NADPH ( chemical energy )  Step 2 phosphorylation  The Light Energy is Converted to Chemical Energy, which is Temporarily Stored in ATP (ADP + Pi + energy )
  14. 14. PHASE 2. DARK REACTION  Other names : Calvin Cycle Light Independent Reaction Carbon Fixation or C3 Fixation  Does not require light  Location Occurs in stroma of chloroplast  Requirments ATP and NADPH as a fuel CO2 (air )  Product glucose sugar  Step CO2 fixation in glucose sugar  The Chemical Energy Stored in ATP and NADPH powers the formation of Organic Compound glucose (Sugar) using CO2
  17. 17. DEFINITION  In the process of photosynthesis, the phosphorylation of ADP to ATP using the energy of sunlight is called photophosphorylation.  In photophosphorylation, light energy is used to create a high-energy electron donor and a lower energy electron acceptor. Electrons then move spontaneously from donor to acceptor through an electron transport chain.  TYPES OF PHOTOPHOSPHORYLATION  There are two types of photophosphorylation  1.Cyclic photophosphorylation  2.Non-cyclic photophosphorylation
  18. 18. CYCLIC PHOTOPHOSPHORYLATION  In bacterial photosynthesis, a single photosystem is involved.  When an electron is energized by absorption of light, it is ejected from the photosystem reaction centre.  The electron then passes down through an electron transport system, and finally to the reaction centre.  The energy released during this electron transport is used to produce ATP.  Since the excited electron returns to the reaction centre, this mechanism for making ATP is called cyclic photophosphorylation.  No reducing power needed for biosynthesis generated in this process
  19. 19. CYCLIC
  20. 20. NON-CYCLIC PHOTOPHOSPHORYLATION  Plants and cyanobacteria utilize two photosystems which work sequentially to produce both energy and reducing power.  First, a photon of light ejects a high electron from photosystem II.  The electron lost from photosystem II does not return to photosystem II, but is replaced by an electron generated from the enzymatic splitting of water and the release of oxygen.
  21. 21.  The electron then travels from the excited reaction centre of photosystem II down an electron transport chain and finally to the reaction centre of photosystem I.  This transport system generates a photon motive force that is used to produce ATP.  Since the excited electron does not return to photosystem II, this mechanism for making ATP is called non-cyclic photophosphorylation.
  22. 22. NON-CYCLIC
  24. 24. INTRODUCTION  Photosystems are involved in light reaction  Photosystems are functional and structural units of protein complexes involved present in thalykoid membrane of GRANUM.  Both photosystems work together.  Together carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons.  They are found in the thylakoid membranes of plants, algae and cyanobacteria (in plants and algae these are located in the chloroplasts), or in the cytoplasmic membrane of photosynthetic bacteria.  There are two kinds of photosystems: II and I.  The flow of electrons occurs in two ways ---Non-Cyclic Pthway: passes through both photosystems (Z- scheme) ---Cyclic Pathway : occurs only in PSI.
  25. 25.  Each photosystem consists of two parts:  Antenna Complex : Cluster of Chlorophyll a,b and carotenoid molecules which gather light ad transfer it to reaction centre.  Reaction Centre: Has chlorophyll "a" molecules slightly slightly different from other chlorophyll molecules.
  26. 26. PHOTOSYSTEM II  Absorbs light of 700nm  Water splits, electrons enter PSII reaction centre.  When P680 gains energy, 2 electrons become excited and leave the molecule.  The electrons are readily captured by primary electron acceptor  Primary electron acceptor>> Plastoquinone>>Cyt b>>Cyt f>> Plastocyanine  PHOTOPHOSPHORYLATION:  As the electron pass through the electron trasport chain its energy is released and is used by thylakoid membrane to synthesize ATP.
  27. 27. PHOTOSYSTEM I (P680)  Absorbs light of 680nm  The electrons from photosystem II finally reaches the reaction centre of photosystem I.  Then electrons move to the primary electron acceptor.  From here they pass to Ferrodoxin (Fd).  The elctrons then passes from Ferrodoxin to NADP,taking H+ and form NADPH2.
  29. 29. OVERALL RESULT OF Z-SCHEME.  The result of the non-cyclic electron flow is that water is oxidzed yielding H+,e- and O2.  ATP is produced.  NADP+ becomes NADPH2.  The hydrogen and Energy of NADPH2 and ATP produced in the light reaction are used in dark reaction.
  31. 31.  the free energy of cleavage of ~P bonds of ATP, and reducing power of NADPH, are used to fix and reduce CO2 to form carbohydrate.  Carbon atoms from CO2 are bonded, or fixed, into organic compounds = carbon fixation.  THIS OCCURS IN THE STROMA
  32. 32.  An enzyme (rubisco), combines CO2 with a 5- carbon sugar RuBP  The product, 6-C sugar, immediately splits into 2, -3C molecules (PGA) PGA– Phosphoglyceric Acid
  33. 33. PGA is converted to another 3- Carbon molecule PGAL in a 2 part process:  Each PGA receives a P group from ATP  The resulting compound receives a proton from NADPH and releases the P, producing PGAL ( ADP & NADP+ return to light rxn., to make ATP and NADPH)
  34. 34.  Most of the PGAL is converted back to RuBP  Requires a P from another ATP  Some PGAL leave and used by plants create organic compounds
  35. 35. BALANCE SHEET FOR PHOTOSYNTHESIS  How much ATP & NADH are required to make 1 molecule of PGA from carbon dioxide?  Each turn fixes one CO2  PGAL is a 3-C molecule (takes 3 turns to make each molecule)  Each turn of the cycle:  3 ATP ( 2 in step 2 & 1 in step 3)  2 NADPH (step 3)
  36. 36. C4 and CAM pathway
  37. 37. C4 PLANT OR PHOTOSYNTHESIS  What is C4 plant or photosynthesis ?  Present particularly in monocot  Presence of Kranz anatomy  PEP is the initial acceptor  First product is OAA ( 4 carbon compound / acid)  Oxygen does not inhibit this process
  38. 38. C4 (CONT.)  Photorespiration is low  Rate of transpiration is low  No wastage of carbon dioxide  Well adapted in xeric condition  Distinct division of labour  Requires more ATP than C3
  39. 39. CAM PATHWAY  It is an adaptation of plants in arid condition  CAM plants use both C3 and C4 pathways  CAM plants use only mesophyll cells  Stomata are close during the day  Prevents carbon dioxide entrance as well while preventing water loss  Stomata opens at night in cooler temprature
  40. 40. CAM PATHWAY (CONT.)  Carbon dioxide diffuses  Carbon dioxide combines with PEP forming OAA  OAA reduce to form Malate  Malate stored in vacuoles  Photosynthesis commences during the day  Transport of malate to cytoplasm
  41. 41. CAM PATHWAY(CONT.)  Formation of pyruvate  Entrance of carbon dioxide to chloroplast  Formation of starch  Example :  Fleshy succulants (stone crops )  ferns
  42. 42. PHOTOSYNTHESIS group members Maryam wahab Hina zamir Amna javed Maleeha inayat Saleha qazi Yusra shair 7th may,2015