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  1. 1. CHAPTER 7 Photosynthesis: Using Light to Make Food
  2. 2. <ul><li>Light is central to the life of a plant </li></ul><ul><li>Photosynthesis is the most important chemical process on Earth </li></ul><ul><ul><li>It provides food for virtually all organisms </li></ul></ul><ul><li>Plant cells convert light into chemical signals that affect a plant’s life cycle </li></ul>Life in the Sun
  3. 3. <ul><li>Light can influence the architecture of a plant </li></ul><ul><ul><li>Plants that get adequate light are often bushy, with deep green leaves </li></ul></ul><ul><ul><li>Without enough light, plants become tall and spindly with small pale leaves </li></ul></ul><ul><li>Too much sunlight can damage a plant </li></ul><ul><ul><li>Chloroplasts and carotenoids help to prevent such damage </li></ul></ul>
  4. 4. <ul><li>Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water </li></ul>AN OVERVIEW OF PHOTOSYNTHESIS Carbon dioxide Water Glucose Oxygen gas PHOTOSYNTHESIS
  5. 5. <ul><li>Plants, some protists, and some bacteria are photosynthetic autotrophs </li></ul><ul><ul><li>They are the ultimate producers of food consumed by virtually all organisms </li></ul></ul>7.1 Autotrophs are the producers of the biosphere
  6. 6. <ul><li>On land, plants such as oak trees and cacti are the predominant producers </li></ul>Figure 7.1A Figure 7.1B
  7. 7. <ul><li>In aquatic environments, algae and photosynthetic bacteria are the main food producers </li></ul>Figure 7.1C Figure 7.1D
  8. 8. <ul><li>In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts </li></ul><ul><li>A chloroplast contains: </li></ul><ul><ul><li>stroma, a fluid </li></ul></ul><ul><ul><li>grana, stacks of thylakoids </li></ul></ul><ul><li>The thylakoids contain chlorophyll </li></ul><ul><ul><li>Chlorophyll is the green pigment that captures light for photosynthesis </li></ul></ul>7.2 Photosynthesis occurs in chloroplasts
  9. 9. <ul><li>The location and structure of chloroplasts </li></ul>Figure 7.2 LEAF CROSS SECTION MESOPHYLL CELL LEAF Chloroplast Mesophyll CHLOROPLAST Intermembrane space Outer membrane Inner membrane Thylakoid compartment Thylakoid Stroma Granum Stroma Grana
  10. 10. <ul><li>Stomata, open and closed </li></ul><ul><li>Site of gas exchange O 2 , H 2 O and CO 2 </li></ul>Figure 7.2x2
  11. 11. <ul><li>The O 2 liberated by photosynthesis is made from the oxygen in water molecules </li></ul>7.3 Plants produce O 2 gas by splitting water Figure 7.3A
  12. 12. Figure 7.3B Figure 7.3C Experiment 1 Experiment 2 Not labeled Labeled Reactants: Products:
  13. 13. <ul><li>Water molecules are split apart and electrons and H + ions are removed, leaving O 2 gas </li></ul><ul><ul><li>These electrons and H + ions are transferred to CO 2 , producing sugar </li></ul></ul>7.4 Photosynthesis is a redox process, as is cellular respiration Figure 7.4A Figure 7.4B Reduction Oxidation Oxidation Reduction
  14. 14. <ul><li>The complete process of photosynthesis consists of two linked sets of reactions: </li></ul><ul><ul><li>the light reactions (the ‘photo’ part) </li></ul></ul><ul><ul><li>and the Calvin cycle (the ‘synthesis’ part) </li></ul></ul><ul><li>The light reactions convert light energy into chemical energy and produce O 2 </li></ul><ul><li>The Calvin cycle assembles sugar molecules from CO 2 using the high energy products of the light reactions </li></ul>7.5 Overview: Photosynthesis occurs in two stages linked by ATP and NADPH
  15. 15. <ul><li>An overview of photosynthesis </li></ul>Figure 7.5 Light Chloroplast LIGHT REACTIONS (in grana) CALVIN CYCLE (in stroma) Electrons H 2 O O 2 CO 2 NADP + ADP + P Sugar ATP NADPH
  16. 16. <ul><li>Certain wavelengths of visible light drive the light reactions of photosynthesis </li></ul>7.6 Visible radiation drives the light reactions THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY Gamma rays X-rays UV Infrared Micro- waves Radio waves Visible light Wavelength (nm) Figure 7.6A
  17. 17. Figure 7.6B Light Chloroplast Reflected light Absorbed light Transmitted light
  18. 18. <ul><li>Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons </li></ul><ul><li>The excited electrons are passed from the primary electron acceptor to electron transport chains </li></ul><ul><ul><li>Their energy ends up in ATP and NADPH </li></ul></ul>7.8 In the light reactions, electron transport chains generate ATP, NADPH, and O 2
  19. 19. <ul><li>The Calvin cycle occurs in the chloroplast’s stroma </li></ul><ul><ul><li>This is where carbon fixation takes place and sugar is manufactured </li></ul></ul>7.10 ATP and NADPH power sugar synthesis in the Calvin cycle THE CALVIN CYCLE: CONVERTING CO 2 TO SUGARS INPUT Figure 7.10A OUTPUT: CALVIN CYCLE
  20. 20. <ul><li>The Calvin cycle constructs G3P using </li></ul><ul><ul><li>carbon from atmospheric CO 2 </li></ul></ul><ul><ul><li>electrons and H + from NADPH </li></ul></ul><ul><ul><li>energy from ATP </li></ul></ul><ul><li>Energy-rich sugar is then converted into glucose </li></ul>
  21. 21. <ul><li>Details of the Calvin cycle </li></ul>Figure 7.10B INPUT: Step Carbon fixation. In a reaction catalyzed by rubisco, 3 molecules of CO 2 are fixed. 1 1 Step Energy consumption 2 3 P P P 6 6 2 ATP 6 ADP + P 6 NADPH 6 NADP + 6 P G3P Step Release of one molecule of G3P. 3 CALVIN CYCLE 3 OUTPUT: 1 P Glucose and other compounds G3P Step Regeneration of RuBP. 4 G3P 4 3 ADP 3 ATP 3 CO 2 5 P RuBP 3-PGA
  22. 22. <ul><li>A summary of the chemical processes of photo-synthesis </li></ul>7.11 Review: Photosynthesis uses light energy to make food molecules PHOTOSYNTHESIS REVIEWED AND EXTENDED Figure 7.11 Light Chloroplast Photosystem II Electron transport chains Photosystem I CALVIN CYCLE Stroma Electrons LIGHT REACTIONS CALVIN CYCLE Cellular respiration Cellulose Starch Other organic compounds
  23. 23. <ul><li>Many plants make more sugar than they need </li></ul><ul><ul><li>The excess is stored in roots, tuber, and fruits </li></ul></ul><ul><ul><li>These are a major source of food for animals </li></ul></ul>
  24. 24. <ul><li>Most plants are C 3 plants, which take CO 2 directly from the air and use it in the Calvin cycle </li></ul><ul><ul><li>In these types of plants, stomata on the leaf surface close when the weather is hot </li></ul></ul><ul><ul><li>This causes a drop in CO 2 and an increase in O 2 in the leaf </li></ul></ul><ul><ul><li>Photorespiration may then occur </li></ul></ul><ul><ul><ul><li>No sugar or ATP </li></ul></ul></ul>7.12 C 4 and CAM plants have special adaptations that save water
  25. 25. <ul><li>Photorespiration in a C 3 plant </li></ul>CALVIN CYCLE 2-C compound Figure 7.12A
  26. 26. <ul><li>Some plants have special adaptations that enable them to save water </li></ul>CALVIN CYCLE 4-C compound Figure 7.12B <ul><ul><li>Special cells in C 4 plants—corn and sugarcane—incorporate CO 2 into a four-carbon molecule </li></ul></ul><ul><ul><li>This molecule can then donate CO 2 to the Calvin cycle </li></ul></ul>3-C sugar
  27. 27. <ul><li>The CAM plants—pineapples, most cacti, and succulents—employ a different mechanism </li></ul>CALVIN CYCLE 4-C compound Figure 7.12C <ul><ul><li>They open their stomata at night and make a four-carbon compound </li></ul></ul><ul><ul><li>It is used as a CO 2 source by the same cell during the day </li></ul></ul>3-C sugar Night Day
  28. 28. <ul><li>Due to the increased burning of fossil fuels, atmospheric CO 2 is increasing </li></ul><ul><ul><li>CO 2 warms Earth’s surface by trapping heat in the atmosphere </li></ul></ul><ul><ul><li>This is called the greenhouse effect </li></ul></ul>7.13 Human activity is causing global warming; photosynthesis moderates it PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE
  29. 29. Figure 7.13A & B Sunlight ATMOSPHERE Radiant heat trapped by CO 2 and other gases
  30. 30. <ul><li>Because photosynthesis removes CO 2 from the atmosphere, it moderates the greenhouse effect </li></ul><ul><ul><li>Unfortunately, deforestation may cause a decline in global photosynthesis </li></ul></ul>
  31. 31. <ul><li>Mario Molino received a Nobel Prize in 1995 for his work on the ozone layer </li></ul><ul><li>His research focuses on how certain pollutants (greenhouse gases) damage that layer </li></ul>7.14 Talking About Science: Mario Molina talks about Earth’s protective ozone layer Figure 7.14A
  32. 32. <ul><li>The O 2 in the atmosphere results from photosynthesis </li></ul><ul><ul><li>Solar radiation converts O 2 high in the atmosphere to ozone (O 3 ) </li></ul></ul><ul><ul><li>Ozone shields organisms on the Earth’s surface from the damaging effects of UV radiation </li></ul></ul>
  33. 33. <ul><li>Industrial chemicals called CFCs have hastened ozone breakdown, causing dangerous thinning of the ozone layer </li></ul>Figure 7.14B Sunlight Southern tip of South America <ul><li>International restrictions on these chemicals are allowing recovery </li></ul>Antarctica