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Chapter 6 Where It Starts – Photosynthesis

Chapter 6 Where It Starts – Photosynthesis. 6.1 Biofuels. Coal, petroleum, and natural gas are fossil fuels – the remains of ancient forests, a limited resource Biofuels such as oils, gases, or alcohols are made from organic matter that is not fossilized – a renewable resource

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Chapter 6 Where It Starts – Photosynthesis

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  1. Chapter 6Where It Starts – Photosynthesis

  2. 6.1 Biofuels • Coal, petroleum, and natural gas are fossil fuels – the remains of ancient forests, a limited resource • Biofuels such as oils, gases, or alcohols are made from organic matter that is not fossilized – a renewable resource • In the United States, biofuels are produced mainly from food crops such as corn, soybeans, and sugarcane • Researchers are looking for ways to use non-food-plant matter such as switchgrass and agricultural wastes

  3. Biofuels Research

  4. Autotrophs and Heterotrophs • Autotrophsharvest energy directly from the environment, and obtain carbon from inorganic molecules • Plants and most other autotrophs make their own food by photosynthesis, a process which uses the energy of sunlight to assemble carbohydrates from carbon dioxide and water • Animals and other heterotrophsget energy and carbon by breaking down organic molecules assembled by other organisms

  5. 6.2 Sunlight as an Energy Source • Energy flow through nearly all ecosystems on Earth begins when photosynthesizers intercept energy from the sun • Photosynthetic organisms use pigments to capture the energy of sunlight and convert it to chemical energy – the energy stored in chemical bonds

  6. 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 wavelength have greater energy

  7. Electromagnetic Spectrum of Radiant Energy range of heat escaping from Earth’s surface longest wavelengths (lowest energy) shortest wavelengths (highest energy) range of most radiation reaching Earth’s surface visible light gamma rays radiation infrared near-infrared radiation ultraviolet radiation x-rays microwaves radio waves 400 nm 500 nm 600 nm 700 nm

  8. Wavelength and Energy

  9. Pigments: The Rainbow Catchers • Different wavelengths form colors of the rainbow • Photosynthesis uses wavelengths of 380-750 nm • 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)

  10. 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

  11. Some Photosynthetic Pigments

  12. Take-Home Message:How do photosynthesizers absorb light? • Energy radiating from the sun travels through space in waves and is organized as packets called photons • The spectrum of radiant energy from the sun includes visible light; humans perceive different wavelengths of visible light as different colors; the shorter the wavelength, the greater the energy • Pigments absorb light at specific wavelengths; photosynthetic species use pigments such as chlorophyll a to harvest the energy of light for photosynthesis

  13. 6.3 Exploring the Rainbow • Photosynthetic pigments work together to harvest light of different wavelengths • Engelmann identified colors of light that drive photosynthesis (violet and red) by using a prism to divide light into colors – algae using these wavelengths gave off the most oxygen

  14. Photosynthesis and Wavelengths of Light bacteria alga 400 nm 500 nm 600 nm 700 nm Wavelength

  15. ANIMATED FIGURE: T. Englemann's experiment

  16. Absorption Spectra • Most photosynthetic organisms use a combination of pigments to drive photosynthesis • An absorption spectrum shows which wavelengths each pigment absorbs best • Organisms in different environments use different pigments

  17. Absorption Spectra chlorophyll b phycoerythrobilin phycocyanobilin β-carotene chlorophyll a Light absorption 400 nm 500 nm 600 nm 700 nm Wavelength

  18. Take-Home Message: Why do cells use more than one photosynthetic pigment? • A combination of pigments allows a photosynthetic organism to most efficiently capture the particular range of light wavelengths that reaches the habitat in which it evolved

  19. 6.4 Overview of Photosynthesis • In plants and other photosynthetic eukaryotes, photosynthesis occurs in chloroplasts • Photosynthesis occurs in two stages

  20. Two Stages of Photosynthesis • Light-dependent reactions (noncyclic pathway) • First stage of photosynthesis • Light energy is transferred to ATP and NADPH • Water molecules are split, releasing O2 • Light-independent reactions • Second stage of photosynthesis • Energy in ATP and NADPH drives synthesis of glucose and other carbohydrates from CO2 and water

  21. Summary: Photosynthesis 6CO2 + 6H2O → light energy → C6H12O6 + 6O2

  22. The Chloroplast • Chloroplast • An organelle that specializes in photosynthesis in plants and many protists • Thylakoid membrane • Folded membrane that make up thylakoids • Contains clusters of light-harvesting pigments that absorb photons of different energies and convert light energy into chemical energy (first stage of photosynthesis)

  23. The Chloroplast • Stroma • A semifluid matrix surrounded by the two outer membranes of the chloroplast • Sugars are built in the stroma (second stage of photosynthesis)

  24. two outer membranes of chloroplast stroma part of thylakoid membrane system: thylakoid compartment, cutaway view Figure 6-5b p105

  25. INTERACTION: Structure of a chloroplast To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

  26. Take-Home Message: Where do the reactions of photosynthesis take place? • In the first stage of photosynthesis, light energy drives the formation of ATP and NADPH, and oxygen is released; in eukaryotic cells, these light-dependent reactions occur at the thylakoid membrane of chloroplasts • The second stage of photosynthesis, the light-independent reactions, occur in the stroma of chloroplasts; ATP and NADPH drive the synthesis of carbohydrates

  27. 3D ANIMATION: Photosynthesis Bio Experience 3D

  28. 6.5 Light-Dependent Reactions • Light-dependent reactions convert light energy to the energy of chemical bonds • Photons boost electrons in pigments to higher energy levels • Light-harvesting complexes absorb the energy • Electrons are released from special pairs of chlorophyll a molecules in photosystems • Electrons may be used in noncyclic or cyclic pathways of ATP formation

  29. Figure 6-7 p106

  30. The Thylakoid Membrane photosystem light-harvesting complex

  31. The Noncyclic Pathway • Photosystems (type II and type I) contain “special pairs” of chlorophyll a molecules that eject electrons • Electrons lost from photosystem II are replaced by photolysisof water molecules – the process by which light energy breaks down a water molecule into hydrogen and oxygen • Electrons lost from a photosystem enter an electron transfer chain (ETC) in the thylakoid membrane

  32. The Noncyclic Pathway • In the ETC, 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 • ATP is formed in the stroma by chemiosmosis, or electron transfer phosphorylation

  33. The Noncyclic Pathway • Electrons from the first electron transfer chain (from photosystem II) are accepted by photosystem I • Electrons ejected from photosystem I enter a different electron transfer chain in which the coenzyme NADP+ accepts the electrons and H+, forming NADPH • ATP and NADPH are the energy products of light-dependent reactions in the noncyclic pathway

  34. Noncyclic Pathway of Photosynthesis H+ to second stage of reactions light energy electron transfer chain light energy ADP + Pi ATP synthase photosystem I photosystem II thylakoid compartment stroma

  35. The 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

  36. Take-Home Message: What happens during the light-dependent reactions of photosynthesis? • In light-dependent reactions, chlorophylls and other pigments in thylakoid membrane transfer light energy to photosystems • Photosystems eject electrons that enter electron transfer chains in the membrane; electron flow through ETCs sets up hydrogen ion gradients that drive ATP formation • In the noncyclic pathway, oxygen is released and electrons end up in NADPH • A cyclic pathway involving only photosystem I allows the cell to continue making ATP when the noncyclic pathway is not running; NADPH does not form; O2 is not released

  37. 3D ANIMATION: Photophosphorylation

  38. ANIMATED FIGURE: Noncyclic pathway of electron flow To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

  39. 6.6 Energy Flow in Photosynthesis • Energy flow in the light-dependent reactions is an example of how organisms harvest energy from their environment

  40. Photophosphorylation • Photophosphorylationis a light-driven reaction that attaches a phosphate group to a molecule • In noncyclic photophosphorylation, electrons move from water to photosystem II, to photosystem I, to NADPH • In cyclic photophosphorylation, electrons cycle within photosystem I

  41. Excited P700 Excited P680 energy P700 (photosystem I) light energy light energy P680 (photosystem II) Energy flow in the noncyclic reactions of photosynthesis Stepped Art Figure 6-9a p108

  42. Excited P700 energy P700 (photosystem I) light energy Energy flow in the cyclic reactions of photosynthesis Stepped Art Figure 6-9b p108

  43. Take-Home Message: How does energy flow during the reactions of photosynthesis? • Light provides energy inputs that keep electrons flowing through electron transfer chains • Energy lost by electrons as they flow through the chains sets up a hydrogen ion gradient that drives the synthesis of ATP alone, or ATP and NADPH

  44. 6.7 Light-Independent Reactions • The cyclic, light-independent reactions of the Calvin-Benson cycle are the “synthesis” part of photosynthesis • Calvin-Benson cycle • Enzyme-mediated reactions that build sugars in the stroma of chloroplasts

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

  46. The Calvin-Benson Cycle • The enzyme rubisco attaches 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

  47. 4 2 other molecules 3 glucose 1 Calvin– Benson Cycle Stepped Art Figure 6-10 p109

  48. ANIMATED FIGURE: Photosynthesis overview To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

  49. Take-Home Message: What happens in light-independent reactions of photosynthesis? • Light-independent reactions of photosynthesis run on the bond energy of ATP and energy of electrons donated by NADPH; both formed in the light-dependent reactions • Collectively called the Calvin–Benson cycle, these carbon-fixing reaction use hydrogen (from NADPH), and carbon and oxygen (from CO2) to build sugars

  50. 6.8 Adaptations: Different Carbon-Fixing Pathways • Environments differ, and so do details of photosynthesis: • C3 plants • C4 plants • CAM plants

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