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Advanced Biology Photosynthesis Notes

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  2. 2. THE PROCESS THAT FEEDS THE BIOSPHERE <ul><li>Photosynthesis is a process that converts solar energy into chemical energy. </li></ul><ul><li>Directly or indirectly, photosynthesis nourishes almost the entire living world. </li></ul>
  3. 3. <ul><li>Autotrophs sustain themselves without eating anything derived from other organisms. </li></ul><ul><li>Autrophs are producers of our biosphere. </li></ul><ul><li>Almost all plants are photoautotrophs, using sunlight to make organic </li></ul><ul><li>molecules from inorganic </li></ul><ul><li>molecules. </li></ul>Monotropa uniflora
  4. 5. <ul><li>Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes. </li></ul><ul><li>These organisms not only feed themselves but also the entire living planet. </li></ul>
  5. 6. LE 10-2 LE 10-2 Plants Unicellular protist Multicellular algae Cyanobacteria Purple sulfur bacteria 10 µm 1.5 µm 40 µm
  6. 7. <ul><li>Heterotrophs obtain their organic matter from other organisms. </li></ul><ul><li>Heterotrophs are consumers of the biosphere. </li></ul><ul><li>Almost all heterotrophs, including humans, depend on photoautrophs for food and oxygen. </li></ul>
  7. 8. Converting Light Energy to the Chemical Energy of Food <ul><li>Chloroplasts are organelles that are responsible for feeding the vast majority of organisms. </li></ul><ul><li>Chloroplasts are present in a variety of photosynthesizing organisms. </li></ul>
  8. 9. <ul><li>Leaves are the organs of plants that play a major role in photosynthesis. </li></ul><ul><li>Typical Leaf Structure: </li></ul>
  9. 10. <ul><li>Cuticle is a waxy or fatty layer of varying thickness on the outer walls of the epidermis. </li></ul><ul><li>Epidermis is the exterior tissue, usually 1 cell layer thick, of leaves, stems, and roots. </li></ul><ul><li>Palisade Mesophyll is mesophyll having 1 or more relatively uniform rows tightly packed walled cells. </li></ul><ul><ul><li>Palisade Mesophyll=Parenchyma Cells </li></ul></ul><ul><ul><li>Mesophyll is tissue between the upper and lower epidermis. </li></ul></ul>
  10. 11. <ul><li>Spongy Mesophyll is mesophyll having loosely arranged cells due to numerous air spaces found near the lower epidermis. </li></ul><ul><li>Vascular Bundle is a strand of tissue composed mostly of xylem and phloem contained usually in a bundle sheath. </li></ul><ul><ul><li>Phloem is the tissue that transports food (sugar) in plants. </li></ul></ul><ul><ul><li>Xylem is the tissue that transports water and dissolved minerals in plants. </li></ul></ul><ul><ul><ul><li>Cohesion-Adhesion Hypothesis </li></ul></ul></ul>
  11. 12. <ul><li>Stomata are pores or openings in the lower epidermis of leaves that regulate gas exchange between plants and the atmosphere. </li></ul><ul><li>Stomata are controlled by specialized cells called guard cells, that expand and contract via osmosis. </li></ul>
  12. 14. Chloroplasts <ul><li>Chloroplasts are found mainly in cells of the mesophyll. </li></ul><ul><li>A typical mesophyll cells has 30-40 chloroplasts. </li></ul>
  13. 15. <ul><li>Stroma is a region containing the bulk volume of chloroplasts’, and contains enzymes that play a role in the carbon-fixation reactions (carbohydrate synthesis) </li></ul><ul><li>Thylakoids are coin-shaped membranous sacks whose contents include pigments that play a role in light reactions. </li></ul><ul><ul><li>Grana are stacks of thylakoids </li></ul></ul>
  14. 16. LE 10-3 Leaf cross section Vein Mesophyll Stomata CO 2 O 2 Mesophyll cell Chloroplast 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoid Granum Stroma 1 µm
  15. 17. The Nature of Light <ul><li>Chloroplast are solar powered chemical factories. </li></ul><ul><li>Light is a form of electromagnetic energy (electromagnetic radiation); that travels in rythmic waves. </li></ul><ul><li>Wavelength is the distance between crests of waves; determines the type of electromagnetic energy </li></ul>
  16. 18. <ul><li>Light also behaves it consists of discrete particles called photons. </li></ul><ul><li>The electromagnetic spectrum is the entire range of electromagnetic energy (radiation). </li></ul><ul><li>Visible light, colors we can see, drives photosynthesis. </li></ul>
  17. 20. LE 10-6 Visible light Gamma rays X-rays UV Infrared Micro- waves Radio waves 10 –5 nm 10 –3 nm 1 nm 10 3 nm 10 6 nm 1 m (10 9 nm) 10 3 m 380 450 500 550 600 650 700 750 nm Longer wavelength Lower energy Shorter wavelength Higher energy
  18. 21. Photosynthetic Pigments <ul><li>Pigments are substances that absorb light </li></ul><ul><li>Different pigments absorb different wavelengths of light. Some light is reflected or transmitted. </li></ul><ul><ul><li>Ex: Leaves are green because chlorophyll reflects and transmits green light. </li></ul></ul>
  19. 22. LE 10-7 Chloroplast Light Reflected light Absorbed light Transmitted light Granum
  20. 23. <ul><li>Chlorophyll a </li></ul><ul><ul><li>Primary photosynthetic pigment of all higher plants, blue-green algae, and cyanobacter. </li></ul></ul><ul><ul><li>Blue-Green in color. </li></ul></ul><ul><li>Chlorophyll b </li></ul><ul><ul><li>An accessory pigment found in virtually all higher plants and green algae. </li></ul></ul><ul><ul><li>Broadens the spectrum of color for photosynthesis </li></ul></ul><ul><ul><li>Yellow-Green in color </li></ul></ul><ul><li>Carotenoids </li></ul><ul><ul><li>Accessory pigments that absorb excessive light that would damage chlorophyll </li></ul></ul><ul><ul><ul><li>Carotenes-orange, reds, yellows </li></ul></ul></ul><ul><ul><ul><li>Xanthophylls-yellow </li></ul></ul></ul>
  21. 24. LE 10-10 CH 3 CHO in chlorophyll a in chlorophyll b Porphyrin ring: light-absorbing “ head” of molecule; note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown
  22. 25. Engelmann’s Experiment <ul><li>The action spectrum of photosynthesis was first demonstrated in 1883 by Thomas Engelmann. </li></ul><ul><ul><li>Exposed different segments of filamentous alga to different wavelengths. </li></ul></ul><ul><ul><ul><li>Favorable wavelengths produced excess O 2 </li></ul></ul></ul><ul><ul><ul><li>Aerobic bacteria clustered along the algae as a measure of O 2 production. </li></ul></ul></ul>
  23. 26. LE 10-9a Chlorophyll a Chlorophyll b Carotenoids Wavelength of light (nm) Absorption spectra Absorption of light by chloroplast pigments 400 500 600 700
  24. 27. LE 10-9b Action spectrum Rate of photo- synthesis (measured by O 2 release)
  25. 28. LE 10-9c Engelmann’s experiment 400 500 600 700 Aerobic bacteria Filament of algae
  26. 29. The Reactions <ul><li>Scientist’s and their contributions </li></ul><ul><ul><li>Helmont </li></ul></ul><ul><ul><li>Ingenhousz </li></ul></ul><ul><ul><li>Priestly </li></ul></ul><ul><li>Photosynthesis can be summarized using the following equation: </li></ul><ul><li>6 CO 2 + 6 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 </li></ul>
  27. 30. The Reactions <ul><li>Photosynthesis is a biochemical pathways involving a series of redox reactions. </li></ul><ul><ul><li>Water is oxidized and carbon dioxide is reduced. </li></ul></ul><ul><li>Two reactions occur: </li></ul><ul><ul><li>Light Reactions (thylakoids) “Photo” </li></ul></ul><ul><ul><li>Calvin Cycle (Stroma) “synthesis” </li></ul></ul>
  28. 31. The Reactions <ul><li>The light reactions (in the thylakoids): </li></ul><ul><ul><li>Split water </li></ul></ul><ul><ul><li>Produced oxygen gas </li></ul></ul><ul><ul><li>Produced ATP (via chemiosmosis) </li></ul></ul><ul><ul><li>Form NADPH, an electron carrier </li></ul></ul><ul><li>The Calvin Cycle (in the stroma): </li></ul><ul><ul><li>Forms the sugar from carbon dioxide using ATP and NADPH. </li></ul></ul>
  29. 32. LE 10-5_3 H 2 O LIGHT REACTIONS Chloroplast Light ATP NADPH O 2 NADP + CO 2 ADP P + i CALVIN CYCLE [CH 2 O] (sugar)
  30. 33. Excitation of Chlorophyll by Light <ul><li>When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable </li></ul><ul><li>When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence. </li></ul><ul><li>If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat </li></ul>
  31. 34. LE 10-11 Excited state Heat Photon (fluorescence) Ground state Chlorophyll molecule Photon Excitation of isolated chlorophyll molecule Fluorescence Energy of electron e –
  32. 35. What is a photosystem? <ul><li>A photosystem consists of a reaction center surrounded by light-harvesting complexes </li></ul><ul><li>The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center </li></ul>
  33. 36. <ul><li>A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a </li></ul><ul><li>Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions </li></ul>
  34. 37. LE 10-12 Thylakoid Photon Light-harvesting complexes Photosystem Reaction center STROMA Primary electron acceptor e – Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Thylakoid membrane
  35. 38. <ul><li>There are two types of photosystems: </li></ul><ul><ul><li>Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm </li></ul></ul><ul><ul><li>Photosystem I is best at absorbing a wavelength of 700 nm </li></ul></ul><ul><li>The two photosystems work together to use light energy to generate ATP and NADPH </li></ul>
  36. 39. Noncyclic Electron Flow <ul><li>During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic </li></ul><ul><li>Noncyclic electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH </li></ul>
  37. 40. LE 10-13_5 Light P680 e – Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H 2 O CO 2 Energy of electrons O 2 e – e – + 2 H + H 2 O O 2 1 / 2 Pq Cytochrome complex Electron transport chain Pc ATP P700 e – Primary acceptor Photosystem I (PS I) e – e – Electron Transport chain NADP + reductase Fd NADP + NADPH + H + + 2 H + Light
  38. 41. Cyclic Electron Flow <ul><li>Cyclic electron flow uses only photosystem I and produces only ATP </li></ul><ul><li>Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle </li></ul>
  39. 42. LE 10-15 Photosystem I Photosystem II ATP Pc Fd Cytochrome complex Pq Primary acceptor Fd NADP + reductase NADP + NADPH Primary acceptor
  40. 43. Chemiosmosis <ul><li>Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy </li></ul><ul><li>Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP </li></ul><ul><li>The spatial organization of chemiosmosis differs in chloroplasts and mitochondria </li></ul>
  41. 44. LE 10-16 MITOCHONDRION STRUCTURE Intermembrane space Membrane Electron transport chain Mitochondrion Chloroplast CHLOROPLAST STRUCTURE Thylakoid space Stroma ATP Matrix ATP synthase Key H + Diffusion ADP + P H + i Higher [H + ] Lower [H + ]
  42. 45. <ul><li>Water is split by photosystem II on the side of the membrane facing the thylakoid space </li></ul><ul><li>The diffusion of H + from the thylakoid space back to the stroma powers ATP synthase </li></ul><ul><li>ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place </li></ul>
  43. 46. LE 10-17 STROMA (Low H + concentration) Light Photosystem II Cytochrome complex 2 H + Light Photosystem I NADP + reductase Fd Pc Pq H 2 O O 2 +2 H + 1 / 2 2 H + NADP + + 2H + + H + NADPH To Calvin cycle THYLAKOID SPACE (High H + concentration) STROMA (Low H + concentration) Thylakoid membrane ATP synthase ATP ADP + P H + i [CH 2 O] (sugar) O 2 NADPH ATP ADP NADP + CO 2 H 2 O LIGHT REACTIONS CALVIN CYCLE Light
  44. 47. The Calvin Cycle <ul><li>The Calvin cycle, regenerates its starting material after molecules enter and leave the cycle </li></ul><ul><li>The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH </li></ul><ul><li>Carbon enters the cycle as CO 2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P) or PGAL </li></ul>
  45. 48. <ul><li>The Calvin cycle has three phases: </li></ul><ul><ul><li>Carbon fixation (catalyzed by rubisco) </li></ul></ul><ul><ul><li>Reduction </li></ul></ul><ul><ul><li>Regeneration of the CO 2 acceptor (RuBP) </li></ul></ul>
  46. 49. LE 10-18_1 [CH 2 O] (sugar) O 2 NADPH ATP ADP NADP + CO 2 H 2 O LIGHT REACTIONS CALVIN CYCLE Light Input 3 CO 2 (Entering one at a time) Rubisco 3 P P Short-lived intermediate Phase 1: Carbon fixation 6 P 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 P P Ribulose bisphosphate (RuBP)
  47. 50. LE 10-18_2 [CH 2 O] (sugar) O 2 NADPH ATP ADP NADP + CO 2 H 2 O LIGHT REACTIONS CALVIN CYCLE Light Input CO 2 (Entering one at a time) Rubisco 3 P P Short-lived intermediate Phase 1: Carbon fixation 6 P 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 P P Ribulose bisphosphate (RuBP) 3 6 NADP + 6 6 NADPH P i 6 P 1,3-Bisphosphoglycerate P 6 P Glyceraldehyde-3-phosphate (G3P) P 1 G3P (a sugar) Output Phase 2: Reduction Glucose and other organic compounds
  48. 51. LE 10-18_3 [CH 2 O] (sugar) O 2 NADPH ATP ADP NADP + CO 2 H 2 O LIGHT REACTIONS CALVIN CYCLE Light Input CO 2 (Entering one at a time) Rubisco 3 P P Short-lived intermediate Phase 1: Carbon fixation 6 P 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 P P Ribulose bisphosphate (RuBP) 3 6 NADP + 6 6 NADPH P i 6 P 1,3-Bisphosphoglycerate P 6 P Glyceraldehyde-3-phosphate (G3P) P 1 G3P (a sugar) Output Phase 2: Reduction Glucose and other organic compounds 3 3 ADP ATP Phase 3: Regeneration of the CO 2 acceptor (RuBP) P 5 G3P
  49. 52. Alternative mechanisms <ul><li>Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis </li></ul><ul><li>On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis </li></ul><ul><li>The closing of stomata reduces access to CO 2 and causes O 2 to build up </li></ul><ul><li>These conditions favor a seemingly wasteful process called photorespiration </li></ul>
  50. 53. Photorespiration: An Evolutionary Relic? <ul><li>In most plants (C 3 plants), initial fixation of CO 2 , via rubisco, forms a three-carbon compound </li></ul><ul><li>In photorespiration, rubisco adds O 2 to the Calvin cycle instead of CO 2 </li></ul><ul><li>Photorespiration consumes O 2 and organic fuel and releases CO 2 without producing ATP or sugar </li></ul>
  51. 54. <ul><li>Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O 2 and more CO 2 </li></ul><ul><li>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 </li></ul>
  52. 55. C 4 Plants <ul><li>C 4 plants minimize the cost of photorespiration by incorporating CO 2 into four-carbon compounds in mesophyll cells </li></ul><ul><li>These four-carbon compounds are exported to bundle-sheath cells, where they release CO 2 that is then used in the Calvin cycle </li></ul>
  53. 56. LE 10-19 Photosynthetic cells of C 4 plant leaf Mesophyll cell Bundle- sheath cell Vein (vascular tissue) C 4 leaf anatomy Stoma Bundle- sheath cell Pyruvate (3 C) CO 2 Sugar Vascular tissue CALVIN CYCLE PEP (3 C) ATP ADP Malate (4 C) Oxaloacetate (4 C) The C 4 pathway CO 2 PEP carboxylase Mesophyll cell
  54. 57. CAM Plants <ul><li>CAM plants open their stomata at night, incorporating CO 2 into organic acids </li></ul><ul><li>Stomata close during the day, and CO 2 is released from organic acids and used in the Calvin cycle </li></ul>
  55. 58. LE 10-20 Bundle- sheath cell Mesophyll cell Organic acid C 4 CO 2 CO 2 CALVIN CYCLE Sugarcane Pineapple Organic acids release CO 2 to Calvin cycle CO 2 incorporated into four-carbon organic acids (carbon fixation) Organic acid CAM CO 2 CO 2 CALVIN CYCLE Sugar Spatial separation of steps Temporal separation of steps Sugar Day Night
  56. 59. The Importance of Photosynthesis: A Review <ul><li>The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds </li></ul><ul><li>Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells </li></ul><ul><li>In addition to food production, photosynthesis produces the oxygen in our atmosphere </li></ul>