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CHAPTER 7 Photosynthesis: Using Light to Make Food

CHAPTER 7 Photosynthesis: Using Light to Make Food. Modules 7.1 – 7.5. photosynthesis. Life in the Sun. Light is central to the life of a plant Photosynthesis is the most important chemical process on Earth Primary Function: It provides food for virtually all organisms

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CHAPTER 7 Photosynthesis: Using Light to Make Food

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  1. CHAPTER 7Photosynthesis:Using Light to Make Food Modules 7.1 – 7.5

  2. photosynthesis

  3. Life in the Sun • Light is central to the life of a plant • Photosynthesis is the most important chemical process on Earth • Primary Function: It provides food for virtually all organisms • Plant cells convert light into chemical signals that affect a plant’s life cycle • Seed germination, flower production

  4. 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 • Too much UV = DNA damage, tumors • Light can influence the architecture of a plant

  5. 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 • Look at the formula: should look familiar (kind of) Carbondioxide Water Glucose Oxygengas PHOTOSYNTHESIS

  6. 7.1 Autotrophs are the producers of the biosphere • (TQ) Plants, some protists (algae/kelp), and some bacteria (even purple) are photosynthetic autotrophs. Animals and fungi are NOT photosynthetic • They are the ultimate producers of food consumed by virtually all organisms • Producers make organic (C6H12O6 or glucose) food material from very simple raw materials. • Raw materials (water and light, no C = inorganic) • New discovery: Animal incorporated chloroplasts.

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

  8. In aquatic environments, algae (kelp) and photosynthetic bacteria are the main food producers Figure 7.1C Figure 7.1D

  9. 7.2 Photosynthesis occurs in chloroplasts • In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts. • Chloroplasts (concentrated in mesophyll tissue ) contains: • stroma, a fluid • grana, stacks of thylakoids • Stomata, small pores allowing gas exchange • The thylakoids contain chlorophyll • Chlorophyll is the green pigment that captures light for photosynthesis

  10. 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

  11. 7.3 Plants produce O2 gas by splitting water • The O2 liberated by photosynthesis is made from the oxygen in water Figure 7.3A

  12. Experiment 1 Pink letters represent radioactive tracers. Good test application question. Notlabeled Experiment 2 Labeled Figure 7.3B Reactants: Hint: Make sure you know which reactants contribute to which products. Products: Figure 7.3C

  13. 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 Lose of electrons and the gaining of electrons “LEO-GER” Review definition of oxidation and reduction Reduction Oxidation Figure 7.4A Oxidation Reduction Figure 7.4B

  14. Name a gas released as by-product of light dependent reactions of photosynthesis • Name the molecule that is the source of this gas • Why is oxygen removed from the molecule named in 2B? 1. O2 2. H20 3. H needed to build glucose molecules

  15. Redox: cellular respiration vs. photosynthesis • (TQ) understand both types of redox reactions: CR and photosynthesis • The directions are reversed • Photosynthesis: water is oxidized to O2, CO2 is reduced to sugar • Goes “uphill,” electrons gaining energy from captured light energy by chlorophyll • Cell Resp.: sugar oxidized to CO2, O2 reduced to H2O • Goes “downhill,” electrons lose PE

  16. 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

  17. H2O CO2 Chloroplast • An overview of photosynthesis ** Know Products and Reactants for both stages Light NADP+ ADP+ P LIGHTREACTIONS(in grana) CALVINCYCLE(in stroma) ATP Electrons NADPH O2 Sugar Figure 7.5

  18. Light reaction Calvin cycle Occurs in the stroma Carbon fixation is the incorporation of carbon into organic compounds Uses ATP as energy to run some steps of Calvin and NADPH electrons get oxidized. Ultimately makes sugar Can run during daytime • Occurs in thylakoid • Light energy used to make ATP from ADP + P • Light energy used to make NADPH from NADP+. Temporary storage of energized electrons • No sugar made

  19. ?: Name two molecules that are produced during the light dependent reactions of photosynthesis and serve as temporary sites for energy storage? Ans: NADPH and ATP

  20. 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

  21. ?: Which has the most energy: one photon of green light, one photon of red light, or one photon of blue light? Ans: Green light (transmitted and reflected by pigments = least likely absorbed)

  22. Reflectedlight Light Chloroplast Absorbedlight Transmittedlight Figure 7.6B

  23. 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

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

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

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

  27. Tracking the capture of solar power • Photon = light energy • (TQ) Pigment molecule absorbing light has an electron gain energy and move from ground to excited state. • Sometimes because the electron it is unstable it than loses energy and falls back to ground state or some energy released as heat • (TQ) Excited electron moved to neighboring molecule, called primary electron acceptor, and is reduced (while chlorophyll gets oxidized) • (TQ) Location of electron donation is called the reaction center, but only 1 chlorophyll a actually donates electrons. (TQ) Antenna molecules are other pigment molecules collecting light and funneling to reaction center • (TQ) Location is the thylakoid membrane

  28. Photosystems • Photosystem 1 = chlorophyll a is P700 (wavelength red light) • Photosystem 2 = chlorophyll a is P680 (more orange than red wavelength) • (TQ) During the fall many trees break down chlorophyll in their leaves. The color of leaves is due to carotenoids

  29. 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 (similar to those in cellular respiration • Their energy ends up in ATP and NADPH • Photosystem II – redox reactions shuttle down etc and electrons lose energy to form ATP (by chemiosmosis) • Photosytem I – NADP+ gets reduced to form NADPH

  30. Big idea • Where do the electrons come from that keep the light reactions running? • NADPH requires 2 electrons from PS I, PS I gets electrons from PS II, PS II gets electrons from the splitting of water. • H+ stays in chloroplast, but O combines with another split O = O2 So what is the overall result of blocking reaction centers? No donation of excited electrons to primary electron acceptor

  31. 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

  32. 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 (Look familiar?)to make ATP • In the stroma, the H+ ions combine with NADP+ to form NADPH (similar to role of O2 in Cell. Resp.)

  33. The production of ATP by chemiosmosis in photosynthesis (photophosphorylation) • Final electron acceptor is NADP+ (unlike O2 for Cell Resp.) Thylakoidcompartment(high H+) Light Light Thylakoidmembrane Antennamolecules Stroma(low H+) ELECTRON TRANSPORT CHAIN PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE Figure 7.9

  34. 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:

  35. 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

  36. 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 Enzyme rubisco combines 5C +1C to make 3C PGA RuBP 3-PGA 6 ATP 3 ADP Step Energy consumption and redox. 2 6 ADP + P CALVINCYCLE 3 ATP 2 6 ATP consumed, NADPH oxidized 4 NADPH 6 NADP+ Step Release of one molecule of G3P. 3 5 P 6 P G3P G3P For every 6 PGA, 1 leaves, and 5 remain. For each 2 leaving = 1 glucose 3 Step Regeneration of RuBP. 4 Glucoseand other compounds OUTPUT: 1 P 5C G3P uses ATP to go back to 3C RuBP G3P Figure 7.10B

  37. Rubisco most common enzyme in the world • Photorespiration may then occur • Not a good thing due to the breakdown of rubisco

  38. 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

  39. 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

  40. 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 • Not a good thing due to the breakdown of rubisco

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

  42. 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

  43. 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

  44. 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

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

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

  47. 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

  48. 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

  49. 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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