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Photosynthesis

Photosynthesis. Chapter 6. Carbon and Energy Sources. Photoautotrophs Carbon source is carbon dioxide Energy source is sunlight Heterotrophs Get carbon and energy by eating autotrophs or one another. Photosynthesis Energy-storing pathway Releases oxygen Requires carbon dioxide.

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Photosynthesis

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  1. Photosynthesis Chapter 6

  2. Carbon and Energy Sources • Photoautotrophs • Carbon source is carbon dioxide • Energy source is sunlight • Heterotrophs • Get carbon and energy by eating autotrophs or one another

  3. Photosynthesis Energy-storing pathway Releases oxygen Requires carbon dioxide Aerobic Respiration Energy-releasing pathway Requires oxygen Releases carbon dioxide Linked Processes

  4. Autotrophs are the producers of the biosphere • Photoautotrophs capture sunlight energy and use it to carry out photosynthesis. • Plants, algae, some protists, and bacteria are photosynthetic autotrophs • They are the ultimate producers of food consumed by virtually all organisms

  5. On land, plants such as oak trees and cacti are the predominant producers

  6. In aquatic environments, algae and photosynthetic bacteria are the main food producers

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

  8. Location and Structure of Chloroplasts

  9. Photosynthesis Equation LIGHT ENERGY 12H2O + 6CO2 6O2 + C2H12O6 + 6H2O

  10. Photosynthesis is a redox process, as is cellular respiration Reduction Oxidation • 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

  11. Two Stages of Photosynthesis

  12. Visible Light • Humans perceive different wavelengths as different colors • Violet (380 nm) to red (750 nm) • Longer wavelengths, lower energy • Shorter wavelengths, higher energy

  13. Pigments • Light-absorbing molecules • Absorb some wavelengths and transmit others • Color you see are the wavelengths NOT absorbed chlorophyll a chlorophyll b Wavelength (nanometers)

  14. Excitation of Electrons • Excitation occurs only when the quantity of energy in an incoming photon matches the amount of energy necessary to boost the electrons of that specific pigment • Amount of energy needed varies among pigment molecules

  15. Pigments in Photosynthesis • Bacteria • Pigments found in plasma membranes • Plants • Pigments embedded in thylakoid membrane system • Pigments and proteins organized into photosystems • Photosystems located next to electron transport systems • Pigments include chlorophyll a, chlorophyll b and carotenoids

  16. Light-Dependent Reactions • Pigments absorb light energy, give up e- which enter electron transport systems • Water molecules are split, ATP and NADPH are formed, and oxygen is released • Pigments that give up electrons get replacements from photosystem

  17. Photosystem Function:Pigments • When excited by light energy, these pigments transfer energy to adjacent pigment molecules • Each transfer involves energy loss

  18. Photosystem Function: Reaction Center • Energy is reduced to level that can be captured by molecule of chlorophyll a • This molecule (P700 or P680) is the reaction center of a photosystem • Reaction center accepts energy and donates electron to acceptor molecule

  19. Light Dependent Reactions

  20. Making Sugar from CO2: The Calvin Cycle • The Calvin–Benson cycle makes sugar from CO2. • Overall reactants • Carbon dioxide • ATP • NADPH • Overall products • Glucose • ADP • NADP+

  21. Making Sugar from CO2: The Calvin Cycle • The Calvin cycle has four phases: • fixation of CO2 • Energy consumption and reduction • carbohydrate production (release of G3P) • regeneration of RuBP.

  22. Calvin Cycle

  23. Using the Products of Photosynthesis • Phosphorylated glucose is the building block for: • sucrose • The most easily transported plant carbohydrate • starch • The most common storage form in plants

  24. Photosynthesis Summary

  25. Photorespiration in C3 Plants • On hot, dry days stomata close • Inside leaf • O2 levels rise • CO2 levels drop • Rubisco attaches RuBP to oxygen instead of carbon dioxide • Only one PGA forms instead of two

  26. Photorespiration in C4 Plants • Carbon dioxide is fixed twice • In mesophyll cells, carbon dioxide is fixed to form four-carbon compound (oxaloacetate) • Carbon dioxide is released and fixed again in Calvin cycle

  27. Photorespiration in CAM Plants • Carbon is fixed twice (in same cells) • Night • Carbon dioxide is fixed to form organic acids • Day • Carbon dioxide is released and fixed in Calvin cycle

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