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Photosynthesis. AP Biology Ms. Haut. Introduction. Photosynthesis is the process that converts solar energy into chemical energy Directly or indirectly, photosynthesis nourishes almost the entire living world

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  1. Photosynthesis AP Biology Ms. Haut

  2. Introduction • Photosynthesis is the process that converts solar energy into chemical energy • Directly or indirectly, photosynthesis nourishes almost the entire living world • Photosynthesis—process in which some of the solar energy is captured by plants (producers) and transformed into glucose molecules used by other organisms (consumers). 6CO2 + 6H2O C6H12O6 + 6O2 Light energy enzymes

  3. Glucose is the main source of energy for all life. The energy is stored in the chemical bonds. • Cellular Respiration—process in which a cell breaks down the glucose so that energy can be released. This energy will enable a cell to carry out its activities. C6H12O6 + 6O2 6CO2 + 6H2O + energy enzymes

  4. Autotroph—organisms that synthesize organic molecules from inorganic materials (a.k.a. producers) • Photoautotrophs—use light as an energy source (plants, algae, some prokaryotes) • Chemoautotrophs—use the oxidation of inorganic substances (some bacteria) • Heterotroph—organisms that acquire organic molecules from compounds produced by other organisms (a.k.a. consumers)

  5. Thylakoids trap sunlight • Sunlight = electromagnetic energy • Wavelike properties • Particlelike properties (photon) Light may be reflected, transmitted, or absorbed when it contacts matter

  6. Photosynthetic Pigments: The Light Receptors • Pigments are substances that absorb visible light • Different pigments absorb different wavelengths • Wavelengths that are not absorbed are reflected or transmitted • Leaves appear green because chlorophyll reflects and transmits green light

  7. Accessory Pigments • Absorb light of varying wavelengths and transfer the energy to chlorophyll a • Chlorophyll b -yellow-green pigment • Carotenoids-yellow and orange pigments

  8. An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength • The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis • An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

  9. Photosynthesis: redox process • Endergonic redox process; energy is required to reduce CO2 • Light is the energy source that boosts potential energy of electrons (e-) as they are moved from water to CO2 • When water is split, e- are transformed from the water to CO2, reducing it to sugar

  10. reduction 6CO2 + 6H2O C6H12O6 + 6O2 oxidation

  11. The Two Stages of Photosynthesis: A Preview • Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) • Light reactions (in the thylakoids) split water, release O2, produce ATP, and form NADPH • Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH • The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

  12. Photosynthesis: 2 processes • Light reactions —convert light energy to chemical bond energy in ATP and NADPH • Occurs in thylakoids in chloroplasts • NADP+ reduced to NADPH—temporary energy storage (transferred from water) • Give off O2 as a by-product • Generates ATP by phosphorylating ADP

  13. Photosynthesis: 2 processes • Calvin Cycle —carbon fixation reactions assimilate CO2 and then reduce it to a carbohydrate • Occur in the stroma of the chloroplast • Do not require light directly, but requires products of the light reactions • Incorporates into existing organic molecules and then reduces fixed carbon into carbohydrate • NADPH provides the reducing power • ATP provides chemical energy

  14. Interdependent Reactions Light reactions produce: ATP and NADPH that are used by the Calvin cycle; O2 released Calvin Cycle produces: ADP and NADP+ that are used by the light reactions; glucose produced

  15. Photosystems: light-harvesting complexes in thylakoid membrane • Photosystem: assemblies of several hundred chlorophyll a, chlorophyll b, and carotenoid molecules in the thylakoid membrane • form a light gathering antennae that absorb photons and pass energy from molecule to molecule • Photosystem I—specialized chlorophyll a molecule, P700 • Photosystem II —specialized chlorophyll a molecule, P680

  16. Noncyclic Electron Flow • Light drives the light reactions to synthesize • NADPH and ATP • Includes cooperation of both photosystems, in • which e- pass continuously from water to • NADP+

  17. When photosystem II absorbs light an e- is excited in the reaction center chlorophyll (P680) and gets captured by the primary e- acceptor. • This leaves a hole in the P680

  18. To fill the hole left in P680, an enzyme extracts e- from water and supplies them to the reaction center • A water molecule is split into 2 H+ ions and an oxygen atom, which immediately combines with another oxygen to form O2

  19. Each photoexcited e- passes from primary e- acceptor to photosystem I via an electron transport chain. • e- are transferred to plastoquinone (Pq) and plastocyanin (Pc) (e- carriers)

  20. As e- cascade down the e- transport chain, energy is released and harnessed by the thylakoid membrane to produce ATP (PHOTOPHOSPHORYLATION) • This ATP is used to make glucose during Calvin cycle

  21. When e- reach the bottom of e- transport chain, it fills the hole in the reaction center P700 of photosystem I. • Pre-existing hole was left by former e- that was excited

  22. When photosystem I absorbs light an e- is excited in the reaction center chlorophyll (P700) and gets captured by the primary e- acceptor. • e- are transferred to ferredoxin (Fd) (e- carrier) • NADP+ reductase transfers e- from Fd to NADP+, storing energy in NADPH (reduction reaction) • NADPH provides reducing power for making glucose in Calvin cycle

  23. Cyclic Electron Flow • Only photosystem I is used • Only ATP is produced

  24. Chemiosmosis • Energy released from e- transport chain is used to pump H+ ions (from the split water) from the stroma across the thylakoid membrane to the interior of the thylakoid. • Creates concentration gradient across thylakoid membrane • Process provides energy for chemisomostic production of ATP

  25. H2O CO2 LIGHT NADP+ ADP CALVIN CYCLE LIGHT REACTOR ATP NADPH STROMA (Low H+ concentration) O2 [CH2O] (sugar) Cytochrome complex Photosystem II Photosystem I NADP+ reductase Light 2 H+ 3 NADP+ + 2H+ Fd NADPH + H+ Pq Pc 2 H2O 1⁄2 O2 THYLAKOID SPACE (High H+ concentration) 1 2 H+ +2 H+ To Calvin cycle ATP synthase Thylakoid membrane STROMA (Low H+ concentration) ADP ATP P H+ Figure 10.17 • The light reactions and chemiosmosis: the organization of the thylakoid membrane

  26. Calvin Cycle • Carbon enters the cycle in the form of CO2 and leaves in the form of sugar (glucose) • The cycle spends ATP as an energy source and consumes NADPH as a reducing agent for adding high energy e- to make sugar • For the net synthesis of this sugar, the cycle must take place 2 times

  27. Calvin Cycle

  28. Calvin Cycle

  29. Calvin Cycle

  30. Calvin Cycle • Carbon Fixation: 3 CO2 molecules bind to 3 5-Carbon sugars, ribulose bisphosphate (RuBP) using enzyme called RuBP carboxylase (rubisco) • Produces 6 molecules of a 3-carbon sugar, 3-phosphoglycerate

  31. Calvin Cycle • Carbon Fixation • Reduction: 6 ATP molecules transfer phosphate group to each molecule of 3-phos. to make 1,3-diphosphoglycerate • 6 molecules of NADPH reduce each molecule of 1,3-diphosph. to make glyceraldehyde 3-phosphate (G3P) • One of the G3P exits the cycle to be used by the plant; the other 5 molecules are used to regenerate the CO2 acceptor, RuBP: 3 molecules of ATP are used to convert 5 molecules of G3P into RuBP

  32. Calvin Cycle • 3 more CO2 molecules enter the cycle, following the same chemical pathway to release another G3P from the cycle. • 2 G3P molecules can be used to make glucose

  33. Interdependent

  34. Alternative mechanisms of carbon fixation have evolved in hot, arid climates • Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis • On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis • The closing of stomata reduces access to CO2 and causes O2 to build up • These conditions favor a seemingly wasteful process called photorespiration

  35. Photorespiration: An Evolutionary Relic? • In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound • In photorespiration, rubisco adds O2 to the Calvin cycle instead of CO2 • Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

  36. Photorespiration: An Evolutionary Relic? • Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2 • In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

  37. C4 Plants • C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells • These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle Corn Crab Grass

  38. C4 Plants • PEP carboxylase-high affinity to CO2 and no affinity for O2, thus no photorespiration possible

  39. CAM Plants • CAM plants open their stomata at night, incorporating CO2 into organic acids • Organic acids stored in vacuoles of mesophyll cells until morning, when stomata close • Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle http://ecology.botany.ufl.edu/ecologyf02/graphics/saguaro.GIF

  40. Sugarcane Pineapple CAM C4 CO2 CO2 Night Mesophyll cell CO2 incorporated into four-carbon organic acids (carbon fixation) Organic acid Organic acid Bundle- sheath cell Day CO2 CO2 Organic acids release CO2 to Calvin cycle CALVIN CYCLE CALVIN CYCLE Sugar Sugar Spatial separation of steps Temporal separation of steps LE 10-20

  41. The CAM and C4 pathways: • Are similar in that CO2 is first incorporated into organic intermediates before it enters the Calvin cycle • Differ in that the initial steps of carbon fixation in C4 plants are structurally separate from the Calvin cycle; in CAM plants, the two steps occur at separate times • Regardless of whether the plant uses C3, C4, or CAM pathway, all plants use the Calvin Cycle to produce sugar from CO2

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