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The process of photosynthesis



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The process of photosynthesis

  1. 1. BEIRA HAILU 1
  2. 2. Physical nature of light  To understand photosynthesis one should understand the physical nature of light  Light : is a form of radiant energy , a narrow band of energy with in the continuous electromagnetic spectrum of radiation emitted by the sun  The term ‘light’ describes that portion of the electromagnetic spectrum that cause the physiological sensation of vision in human  Light is defined by the range of wavelengths between 400-700 nm. BEIRA HAILU 2
  3. 3.  Electromagnetic spectrum  Visible radiation (light) 400-700 nm  Infrared radiation - >700 nm  Ultraviolet radiation 100-400 nm  Colour is determined by wavelength of the radiation  ATRIBUTES OF LIGHT  Wave length  Particle property  Important in understanding the biological functions of light BEIRA HAILU 3
  4. 4. Wave property o Characterized by wave length or frequency  Wavelength ()–the distance between successive crests  Frequency ()- number of wave crests passing a point in one second  Frequency is related to wave length as: Frequency =speed of light /wavelength = = C/ BEIRA HAILU 4
  5. 5.  Particle property  Light behaves as if its energy is divided in to particles called photons when it is emitted  Photons carry energy termed quantum and is related to frequency and wave length.  Thus, Eq = hc/=h h=planck’s constant=6.62*10-14 Js photon -1  Quantum energy is inversely proportional to its wavelength BEIRA HAILU 5
  6. 6. Photons of violet end of the spectrum have highest energy while photons of infrared have lowest energy Eg.  Light >1200 nm  low energy content  Too low to mediate chemical reaction  Energy absorbed is converted to heat BEIRA HAILU 6
  7. 7.  2oo-1200 nm  Sufficient to produce a chemical change  PAR is found with in this range Photosynthetically active radiation 400 nm(blue end)-700 nm(red end) Optimum wavelength for driving photosynthesis  Other regions of the spectrum are absorbed by molecule in the atmosphere BEIRA HAILU 7
  8. 8. BEIRA HAILU 8
  9. 9. ABSORPTION SPECTRA  Not all the wavelength of light can be absorbed by the plant pigment  The chlorophyll can absorb waves of certain length with in the range of visible light  Different chlorophylls show different absorption peaks on different region of the band BEIRA HAILU 9
  10. 10. PHOTOSYNTHETIC MOLECULES Plants posses pigment molecules that absorb physiologically useful radiations Called photoreceptors  Process the energy and information content of light into a form that can be used by the plant BEIRA HAILU 10
  11. 11. Principal molecules Chlorophyll Chlorophyll a,b,c,d,e Bacteriochlorophyll a, b Chlorobium 650,660 Carotenoids Phycoblins BEIRA HAILU 11
  12. 12. Chlorophyll  Is primarily responsible for harvesting light energy used in photosynthesis  Chlorophyll structure : has two parts I. Porphyrin head  Cyclic tetrapyrrole  Made up of four nitrogen containing pyrrole rings arranged in cyclic fashion  Magnesium ion is chelated to the four nitrogen atoms in the center of the ring  Loss of Mg ion leads to formation of pheophytin BEIRA HAILU 12
  13. 13.  Requires light for their synthesis  Yellow appearance of etiolated leaves is due to lack of light  The reduction of proto chlorophyll to chlorophyll is accomplished at the expense of light absorbed by the protochlorophyll  The reduction of the bond is catalysed by the enzyme NADPH: protochlorophyll oxidoreductase  Light sensitive part in angiosperm BEIRA HAILU 13
  14. 14. BEIRA HAILU 14
  15. 15. II. Phytol tail  Long, lipid-soluble hydrocarbon tail (20 C alcohol)  Makes the molecule very hydrophobic  Important for orientation and anchoring of chlorophyll molecule in the chlorophyll membrane BEIRA HAILU 15
  16. 16. Difference in chemical structure Ring II: • Chll a: CH3 • Chll b: CHO Ring I • Chll a :CH2=CH • Chll d: O-CHO Chll c : lacks phytol tail BEIRA HAILU 16
  17. 17. Carotenoids  Comprises a family of orange and yellow pigments of most photosynthetic organisms  When chlorophyll pigments are degraded carotenoids account for the brilliant orange and yellow colour  Found in  Carrot roots  Tomato fruit  Green leaves  They are dominantly hydrocarbon s thus are lipid soluble and located either in the chloroplast membrane or in chromoplasts BEIRA HAILU 17
  18. 18. Carotenoids Hydrocarbon carotene -red colour -carotene -carotene Lycopene -tomato Oxygen derivatives xanthophylls - Yellow colour Zeaxanthin Lutein Violaxanthin ?? Anthocyanins (flvonoids) Blue & Red BEIRA HAILU 18
  19. 19. Significance 1. Protect against the photoxidation of chlorophyll molecule by absorbing excess blue light  Acts as preferred substrate in the photosynthesised oxidation  Combine with oxygen (highly reactive form of O2 )to form violaxanthin 2. Absorb and transfer light energy to chlorophyll a BEIRA HAILU 19
  20. 20. Phycoblins • Blue green algae Phycocynins (phycoerythroblin) • Red algae Phycoerythrin (phycocyanoblin) • Blue green and red algae Allophycocyanins (allophycocyanoblin) • Regulates various aspects of growth and developments Phytochromobilin BEIRA HAILU 20
  21. 21.  All the study of these came from the study about pigment–protein complex  They are classified as accessory pigments  The energy harvested by these pigments is transferred to chlorophyll a similar to carotenoids before it is active in photosynthesis BEIRA HAILU 21
  22. 22. Site of photosynthesis  The light–driven metabolism of CO2  In plants photosynthesis takes place primarily in leaves  The process occurs from start to completion in the chloroplast  Chloroplast is highly ordered complex structure that floats free in the cytoplasm of green plants BEIRA HAILU 22
  23. 23. BEIRA HAILU 23
  24. 24. Chemical composition of chloroplast Protein 40-50% Phospholipids 25-30 % Chlorophylls 5-10% Carotenoids 1-2% RNA 5% DNA as fragments BEIRA HAILU 24
  25. 25. BEIRA HAILU 25
  26. 26. Chloroplast structure  Chloroplast is composed of several compartments with its own set of metabolic functions : 1. Outer envelop  The ‘skin’ that holds every thing in.  The external membrane , which is permeable to most substances  Smooth, composed of 2 lipid molecules 2. Inner envelop  The inner membrane, impermeable to most molecules  Contains transport proteins that control the movement of substance in to and out of the chloroplast BEIRA HAILU 26
  27. 27. 3. Thylakoid  System of internal membranes that contain the photosystems and components of the electron transport chain  Site of light reaction of photosynthesis  Organized in to  Compactly arranged regions -most important part  Loosely arranged – grana amellas  Thylakoid enclose a continuous fluid space known as the lumen  Contains ATP synthase , but ATP is not generated BEIRA HAILU 27
  28. 28. 4. Stroma  Forms the matrix of the chloroplast- a protein filled gel that contains soluble enzymes and metabolites  Lamellae in this portion are loosely arranged called stroma lamella  Consists of ribosomes serving as site of protein synthesis  Site for dark reaction of photosynthesis  The major protein in the stroma is the carboxilating enzyme RUBISCO BEIRA HAILU 28
  29. 29. The photosynthetic process  Photo= light , synthesis = putting together  CO2 and water are combined using light energy from sun light to form glucose  An extremely complex process  Oxygen is given off as waste product  Source of oxygen in the atm  Occurs in higher plants, algae, some bacteria BEIRA HAILU 29
  30. 30.  Consists of two key process 1. Removal of H from water 2. Reduction of CO2 by these H atoms to form organic molecules  Photosynthesis is a two-way stage process in the chloroplast 1. Light reaction (light dependent rxn) hill reaction 2. Dark reaction (light independent rxn) BEIRA HAILU 30
  31. 31. Phases of photosynthesis BEIRA HAILU 31
  32. 32. 2. Dark reaction • Occurs in the stroma • Involves utilization of ATP & NADPH • Fixation of CO2 into carbohydrate in the Calvin-Benson cycle (reduction of CO2 into glucose) BEIRA HAILU 32
  33. 33. NADPH ATP NADP+ ADP+ H2O ADP NADP+ CO2 GLUCOSE Light reaction Dark Reaction LIGHT
  34. 34. Events of over all photosynthetic BEIRA HAILU 34
  35. 35. Light reaction (light dependent rxn) or Hill reaction BEIRA HAILU 35
  36. 36. BEIRA HAILU 36 hv hv 2e PS II CYT PS I NADPH +H+ NADP+ +2H+ H2O 1/2O2 + 2H+ Fig. Linear representation of light rxn
  37. 37.  Chloroplasts contain a system of thylakoid membranes.  Embeds six different complexes of integral membrane proteins 1. Photosystem I 2. Photosystem II 3. Light harvesting complexes I 4. Light harvesting complexes II 5. Cytochrome b6 and f complex 6. ATP synthase BEIRA HAILU 37
  38. 38. I. Photosystems  They are multicellular complex  Two photosystems  PS I and PS II  Each photosystem is consist of a. Antennae  Light harvesting system  Chlorophyll a, b and carotenoids  Light travels from antennae to inner antennae and to reaction center BEIRA HAILU 38
  39. 39. b. Reaction center  Reaction center consists of special chlorophyll involved in:  Charge separation  Electron transfer  In PS II the reaction center chlorophyll is P680  In PS II the reaction center chlorophyll is P700  Subscripts – absorption maxima BEIRA HAILU 39
  40. 40. PS I PS II  12 protein molecules  96 molecules of chll a  2 molecules of rxn center chll P700  4 accessory molecules  90 molecules that serve as antenna pigments  22 carotenoids molecule  4 lipids molecules  3 cluster of Fe4S4  2 phylloquinones  >20 different protein molecules  50 chlorophyll a molecule  2 molecules of the rxn center chll P680  2 accessory molecules close to them  2 molecules of pheophytin  Antenna pigments  Half dozen carotenoids molecule  2 molecules of plastoquinone BEIRA HAILU 40
  41. 41. II. Light harvesting complex  These are chlorophyll-protein complexes  Function extended antenna systems for harvesting additional light energy  Important Role  Dynamic regulation of energy distribution and Electron transport BEIRA HAILU 41
  42. 42. • Associated with PS I • Small, has chll a/b ratio of 4/1 LHCI • Associated with PS II • Has chll a/b ratio of about ½ • Also contain the xanthophyll LHCII BEIRA HAILU 42
  43. 43. III. Cytochrome b/f complexes are uniformly distributed through out both regions IV. ATP synthase BEIRA HAILU 43
  44. 44. PHOTOPHOSPHORYLATION  Light-driven production of ATP by chloroplast: a. Noncyclic Photophosphorylation b. Cyclic Photophosphorylation c. Pseudocyclic Photophosphorylation BEIRA HAILU 44
  45. 45. Non-cyclic electron transport • Electron flow from water to NADP+ (Final electron acceptor) • ATP formation at one location only (Non- cyclic Photophosphorylation) • Both photosystems involve • Water as primary electron source (oxidation of water in the thylakoid lumen) • NADP+ is reduced to form NADPH BEIRA HAILU 45
  46. 46. BEIRA HAILU 46 Noncyclic Photophosphorylation (Z-scheme)
  47. 47.  Oxidation of water as the primary source of electrons  The reduction of the final electron acceptor NADP+  Photophosphorylation (ATP synthesis)  Electrons flow from water to NADP+  Large vertical arrows represent the input of light energy into the system  NADP+ is reduced to NADPH on the stroma side of the membrane BEIRA HAILU 47
  48. 48. ATP synthase Photosystem I Cytochrome b6 /f complex Photosystem II Organization of the photosynthetic electron transport system in the thylakoid membrane involves: 48BEIRA HAILU
  49. 49. 49BEIRA HAILU
  50. 50. Cyclic electron transport • Electrons from reduced feredoxin is transferred back to plastoquinone • Occurs when NADP+ is not available in its oxidized form to trap electrons • No oxidation of water is involved • ATP formation at two locations (cyclic Photophosphorylation) • Only PS I involves • Occurs when chlorophyll molecules are exposed to light energy >680 nm BEIRA HAILU 50
  51. 51. BEIRA HAILU 51
  52. 52. Pseudocyclic Photophosphorylation  This path requires both photosystems  the ferredoxin passes the electrons to molecular oxygen which act as the electron accepter thereby forming hydrogen peroxide  Is called Mehler reaction  By the action of hydrogen peroxide the reduced oxygen is graded thus giving rise to superoxide radical  molecular hydrogen which reacts with superoxide radical and give rise to very dangerous hydrogen peroxide BEIRA HAILU 52
  53. 53.  There is no net oxygen exchange (take-up & evolved)  So, here electrons come from water to oxygen and back to water but the same electrons are not recycled like the cyclic flow do and for this reason that is why is also not referred to as cyclic flow.  This flow takes place when oxygen concentrations are very high or when carbon dioxide fixation is very low 53BEIRA HAILU
  54. 54. BEIRA HAILU 54
  55. 55. Overall light reaction BEIRA HAILU 55
  56. 56. ATP + NADPH NADP+ Triose phosphate CO2 +H2O (CH2O)2 BEIRA HAILU 56
  57. 57. The reactions catalyzing the reduction of CO2 to carbohydrate are coupled to the consumption of NADPH & ATP by enzymes in the stroma Stroma reactions are long to be independent of light (dark reactions) But this reaction depend on the products of the photochemical processes • Directly regulated by light • Properly referred to as carbon reactions of photosynthesis BEIRA HAILU 57
  58. 58. Cyclic reactions that accomplish fixation and reduction of CO2  There are three types of photosynthesis 1. Calvin cycle (C3) 2. Hatch –slack cycle (C4) 3. Crassulacean acid metabolism (CAM) BEIRA HAILU 58
  59. 59. I. The Calvin cycle All photosynthetic eukaryotes reduce CO2 to carbohydrate via the same basic mechanism:  The photosynthetic carbon reduction (PCR) cycle  Calvin cycle  Reductive pentose phosphate (RPP) cycle  C3 cycle C3 photosynthesis is the typical photosynthesis that most plants use The other cycles are auxiliary to or dependent on the basic Calvin cycle BEIRA HAILU 59
  60. 60. Carboxylation Reduction Regeneration BEIRA HAILU 60
  61. 61. BEIRA HAILU 61
  62. 62. BEIRA HAILU 62
  63. 63. The temporary chemical (ATP) reducing (NADPH) potentials that were generated in the light reactions are used to reduce PGA to carbonyl (a carbohydrate) called glyceraldehyde-3-phosphate This is a two step reaction sequence • PGA is phosphorylated with ATP to 1,3- bisphosphoglycerate (BPG) • Reduction of BPG to Glyceraldehyde-3- phosphate (GAP, G-3P) through the use of NADPH generated by the light reaction BEIRA HAILU 63 2. Reduction
  64. 64. 2PGA ATP 1,3- bisphospho glycerate NADPH Glyceraldeh yde-3- phosphate BEIRA HAILU 64
  65. 65. 3. Regeneration  The continued uptake CO2 requires the availability CO2 acceptor, ribulose -1,5 bisphosphate  Regeneration of the CO2 acceptor RuBP fromG-3-P  Three molecules of RuBP (15 C total) are formed by reactions that reshuffle the carbons from the five molecules of trios sugar 65BEIRA HAILU
  66. 66. BEIRA HAILU 66 Trios sugar 3 RuBP 6G-3-P
  67. 67. The reshuffling reaction consists 1. Conversion of one G3P to dihydroyaceton-3-phpsphate (DHAP) 2. DHAP undergoes aldol condensation with second molecule of G3P to give fructose-1,6-bisphosphate 3. FBP is hydrolyzed to fructose -6-phosphate 4. F6P is transferred transketolase to a third G3P to give Erythrose-4-phosphate (E- 4-P) and xylulose-5- phosphate (X-5-P) 5. E-4-P combines vial aldolase with a fourth molecule of G3P to give a seven- carbon sugar sedoheptulose-1,7-bisphosphate (SBP) 6. 67BEIRA HAILU
  68. 68. 6. SBP is then hydrolyzed to give sedoheptulose -7-phosphate (S-7-P) 7. S7P donates a two-carbon unit to the fifth(last) molecule G3P and produce ribose-5-phosphate and xylulose-5-phosphate 8. The two xylulose-5-phosphate are converted to 2 molecules of ribulose-5-phosphate (Ru-5-P) sugar by ribulose-5-phosphate epimerase ; the third Ru-5-P is formed from ribose-5-phosphate by ribose-5-phosphate isomerase 9. Phosphorylation of Ru-5-P with ATP to generate RUBP BEIRA HAILU 68
  69. 69. Fig. C3 cycle BEIRA HAILU 69
  70. 70. Summery  Called C3 because the CO2 is first incorporated into a 3-carbon compound.  Stomata are open during the day.  The net product is one molecule of trios sugar per 3CO2 taken  9 ATP & 6 NADPH are consumed per 3CO2  RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO2. BEIRA HAILU 70
  71. 71.  Adaptive Value:  more efficient than C4 and CAM plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy).  Most plants are C3. BEIRA HAILU 71
  72. 72. II. Hatch –slack cycle (C4) There is difference in leaf anatomy between pants that have a C4 carbon cycle(C4 plants) and those that photosynthesis solely via Calvin photosynthetic cycle (3 plants) The cross section of C3 leaf reveals one major cell type that has chloroplast , the mesophyll . BEIRA HAILU 72
  73. 73.  In contrast C4 leaf has two distinct chloroplast- containing cell types:  Mesophyll cells  Bundle sheath cells  Such distinction is called Kranz anatomy  Both are connected by an extensive net work of plasmodesmata , thus providing a pathway for the flow of metabolites between the cell types BEIRA HAILU 73
  74. 74.  The C4 cycle concentrates CO2 in bundle sheath cell  The basic c4 cycle consists of four stages: 1. Fixation of CO2  Carboxylation of phosphoenolpyruvate in the mesophyll cells to form a C4 acid (malate or asparate)  Catalyzed by enzyme called phosphoenolpyruvate carboxylase (PEP case) 2. Transport of the C4 acid (pyruvate or alanine) from mesophyll cells to the bundle sheath cells BEIRA HAILU 74
  75. 75. 3. Decarboxylation  C4 acid is decarboxylated with in the bundle sheath cell  Generation of CO2  CO2 released is reduced to carbohydrate via C3 cycle 4. Regeneration  Transport of C3 acid (pyruvate) formed by decarboxylation back to mesophyll cell  Phosphorylation of pyruvate using ATP to generate CO2 acceptor PEP BEIRA HAILU 75
  76. 76. Fig. Hatch-slack path way BEIRA HAILU 76
  77. 77. Three variations of basic C4 cycle Variation 1. In the c4 acid transported into the bundle sheath cell (asparate or malate)  The 3-carbon acid pyruvate or alanine returned to the mesophyll cell 2. The nature of enzyme that catalyzes the decarboxylation step  Thus their name is after the enzyme that catalyzes their decarboxylation reaction BEIRA HAILU 77
  78. 78. a. NADP-ME type  This is NADP dependent malic enzyme  Found in the chloroplast of bundle sheath  Malate is transported bundle sheath cell  Pyruvate is transported to mesophyll cell  Example: corn, sugarcane, sorghum BEIRA HAILU 78
  79. 79. b. NAD-ME type  NAD dependent malic enzyme  Decarboxylation occurs in the mitochondria  Asparate is transported bundle sheath cell  Alanine is transported to mesophyll cell  Examples : millet, pigweed BEIRA HAILU 79
  80. 80. c. PEP-CK type  Phosphoenol-pyruvate dependent carboxykinase  Decarboxylation occurs in the cytosol of chloroplast  Asparate to bundle sheath cell  Alanine to mesophyll cell BEIRA HAILU 80
  81. 81. Summery Called C4 because the CO2 is first incorporated into a 4-carbon compound. Stomata are open during the day. Uses PEP Carboxylase for the enzyme involved in the uptake of CO2 (HCO3 as substrate ) This enzyme allows CO2 to be taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photosynthesis. Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy) The concentration of CO2 in bundle sheath has an energy cost ; 5ATP and 2NADPH per 1 CO2 consumed BEIRA HAILU 81
  82. 82.  Adaptive Value:  Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration.  Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis.  C4 plants include several thousand species in at least 19 plant families. BEIRA HAILU 82
  83. 83. III. Crassulacean Acid Metabolism  Called CAM after the plant family in which it was first found (Crassulaceae) and because the CO2 is stored in the form of an acid before use in photosynthesis  The type of photosynthesis is similar to C4 cycle in many respects but different in two important features: 1. Formation of c4 acid is both temporally and spatially separated (PEP case and decarboxylase located in the cytosol function at different time 2. A specialized anatomy is not needed BEIRA HAILU 83
  84. 84. During Night  Stomata open for uptake of CO2  At night CO2 is captured by PEP carboxylase in the cytosol  Fixation of CO2 as malic acid temporally and is stored in the vacuole • Acidification of leaf when malic acid is stored in the vacuole BEIRA HAILU 84
  85. 85. During Day  Stomata are closed for reducing water loss  Transportation of malate from vacuole to chloroplast  Decarboxylation (deacidification) occurs , the released CO2 is fixed by the Calvin cycle  Refixation of internally released CO2 by C3 cycle  Since stomata are closed ,internally released can not escape from the leaf BEIRA HAILU 85
  86. 86. 86 HCO3 Phosphoeno l pyruvate Pi OAA malate NADH NAD+ Malate CO2C3 cycle Pyruvatestarch Trios phosphate Chloroplast NADP+ malic dehydogenase PEP case
  87. 87. BEIRA HAILU 87
  88. 88.  Stomata open at night (when rates of water loss are usually lower) and are usually closed during the day.  The CO2 is converted to an acid and stored during the night.  During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis  CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads BEIRA HAILU 88
  89. 89.  Adaptive Value:  Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower  When conditions are extremely arid, CAM plants can just leave their stomata closed night and day.  Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis.  CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells). BEIRA HAILU 89
  90. 90. Photorespiration  Many land plants take up oxygen and release CO2 in the light.  This process is called photorespiration  However, it is normally masked by photosynthesis, which is even faster.  Photorespiration differs from true respiration.  Plants do respire normally with mitochondria that produces ATP and NADH, and occurs mostly in the dark. BEIRA HAILU 90
  91. 91.  In contrast, photorespiration is wasteful and occurs mostly in the light (produces no ATP)  Photorespiration appears to serve no useful purpose.  Its main effect is to reduce the apparent rate of photosynthesis. BEIRA HAILU 91 Phosphoglycolate + phosphoglycerate
  92. 92. Not all plants photorespire  Plants that photorespire 1. Typically show light saturation point (LSP)  Point at which increasing light yields a constant amount of photosynthesis 2. have higher light compensation point (LCP)  Light at which the amount of photosynthesis just equals the amount of respiration BEIRA HAILU 92
  93. 93.  Oxygen inhibition of photosynthesis in plants that photorespire is called Warburg effect  Oxygen acts as antagonistic in photosynthesis and acts in a competitive manner  This is due to the fact that rubisco is not a substrate specific enzyme  i.e. also has an oxygenase function, thus binds oxygen to RuBP although higher affinity for CO2  Favoured by low CO2/O2 ratio BEIRA HAILU 93
  94. 94. BEIRA HAILU 94 Involves three cellular organelles
  95. 95. Reading assignment Factors affecting the process of photosynthesis BEIRA HAILU 95