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06 Lecture Ppt

  1. 1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 6 Pathways of Photosynthesis
  2. 2. Photosynthesis Produces Food and Releases Oxygen
  3. 3. 6.1 Photosynthesizers are autotrophs that produce their own food <ul><li>Photosynthesis converts solar energy into the chemical energy of a carbohydrate </li></ul><ul><ul><li>Photosynthesizers are the base of almost every food chain in the world </li></ul></ul><ul><ul><li>Autotrophs and heterotrophs (consumers) use organic molecules from photosynthesis </li></ul></ul>
  4. 4. Figure 6.1 Photosynthetic organisms
  5. 5. 6.2 In plants, chloroplasts carry out photosynthesis <ul><li>Photosynthesis occurs in green plant parts </li></ul><ul><ul><li>Carbon dioxide enters leaves via stomata </li></ul></ul><ul><ul><li>Water and carbon dioxide diffuse into chloroplasts , the photosynthetic organelles </li></ul></ul><ul><ul><li>Chloroplasts contain the chlorophyll and other pigments that absorb solar energy </li></ul></ul>
  6. 6. Figure 6.2 Leaf structures specialized for photosynthesis
  7. 7. 6.3 Photosynthesis is a redox reaction that releases O 2 <ul><li>Oxidation is the loss of electrons and reduction is the gain of electrons </li></ul><ul><ul><li>Because they always occur together, the entire reaction is a redox reaction </li></ul></ul><ul><li>Photosynthesis is a redox reaction in which hydrogen atoms are transferred from water to carbon dioxide </li></ul>
  8. 8. APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES 6.4 Experiments showed that the O 2 released by photosynthesis comes from water <ul><li>In 1930 C.B. van Niel used the oxygen isotope O 18 to track the path of oxygen production in photosynthesis </li></ul>
  9. 9. 6.5 Photosynthesis involves two sets of reactions <ul><li>The light reactions </li></ul><ul><ul><li>Only occur when solar energy is available </li></ul></ul><ul><ul><li>Chlorophyll molecules absorb solar energy to energize electrons used in ATP production </li></ul></ul><ul><li>The Calvin cycle reactions </li></ul><ul><ul><li>CO 2 is taken up and reduced to a carbohydrate that can be converted to glucose </li></ul></ul><ul><ul><li>ATP and NADPH from light reactions are needed </li></ul></ul>
  10. 10. Figure 6.5A Overview of photosynthesis
  11. 11. First, Solar Energy Is Captured
  12. 12. 6.6 Light reactions begin: Solar energy is absorbed by pigments <ul><li>Solar energy can be described in terms of its wavelength and its energy content </li></ul><ul><li>Most common pigments chlorophylls a and b and carotenoids are capable of absorbing various portions of visible light </li></ul>
  13. 13. <ul><li>Figure 6.6A The electromagnetic spectrum includes visible light </li></ul><ul><li>Figure 6.6B Absorption spectrum of photosynthetic pigments </li></ul>
  14. 14. APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES 6.7 Fall temperatures cause leaves to change color <ul><li>Chlorophyll is not very stable and in the fall, sufficient energy to rebuild chlorophyll is not available </li></ul><ul><li>Chlorophyll in leaves disintegrates, </li></ul><ul><li>and we begin to see yellow and </li></ul><ul><li>orange pigments in the leaves </li></ul>
  15. 15. 6.8 Solar energy boosts electrons to a higher energy level <ul><li>Photosystem I (PS I) & Photosystem II (PS II) consist of antenna molecules and a reaction center </li></ul><ul><li>Antenna molecules absorb light and pass energy to the reaction center </li></ul><ul><li>In reaction center excited electrons are passed to electron acceptors </li></ul>
  16. 16. Figure 6.8 A general model of a photosystem
  17. 17. 6.9 Electrons release their energy as ATP forms <ul><li>Electron transport chain - a series of carriers that pass electrons from one to another </li></ul><ul><li>Each electron transfer releases energy that is ultimately used to make ATP </li></ul><ul><ul><li>ATP synthase complexes - hydrogen ions flow through, providing energy for ATP synthesis </li></ul></ul>
  18. 18. Figure 6.9 High-energy electrons (e − ) release energy as they pass down an electron transport chain
  19. 19. 6.10 During the light reactions, electrons follow a noncyclic pathway
  20. 20. 6.11 The thylakoid membrane is organized to produce ATP and NADPH <ul><li>Chemiosmosis - ATP production is tied to an H + gradient across a membrane </li></ul>
  21. 21. Figure 6.11 Organization of a thylakoid
  22. 22. Second, Carbohydrate Is Synthesized
  23. 23. Fig 6.12 Calvin cycle reactions
  24. 24. 6.12 The Calvin cycle uses ATP and NADPH from the light reactions to produce a carbohydrate <ul><li>Calvin cycle reduces CO 2 from the atmosphere to produce carbohydrate </li></ul><ul><ul><li>CO 2 Fixation – CO 2 combines with RuBP with the help of RuBP carboxylase </li></ul></ul><ul><ul><li>CO 2 Reduction – CO 2 is reduced to a carbohydrate </li></ul></ul>
  25. 25. <ul><ul><li>RuBP Regeneration – every three turns of the Calvin cycle, five molecules of G3P reform three molecules of RuBP </li></ul></ul>
  26. 26. 6.13 In plants, carbohydrate is the starting point for other molecules Figure 6.13 Fates of G3P
  27. 27. C 3 , C 4 , and CAM Photosynthesis Thrive Under Different Conditions
  28. 28. 6.14 C 3 photosynthesis evolved when oxygen was in limited supply <ul><li>In C 3 plants the first detectable molecule after CO 2 fixation is a C 3 molecule 3PG </li></ul><ul><ul><li>Common in plants where temperature and rainfall are moderate </li></ul></ul><ul><li>Under water stress, stomata close, limiting CO 2 and reducing efficiency </li></ul><ul><ul><li>O 2 competes with CO 2 for the active site of RuBP carboxylase, and less C 3 is produced </li></ul></ul>Figure 6.14 Carbon dioxide fixation in C3 plants as exemplified by these wildflowers
  29. 29. 6.15 C 4 photosynthesis boosts CO 2 concentration for RuBP carboxylase <ul><li>C 4 plant - the first detectable molecule following CO 2 fixation is a four carbon molecule </li></ul><ul><li>Avoid the uptake of O 2 by RuBP carboxylase by increasing the amount of CO 2 available to the enzyme </li></ul>
  30. 30. <ul><li>Figure 6.15A Anatomy of a C 3 plant compared to a C 4 plant </li></ul>
  31. 31. Figure 6.15B Carbon dioxide fixation in C 4 plants as exemplified by corn
  32. 32. 6.16 CAM photosynthesis is another alternative to C 3 photosynthesis <ul><li>CAM – crassulacean-acid metabolism </li></ul><ul><ul><li>Partitioning based on time </li></ul></ul><ul><li>CAM plants open their stomata only at night; during the day, the stomata are closed, conserving water </li></ul>
  33. 33. Figure 6.16 Carbon dioxide fixation in CAM plants as exemplified by pineapple
  34. 34. APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES 6.17 Destroying tropical rain forests contributes to global warming <ul><li>Tropical rain forests contribute greatly to uptake of CO 2 because they are the most efficient of all terrestrial ecosystems </li></ul><ul><ul><li>CO 2 in our atmosphere traps radiant heat from the sun and warms the world </li></ul></ul><ul><li>Deforestation in tropical rain forests accounts for 20–30% of all carbon dioxide in the atmosphere </li></ul><ul><ul><li>Burning also removes trees that would ordinarily absorb CO 2 </li></ul></ul>
  35. 35. Figure 6.17 Global warming: Past trends and future predictions
  36. 36. Connecting the Concepts: Chapter 6 <ul><li>General equation for photosynthesis </li></ul><ul><ul><li>6CO 2 + 6H 2 O -> C 6 H 12 O 6 + 6O 2 </li></ul></ul><ul><li>Two separate sets of reactions </li></ul><ul><ul><li>Light reactions (in thylakoid membrane) </li></ul></ul><ul><ul><ul><li>Absorb solar energy and produce NADH and ATP, which are provided to the Calvin cycle to reduce CO 2 </li></ul></ul></ul><ul><ul><li>Calvin cycle reactions (in stroma) </li></ul></ul><ul><ul><ul><li>Reduce carbon dioxide to a carbohydrate </li></ul></ul></ul><ul><li>C 4 and CAM are alternative means of supplying RuBP carboxylase with CO 2 while limiting its exposure to oxygen. </li></ul><ul><li>Photosynthesis keeps the biosphere functioning by supplying energy to all organisms </li></ul><ul><ul><li>Organisms use cellular respiration, which occurs in the mitochondria, to get energy out of carbohydrates </li></ul></ul>