Photosynthesis

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Photosynthesis

  1. 1. 8/21/2013 1 PHOTOSYNTHESIS JJA Obico, Insructor Department of Biology ENERGY REDOX REACTION Energy is transferred from one molecule to another via redox reactions. The reduced form of a molecule thus has a higher level of energy than the oxidized form
  2. 2. 8/21/2013 2 Redox in covalently bonded molecules
  3. 3. 8/21/2013 3 Photosynthesis Trivia On a global scale, photosynthesis makes about 160 billion metric tons of carbohydrate per year 1 metric ton= 1000 kg Equivalent to: a stack of 60 trillion copies of a thick book; 17 stacks of books reaching from earth to sun!!! Photosynthesis TWO STAGES: 1. Light reaction- “photo” - light energy chemical energy (ATP, NADPH) Occurs in the thylakoids- Occurs in the thylakoids 2. Dark reaction – Calvin cycle- “synthesis” - Light independent reactions - Carbon fixation; Makes sugar - Occurs in the stroma Nature of LIGHT A form of electromagnetic energy/ radiation Visible light- segment of EM spectrum vital to life W l h h d b fWavelength- the distance between crests of EM waves
  4. 4. 8/21/2013 4 Pigments Pigments- substances that absorb light Pigments in the chloroplast Chlorophyll a blue green; primary pigmentChlorophyll a- blue-green; primary pigment Chlorophyll b- yellow green; acessory pigment Carotenoids- shades of yellow and orange Absorption spectrum Graph of pigment’s light absorption versus wavelength The light reaction can perform work with those wavelengths of light that are absorbed. In the thylakoid are several pigments that differ in their absorption spectrum. Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green. Other pigments with different structures have different absorption spectra. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.8a
  5. 5. 8/21/2013 5 Collectively, these photosynthetic pigments determine an overall action spectrum for photosynthesis. An action spectrum measures changes in some measure of photosynthetic activity (for example, O2 release) as the wavelength is varied. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.8b The action spectrum of photosynthesis was first demonstrated in 1883 through an elegant experiment by Thomas Engelmann. In this experiment, different segments of a filamentous alga were exposed to different wavelengths of light. Areas receiving wavelengths favorable to photosynthesis should produce excess O2. Engelmann used the abundance of aerobicabundance of aerobic bacteria clustered along the alga as a measure of O2 production. Fig. 10.8c The action spectrum of photosynthesis does not match exactly the absorption spectrum of any one photosynthetic pigment, including chlorophyll a. Only chlorophyll a participates directly in the light reactions but accessory photosynthetic pigments absorb light and transfer energy to chlorophyll a. Chlorophyll b, with a slightly different structure than chlorophyll a, has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a.gy g p y Carotenoids can funnel the energy from other wavelengths to chlorophyll a and also participate in photoprotection against excessive light.
  6. 6. 8/21/2013 6 LIGHT REACTIONS
  7. 7. 8/21/2013 7 Photosystems Chlorophyll is organized along with other proteins and smaller organic molecules into photosystems PHOTOSYSTEMS Has light gathering “antenna complex” consisting of a cluster of chloro a, b and carotenoidmoleculesß Reaction center One of the chloro a Where first light driven chemical reaction of photo occurs Includes PRIMARY ELECTRON ACCEPTOR PHOTOSYSTEM PHOTOSYSTEM I P700 (700 nm wavelength) PHOTOSYSTEM II P680 Two Routes for electron flow 1. Non cyclic predominant route; PS I and PS II ATP and NADPH 2. Cyclic alternative route; makes use of PS I ATP only
  8. 8. 8/21/2013 8 Non cyclic electron flow Cyclic electron flow What is the significance of cyclic electron flow? Noncyclic- produces ATP and NADPH Calvin cycle- consumes more ATP than NADPH A h l l h hNADPH concentration- helps regulate which pathway to take (cyclic or noncyclic) If ATP runs low for Calvin; NADPH accumulates
  9. 9. 8/21/2013 9 Calvin Cycley Dark Reactions/ Light Independent Reaction Calvin cycle Uses ATP and NADPH to convert CO2 to sugar Direct product: Glyceraldehyde-3-phosphate (G3P) NOT glucose 3 Carbon sugarg g Cycle must take place 3X for the net synthesis of glucose Carbon fixation – incorporation of CO2 into organic compounds Calvin cycle 3 Phases 1. Carbon fixation CO2 joins ribulose biphosphate (RuBP) by RuBP carboxylase (Rubisco) Product is a 6-carbon unstable molecule that breaks into 2 3 b l l d 3 h h l t3-carbon molecule named 3-phosphoglycerate 2. Reduction 3- phosphoglycerate receives a phosphate group 1,3 biphosphoglycerate NADPH reduces 1,3 biphosphoglycerate 3. Regeneration of CO2 acceptor (RuBP) Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.17.1
  10. 10. 8/21/2013 10 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.17.2 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.17.3 Alternative mechanisms of carbon fixation Photorespiration C3 plants- first organic product of carbon fixation is a 3- carbon molecule (3 phosphoglycerate) Some plants close stomata during hot, dry days. CO2 entry is prevented; Calvin cycle starves. Inside the leaf, O2 production increases because of photosynthesisphotosynthesis Rubisco can also accept O2 Product splits One 2-carbon compound is exported from the chloroplast Mitochondria and peroxisomes break this product CO2 is released
  11. 11. 8/21/2013 11 Photorespiration is a wasteful processp It does not generate ATP and produces no food C4 plants Preface Calvin cycle with 4-carbon compound Sugarcane and corn Presence of BUNDLE SHEATH CELLS Calvin cycle is confined in the chloroplast of bundle h h llsheath cells Carbon fixation happens in the MESOPHYLL CELLS CO2 joins PEP (Phosphoenolpyruvate) by PEP carboxylase (Pepco) forms oxaloacetate Pepco- higher affinity with CO2 Mesophyll cells- pump CO2 into bundle sheath so Rubisco can function well CAM plants Crassulacean acid metabolism Succulent plants Cacti, pineapple, CrassulaceaeCacti, pineapple, Crassulaceae Open their stomata during night and close during the day Store organic acid into VACUOLES CO2 is release from the organic acid during the day when products of light reactions are available (ATP, NADPH)
  12. 12. 8/21/2013 12

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