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Chapter 7 - Photosynthesis

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  1. Chapter 7 - Photosynthesis

  2. evolution.berkeley.edu/.../images/chicxulub.gif

  3. What Is Photosynthesis? • The ability to capture sunlight energy and convert it to chemical energy. iStockphoto.com/hougaardmalan

  4. The Photosynthetic Equation 6CO2 carbon dioxide + 6H2O water + light energy  sunlight C6H12O6 glucose (sugar) + 6O2 oxygen

  5. Photosynthetic Organisms • Plants, algae, and some prokaryotes • Are autotrophs (“self- feeders”) uni-bielefeld.de

  6. Photosynthesis and Cellular Respiration • Are interconnected • Water, CO2, sugar, and O2are used or produced as byproducts in both processes

  7. Adaptations for Photosynthesis • Leaves • Chloroplasts

  8. Leaves • Flattened leaf shape exposes large surface area to catch sunlight • Epidermis • upper and lower leaf surfaces • Cuticle • waxy, waterproof outer surface • reduces water evaporation

  9. Leaf Anatomy • Stomata • adjustable pores allow for entry of air with CO2 • Mesophyll • inner cell layers that contain majority of chloroplasts • Vascular bundles (veins) • supply water and minerals to the leaf while carrying sugars away from the leaf

  10. Anatomy of a Chloroplast • Chloroplasts • bounded by a double membrane composed of innerand outer membranes • Stroma • semi-fluid medium within the inner membrane • Thylakoids • disk-shaped sacs found within the stroma • in stacks called grana

  11. Location of Photosynthetic Reactions • 2 sets of chemical reactions occur in the: • Thylakoid membranes • Stroma

  12. Light-dependent Reactions • Pigment molecules (e.g. chlorophyll) of the thylakoids capture sunlight energy • Sunlight energy is converted to the energy carrier molecules ATP and NADPH • Oxygen is released as a by-product

  13. Light-independent Reactions • Enzymes in stroma synthesize glucose and other organic molecules using the chemical energy stored in ATP and NADPH

  14. Light-Dependent Reactions • What two energy carrying molecules are used to store captured sunlight energy during light-dependent reactions?

  15. The Energy in Visible Light • Sun radiates electromagnetic energy • Photons (basic unit of light) • packets of energy with different energy levels • short-wavelength photons are very energetic • longer-wavelength photons have lower energies • Visible light is radiation falling between 400-750 nanometers of wavelength

  16. Light Captured by Pigments • Absorptionof certain wavelengths • light is “trapped” • Reflectionof certain wavelengths • light bounces back • Transmissionof certain wavelengths • light passes through

  17. Light Captured by Pigments • Absorbed light drives biological processes when it is converted to chemical energy • Pigment absorbs visible light • Common pigments: • Chlorophyllaandb • absorb violet, blue, and red light but reflect green light (hence they appear green) • Carotenoids • absorb blue and green light but reflect yellow, orange, or red (hence they appear yellow-orange) • Are accessory pigments

  18. Why Do Autumn Leaves Change Colors? autumn-pictures.com

  19. Light-Dependent Reactions • Photosystems within thylakoids • Assemblies of proteins, chlorophyll, & accessory pigments • Two Photosystems • PSII (comes 1st) and PSI (comes 2nd) • Each Photosystem is associated with a chain of electron carriers

  20. Light-Dependent Reactions Steps of the light reactions: • Accessory pigments in Photosystems absorb light and pass energy to reaction centerscontaining chlorophyll • Reaction centers receive energized electrons… • Energized electrons then passed down a series of electron carrier molecules (Electron Transport Chain) • Energy released from passed electrons used to synthesize ATP from ADP and phosphate • Energized electrons also used to makeNADPH from (NADP+) + (H+)

  21. Maintaining Electron Flow • Electrons from PSII flow one-way into PS I • PSII – produces ATP • PSI – produces NADPH

  22. Oxygen • May be used by plant or released into atmosphere

  23. Light-Independent Reactions • NADPH and ATP from light-dependent rxns used to power glucose synthesis • Light not directlynecessary for light-independent rxns if ATP & NADPH available • Light-independent rxns called the Calvin-BensonCycle or C3 Cycle

  24. The C3 Cycle • 6 CO2molecules used to synthesize 1 glucose (C6H12O6) • CO2is captured and linked to ribulose bisphosphate (RuBP) • ATP and NADPH from light dependent rxns used to power C3 reactions

  25. Relationship Between Rxns • “Photo” • capture of light energy (light dependent rxns) • “Synthesis” • glucose synthesis (light-independent rxns) • Light dependent rxns produce ATP and NADPH which is used to drive light-independent rxns • Depleted carriers (ADP and NADP+) return to light-dependent rxns for recharging

  26. The Ideal Leaf • The ideal leaf: • Large surface area to intercept sunlight • Very porous to allow for CO2 entry from air lowcarboneconomy.com biology-blog.com forestry.about.com sbs.utexas.edu

  27. Leaves • Problem: • Substantial leaf porosity leads to substantial water evaporation, causing dehydration stress on the plant • Plants evolved waterproof coating and adjustable pores (stomata) for CO2 entry

  28. When Stomata Are Closed • When stomata close, CO2 levels drop and O2 levels rise • Photorespiration occurs • Carbon fixing enzyme combines O2 instead of CO2 with RuBP

  29. When Stomata Are Closed • Photorespiration: • O2 is used up as CO2 is generated • No useful cellular energy made • No glucose produced • Photorespiration is unproductive and wasteful

  30. When Stomata Are Closed • Hot, dry weather causes stomata to stay closed • O2levels rise as CO levels fall inside leaf • Photorespiration very common under such conditions • Plants may die from lack of glucose synthesis

  31. weedtwister.com

  32. C4 Plants Reduce Photorespiration • “C4 plants” have chloroplasts in bundle sheath cells and mesophyll cells • Bundle sheath cells surround vascular bundles deep within mesophyll • C3plants lack bundle sheath cell chloroplasts

  33. C4 Plants Reduce Photorespiration • C4 plants utilize the C4 pathway • Two-stage carbon fixation pathway • Takes CO2 to chloroplasts in bundle sheath cells

  34. Environmental Conditions • C4 pathway uses up more energy than C3 pathway • C3 plants thrive where water is abundant or if light levels are low (cool, wet, and cloudy climates) • Ex. : most trees, wheat, oats, rice, Kentucky bluegrass • C4 plants thrive when light is abundant but water is scarce (deserts and hot climates) • Ex. : corn, sugarcane, sorghum, crabgrass, some thistles

  35. The End