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Photosynthesis: Using Light to Make Food Life in the Sun

Explore the process of photosynthesis and how plants convert light into chemical energy to produce food. Discover the importance of light for plant growth and the role of chloroplasts. Learn about the two stages of photosynthesis and how they are linked by ATP and NADPH.

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Photosynthesis: Using Light to Make Food Life in the Sun

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  1. CHAPTER 7Photosynthesis:Using Light to Make Food

  2. Life in the Sun • Light is central to the life of a plant • Photosynthesis is the most important chemical process on Earth • It provides food for virtually all organisms • Plant cells convert light into chemical signals that affect a plant’s life cycle

  3. Plants that get adequate light are often bushy, with deep green leaves • Without enough light, plants become tall and spindly with small pale leaves • Too much sunlight can damage a plant • Chloroplasts and carotenoids help to prevent such damage • Light can influence the architecture of a plant

  4. AN OVERVIEW OF PHOTOSYNTHESIS • Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water Carbondioxide Water Glucose Oxygengas PHOTOSYNTHESIS

  5. 7.1 Autotrophs are the producers of the biosphere • Plants, some protists, and some bacteria are photosynthetic autotrophs • They are the ultimate producers of food consumed by virtually all organisms

  6. On land, plants such as oak trees and cacti are the predominant producers Figure 7.1A Figure 7.1B

  7. In aquatic environments, algae and photosynthetic bacteria are the main food producers Figure 7.1C Figure 7.1D

  8. 7.2 Photosynthesis occurs in chloroplasts • In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts • A chloroplast contains: • stroma, a fluid • grana, stacks of thylakoids • The thylakoids contain chlorophyll • Chlorophyll is the green pigment that captures light for photosynthesis

  9. Chloroplast LEAF CROSS SECTION MESOPHYLL CELL LEAF • The location and structure of chloroplasts Mesophyll Intermembrane space CHLOROPLAST Outer membrane Granum Innermembrane Grana Stroma Thylakoidcompartment Stroma Thylakoid Figure 7.2

  10. 7.3 Plants produce O2 gas by splitting water • The O2 liberated by photosynthesis is made from the oxygen in water Figure 7.3A

  11. Experiment 1 Notlabeled Experiment 2 Labeled Figure 7.3B Reactants: Products: Figure 7.3C

  12. 7.4 Photosynthesis is a redox process, as is cellular respiration • Water molecules are split apart and electrons and H+ ions are removed, leaving O2 gas • These electrons and H+ ions are transferred to CO2, producing sugar Reduction Oxidation Figure 7.4A Oxidation Reduction Figure 7.4B

  13. 7.5 Overview: Photosynthesis occurs in two stages linked by ATP and NADPH • The complete process of photosynthesis consists of two linked sets of reactions: • the light reactions and the Calvin cycle • The light reactions convert light energy to chemical energy and produce O2 • The Calvin cycle assembles sugar molecules from CO2 using the energy-carrying products of the light reactions

  14. H2O CO2 Chloroplast • An overview of photosynthesis Light NADP+ ADP+ P LIGHTREACTIONS(in grana) CALVINCYCLE(in stroma) ATP Electrons NADPH O2 Sugar Figure 7.5

  15. THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY 7.6 Visible radiation drives the light reactions • Certain wavelengths of visible light drive the light reactions of photosynthesis Gammarays Micro-waves Radio waves X-rays UV Infrared Visible light Wavelength (nm) Figure 7.6A

  16. Reflectedlight Light Chloroplast Absorbedlight Transmittedlight Figure 7.6B

  17. 7.7 Photosystems capture solar power • Each of the many light-harvesting photosystems consists of: • an “antenna” of chlorophyll and other pigment molecules that absorb light • a primary electron acceptor that receives excited electrons from the reaction-center chlorophyll

  18. Primaryelectron acceptor PHOTOSYSTEM Photon Reaction center Pigmentmoleculesof antenna Figure 7.7C

  19. Fluorescence of isolated chlorophyll in solution Heat Photon(fluorescence) Photon Chlorophyllmolecule Figure 7.7A

  20. Primaryelectron acceptor • Excitation of chlorophyll in a chloroplast Othercompounds Photon Chlorophyllmolecule Figure 7.7B

  21. 7.8 In the light reactions, electron transport chains generate ATP, NADPH, and O2 • Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons • The excited electrons are passed from the primary electron acceptor to electron transport chains • Their energy ends up in ATP and NADPH

  22. In photosystem I, electrons from the bottom of the cascade pass into its P700 chlorophyll • Where do the electrons come from that keep the light reactions running?

  23. Primaryelectron acceptor Electron transport • Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product Primaryelectron acceptor Electron transport chain Photons Energy forsynthesis of PHOTOSYSTEM I PHOTOSYSTEM II Figure 7.8 by chemiosmosis

  24. 7.9 Chemiosmosis powers ATP synthesis in the light reactions • The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane • The flow of H+ back through the membrane is harnessed by ATP synthase to make ATP • In the stroma, the H+ ions combine with NADP+ to form NADPH

  25. The production of ATP by chemiosmosis in photosynthesis Thylakoidcompartment(high H+) Light Light Thylakoidmembrane Antennamolecules Stroma(low H+) ELECTRON TRANSPORT CHAIN PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE Figure 7.9

  26. THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle • The Calvin cycle occurs in the chloroplast’s stroma • This is where carbon fixation takes place and sugar is manufactured INPUT CALVINCYCLE Figure 7.10A OUTPUT:

  27. carbon from atmospheric CO2 • electrons and H+ from NADPH • energy from ATP • Energy-rich sugar is then converted into glucose • The Calvin cycle constructs G3P using

  28. INPUT: 3 In a reaction catalyzed by rubisco, 3 molecules of CO2 are fixed. CO2 Step Carbon fixation. 1 • Details of the Calvin cycle 1 3 P P 6 P RuBP 3-PGA 6 ATP 3 ADP Step Energy consumption and redox. 2 6 ADP + P CALVINCYCLE 3 ATP 2 6 4 NADPH 6 NADP+ Step Release of one molecule of G3P. 3 5 P 6 P G3P G3P 3 Step Regeneration of RuBP. 4 Glucoseand other compounds OUTPUT: 1 P G3P Figure 7.10B

  29. PHOTOSYNTHESIS REVIEWED AND EXTENDED 7.11 Review: Photosynthesis uses light energy to make food molecules • A summary of the chemical processes of photo-synthesis Chloroplast Light Photosystem IIElectron transport chains Photosystem I CALVIN CYCLE Stroma Electrons Cellular respiration Cellulose Starch Other organic compounds LIGHT REACTIONS CALVIN CYCLE Figure 7.11

  30. The excess is stored in roots, tuber, and fruits • These are a major source of food for animals • Many plants make more sugar than they need

  31. 7.12 C4 and CAM plants have special adaptations that save water • Most plants are C3 plants, which take CO2 directly from the air and use it in the Calvin cycle • In these types of plants, stomata on the leaf surface close when the weather is hot • This causes a drop in CO2 and an increase in O2 in the leaf • Photorespiration may then occur

  32. Photorespiration in a C3 plant CALVIN CYCLE 2-C compound Figure 7.12A

  33. Special cells in C4 plants—corn and sugarcane—incorporate CO2 into a four-carbon molecule • This molecule can then donate CO2 to the Calvin cycle • Some plants have special adaptations that enable them to save water 4-C compound CALVIN CYCLE 3-C sugar Figure 7.12B

  34. They open their stomata at night and make a four-carbon compound • It is used as a CO2 source by the same cell during the day • The CAM plants—pineapples, most cacti, and succulents—employ a different mechanism 4-C compound Night Day CALVIN CYCLE 3-C sugar Figure 7.12C

  35. PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE 7.13 Human activity is causing global warming; photosynthesis moderates it • Due to the increased burning of fossil fuels, atmospheric CO2 is increasing • CO2 warms Earth’s surface by trapping heat in the atmosphere • This is called the greenhouse effect

  36. Sunlight ATMOSPHERE Radiant heat trapped by CO2 and other gases Figure 7.13A & B

  37. Because photosynthesis removes CO2 from the atmosphere, it moderates the greenhouse effect • Unfortunately, deforestation may cause a decline in global photosynthesis

  38. 7.14 Talking About Science: Mario Molina talks about Earth’s protective ozone layer • Mario Molino received a Nobel Prize in 1995 for his work on the ozone layer • His research focuses on how certain pollutants (greenhouse gases) damage that layer Figure 7.14A

  39. The O2 in the atmosphere results from photosynthesis • Solar radiation converts O2 high in the atmosphere to ozone (O3) • Ozone shields organisms on the Earth’s surface from the damaging effects of UV radiation

  40. Industrial chemicals called CFCs have hastened ozone breakdown, causing dangerous thinning of the ozone layer Sunlight • International restrictions on these chemicals are allowing recovery Southern tip of South America Antarctica Figure 7.14B

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