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Where It Starts – Photosynthesis

Where It Starts – Photosynthesis. Chapter 7 Part 1. 7.1 Sunlight as an Energy Source. Photosynthetic organisms use pigments to capture the energy of sunlight Photosynthesis The synthesis of organic molecules from inorganic molecules using the energy of light

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Where It Starts – Photosynthesis

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  1. Where It Starts – Photosynthesis Chapter 7 Part 1

  2. 7.1 Sunlight as an Energy Source • Photosynthetic organisms use pigments to capture the energy of sunlight • Photosynthesis • The synthesis of organic molecules from inorganic molecules using the energy of light • Chlorophyll has to be present as an enzyme

  3. Properties of Light • Visible light is part of an electromagnetic spectrum of energy radiating from the sun • Travels in waves • Organized into photons • Wavelength • The distance between the crests of two successive waves of light (nm) • Shorter wavelengths are more energy filled

  4. Electromagnetic Spectrum of Radiant Energy

  5. The Rainbow Catchers • Different wavelengths form colors of the rainbow • Photosynthesis uses wavelengths of 380-750 nm • That’s the range of visible light for us • Pigment • An organic molecule that selectively absorbs light of specific wavelengths • Chlorophyll a • The most common photosynthetic pigment • Absorbs violet and red light (appears green-reflected to us) • Are other photosynthetic pigments (Chlorophyll b, etc.)

  6. Photosynthetic Pigments • Collectively, chlorophyll and accessory pigments absorb most wavelengths of visible light • Certain electrons in pigment molecules absorb photons of light energy, boosting electrons to a higher energy level • Energy is captured and used for photosynthesis

  7. Beautiful fall colors – they were obscured by chlorophyll before Fig. 7-3a, p. 109

  8. 7.2 Exploring the Rainbow • Engelmann identified major colors of light that drive photosynthesis (violet and red) by using a prism to divide light into colors • Algae using these WL gave off the most oxygen • This attracted oxygen-seeking organisms in water • An absorption spectrum shows which wavelengths a pigment absorbs best • Organisms in different environments use different pigments

  9. Fig. 7-4b, p. 110

  10. phycoerythrobilin 100 chlorophyll b phycocyanobilin chlorophyll a β-carotene 80 60 Light absorption (%) 40 20 0 400 500 600 700 Wavelength (nanometers) C Absorption spectra of a few photosynthetic pigments. Line color indicates the characteristic color of each pigment. Fig. 7-4c, p. 110

  11. 7.1-7.2 Key Concepts:The Rainbow Catchers • The flow of energy through the biosphere starts when chlorophylls and other photosynthetic pigments absorb the energy of visible light

  12. 7.3 Overview of Photosynthesis • Chloroplast • An organelle that specializes in photosynthesis in plants and many protists • Made by plant as needed; has active chlorophyll • Stroma and thylakoid of the chloroplast • Sunlight energy is captured in inner thylakoid membrane • Stroma is a semifluid matrix surrounded by the two outer membranes of the chloroplast • Sugars are built in the stroma using energy from thylakoid • Result is C6H12O6, - glucose

  13. Overview of Photosynthesis • Thylakoid membrane • Folded membrane that make up thylakoids • Contains clusters of light-harvesting pigments that absorb photons of different energies • Photosystems (type I and type II) • Groups of molecules that work as a unit to begin the reactions of photosynthesis • Convert light energy into chemical energy

  14. Overview of Photosynthesis • Light-dependent reactions (gotta have sun) • Light energy is transferred to ATP and NADPH • Water molecules are split, releasing O2 • Electrons are released from water split and they are used to “supercharge” the energy storage • Light-independent reaction (sun not needed) • Energy in ATP and NADPH drives synthesis of glucose and other carbohydrates from CO2 and water

  15. Photosynthesis Equation 12H2O + 6CO2 –sun / chlorophyll C6H12O6 + 6O2 + 6H2O

  16. Sites of Photosynthesis in Plants

  17. Fig. 7-5b, p. 111

  18. sunlight CO2 H2O O2 CHLOROPLAST NADPH, ATP light-independent reactions light-dependent reactions NADP+, ADP sugars CYTOPLASM C In chloroplasts, ATP and NADPH form in the light-dependent stage of photosynthesis, which occurs at the thylakoid membrane. The second stage, which produces sugars and other carbohydrates, proceeds in the stroma. Fig. 7-5c, p. 111

  19. 7.4 Light-Dependent Reactions • In the first stage of photosynthesis, light energy drives electrons out of photosystems • The electrons may be used in a noncyclic or cyclic pathway of ATP formation

  20. Capturing Energy for Photosynthesis • Photons boost electrons in pigments to higher energy levels thus storing more sun energy • Light-harvesting complexes absorb the energy • Electrons are released from special pairs of chlorophyll a molecules in photosystems

  21. The Thylakoid Membrane

  22. ADP + Pi ATP Light-dependent reactions (noncyclic pathway) NADP+ NADPH H2O O2 ADP + Pi ATP Light-dependent reactions (cyclic pathway) Fig. 7-6, p. 112

  23. Replacing Lost Electrons • Electrons lost from photosystem II are replaced by photolysis of water molecules, which dissociate into hydrogen ions and oxygen • Photolysis • Process by which light energy breaks down a molecule such as water

  24. Electron Flow In Noncyclic Pathway • Electrons lost from a photosystem enter an electron transfer chain in the thylakoid membrane • Electron transfer chains • Organized arrays of enzymes, coenzymes, and other proteins that accept and donate electrons in a series

  25. Harvesting Electron Energy • Light energy is converted to chemical energy • Entry of electrons from a photosystem into the electron transfer chain is the first step in light-dependent reactions • ATP forms in the stroma • Electron energy is used to build up a H+ gradient across the membrane • H+ flows through ATP synthase, which attaches a phosphate group to ADP

  26. Noncyclic Pathway Of Photosynthesis

  27. Electron Flow In Cyclic Pathway • When NADPH accumulates in the stroma, the noncyclic pathway stalls • A cyclic pathway runs in type I photosystems to make ATP; electrons are cycled back to photosystem I and NADPH does not form

  28. 7.5 Energy Flow In Photosynthesis • Energy flow in the light-dependent reactions is an example of how organisms harvest energy from their environment

  29. Photophosphorylation • Photophosphorylation • A light-driven reaction that attaches a phosphate group to a molecule • Cyclic photophosphorylation • Electrons cycle within photosystem I • Noncyclic photophosphorylation • Electrons move from water to photosystem II, to photosystem I, to NADPH

  30. Energy Flow In Light-Dependent Reactions

  31. 7.3-7.5 Key Concepts:Making ATP And NADPH • Photosynthesis proceeds through two stages in the chloroplasts of plants and many types of protists • In the first stage, sunlight energy is converted to the chemical bond energy of ATP • The coenzyme NADPH forms in a pathway that also releases oxygen

  32. Where It Starts – Photosynthesis Chapter 7 Part 2

  33. 7.6 Light-Independent Reactions: The Sugar Factory • The cyclic, light-independent reactions of the Calvin-Benson cycle are the “synthesis” part of photosynthesis (where sugar is put together) • Calvin-Benson cycle (the “dark side”) • Enzyme-mediated reactions that build sugars in the stroma of chloroplasts • Use energy from radiant energy captured by the chloroplast in light dependent stage

  34. Carbon Fixation • Carbon fixation • Extraction of carbon atoms from inorganic sources (atmosphere) and incorporating them into an organic molecule • Builds glucose from CO2 plus energy bonds • Uses bond energy of molecules formed in light-dependent reactions (stored ATP, NADPH)

  35. The Calvin-Benson Cycle • Enzyme rubisco attaches C from CO2 to RuBP • Forms two 3-carbon PGA molecules • PGAL is formed • PGAs receive a phosphate group from ATP, and hydrogen and electrons from NADPH • Two PGAL combine to form a 6-carbon sugar • Rubisco is regenerated with each “turn of wheel” • But takes 6 “wheel turns” to complete sugar for normal growing conditions, with one carbon added to growing glucose with each turn

  36. Inputs And Outputs Of Calvin-Benson Cycle

  37. Calvin-Benson Cycle Summary Each turn adds 1 carbon to sugar and rebuilds 5 carbon RuBP

  38. 7.7 Adaptations: Different Carbon-Fixing Pathways • Environments differ, and so do details of photosynthesis • C3 plants – most plants, with wilting when water becomes in short supply, closing stomata • C4 plants – drought resistant plants that can store carbon in two places and resist wilting longer • CAM plants – desert plants, require little water due to ability to fix carbon only at night and thus keep stomata closed during day

  39. Stomata • Stomata (not stroma) • Small openings through the waxy cuticle covering epidermal surfaces of leaves and green stems especially on lower surfaces • Allow CO2 in and O2 and water out • Close on dry days to minimize water loss; this stops carbon attainment by preventing intake of CO2 from air and leads to wilting • There are C3, C4, and CAM plants based upon ability to deal with drought conditions

  40. C3 Plants • C3 plants • Plants that use only the Calvin–Benson cycle to fix carbon • Most plants are C3 • Forms 3-carbon PGA in mesophyll cells • Used by most plants, but inefficient in dry weather when stomata are closed • Examples: most flowering and vegetable plants

  41. Photorespiration • When stomata are closed, CO2 needed for light-independent reactions can’t enter, O2 produced by light-dependent reactions can’t leave • Photorespiration • At high O2 levels, rubisco attaches to oxygen instead of carbon • CO2 is produced rather than fixed • So efficiency lost and more “turns” needed to make sugar product

  42. C4 Plants • C4 plants • Plants that have an additional set of reactions for sugar production on dry days when stomata are closed; compensates for inefficiency of rubisco • Forms 4-carbon oxaloacetate in mesophyll cells, then bundle-sheath cells make sugar • Examples: Corn, switchgrass, bamboo

  43. C3 And C4 Plant Leaves

  44. CAM Plants • CAM plants (Crassulacean Acid Metabolism) • Plants with an alternative carbon-fixing pathway that allows them to conserve water in climates where days are hot • Forms 4-carbon oxaloacetate at night, which is later broken down to CO2 for sugar production • Example: succulents, cactuses

  45. A CAM Plant Many People Grow • Jade plant (Crassula argentea)

  46. C3, C4, And CAM Reaction Summary

  47. 7.6-7.7 Key Concepts:Making Sugar By Photosynthesis • The second stage is the “synthesis” part of photosynthesis, in which sugars are assembled from CO2 • The reactions use ATP and NADPH that form in the first stage of photosynthesis as radiant energy is captured from sunlight • Details of the reactions vary among organisms

  48. 7.8 Photosynthesis And Atmosphere • The evolution of photosynthesis dramatically and permanently changed Earth’s atmosphere • Oxygen comes only from photosynthesis and prior to development in plants the atmosphere did not contain oxygen

  49. Food Sources And Consumers • Autotrophs • Organisms that make their own food using energy from the environment and inorganic carbon • Heterotrophs • Organisms that get energy and carbon from organic molecules assembled by other organisms that are autotrophs or feed on autotrophs

  50. Two Kinds Of Autotrophs • Chemoautotrophs • Extract energy and carbon from simple molecules in the environment (hydrogen sulfide, methane) • Used before the atmosphere contained oxygen • Photoautotrophs • Use photosynthesis to make food from CO2 and water, releasing O2 • Allowed oxygen to accumulate in the atmosphere

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