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Photosynthesis

Photosynthesis. Chapter 8. Photosynthesis Overview. Energy for all life on Earth ultimately comes from photosynthesis 6CO 2 + 12H 2 O C 6 H 12 O 6 + 6H 2 O + 6O 2 Oxygenic photosynthesis is carried out by Cyanobacteria 7 groups of algae All land plants – chloroplasts.

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Photosynthesis

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  1. Photosynthesis Chapter 8

  2. Photosynthesis Overview • Energy for all life on Earth ultimately comes from photosynthesis 6CO2 + 12H2O C6H12O6 + 6H2O + 6O2 • Oxygenic photosynthesis is carried out by • Cyanobacteria • 7 groups of algae • All land plants – chloroplasts

  3. Chloroplast • Thylakoid membrane – internal membrane • Contains chlorophyll and other photosynthetic pigments • Pigments clustered into photosystems • Grana – stacks of flattened sacs of thylakoid membrane • Stroma lamella – connect grana • Stroma – semiliquid surrounding thylakoid membranes

  4. Stages • Light-dependent reactions • Require light • Capture energy from sunlight • Make ATP and reduce NADP+ to NADPH • Carbon fixation reactions or light-independent reactions • Does not require light • Use ATP and NADPH to synthesize organic molecules from CO2 Animation

  5. Discovery of Photosynthesis • Jan Baptista van Helmont (1580–1644) • Demonstrated that the substance of the plant was not produced only from the soil • Joseph Priestly (1733–1804) • Living vegetation adds something to the air • Jan Ingen-Housz (1730–1799) • Proposed plants carry out a process that uses sunlight to split carbon dioxide into carbon and oxygen (O2 gas)

  6. F.F. Blackman (1866–1947) • Came to the startling conclusion that photosynthesis is in fact a multistage process, only one portion of which uses light directly • Light versus dark reactions • Enzymes involved

  7. C. B. van Niel (1897–1985) • Found purple sulfur bacteria do not release O2 but accumulate sulfur • Proposed general formula for photosynthesis • CO2 + 2 H2A + light energy → (CH2O) + H2O + 2 A • Later researchers found O2 produced comes from water • Robin Hill (1899–1991) • Demonstrated Niel was right that light energy could be harvested and used in a reduction reaction

  8. Pigments • Molecules that absorb light energy in the visible range • Light is a form of energy • Photon – particle of light • Acts as a discrete bundle of energy • Energy content of a photon is inversely proportional to the wavelength of the light • Photoelectric effect – removal of an electron from a molecule by light

  9. Absorption spectrum • When a photon strikes a molecule, its energy is either • Lost as heat • Absorbed by the electrons of the molecule • Boosts electrons into higher energy level • Absorption spectrum – range and efficiency of photons molecule is capable of absorbing

  10. Many pigments • Organisms have evolved a variety of different pigments • Only two general types are used in green plant photosynthesis • Chlorophylls • Carotenoids • In some organisms, other molecules also absorb light energy

  11. Chlorophylls • Chlorophyll a • Main pigment in plants and cyanobacteria • Only pigment that can act directly to convert light energy to chemical energy • Absorbs violet-blue and red light • Chlorophyll b • Accessory pigment or secondary pigment absorbing light wavelengths that chlorophyll a does not absorb

  12. Structure of chlorophyll • Porphyrin ring • Complex ring structure with alternating double and single bonds • Magnesium ion at the center of the ring • Photons excite electrons in the ring • Electrons are shuttled away from the ring

  13. Action spectrum • Relative effectiveness of different wavelengths of light in promoting photosynthesis • Corresponds to the absorption spectrum for chlorophylls

  14. Other Pigments • Carotenoids • Carbon rings linked to chains with alternating single and double bonds • Can absorb photons with a wide range of energies • Also scavenge free radicals – antioxidant • Protective role • Phycobiloproteins • Important in low-light ocean areas

  15. Photosystem Organization • Antenna complex • Hundreds of accessory pigment molecules • Gather photons and feed the captured light energy to the reaction center • Reaction center • 1 or more chlorophyll a molecules • Passes excited electrons out of the photosystem

  16. Antenna complex • Also called light-harvesting complex • Captures photons from sunlight and channels them to the reaction center chlorophylls • In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins

  17. Reaction center • Transmembrane protein–pigment complex • When a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level • Light-energized electron can be transferred to the primary electron acceptor, reducing it • Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule

  18. Light-Dependent Reactions • Primary photoevent • Photon of light is captured by a pigment molecule • Charge separation • Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule • Electron transport • Electrons move through carriers to reduce NADP+ • Chemiosmosis • Produces ATP Capture of light energy

  19. Cyclic photophosphorylation • In sulfur bacteria, only one photosystem is used • Generates ATP via electron transport • Anoxygenic photosynthesis • Excited electron passed to electron transport chain • Generates a proton gradient for ATP synthesis

  20. Chloroplasts have two connected photosystems • Oxygenic photosynthesis • Photosystem I (P700) • Functions like sulfur bacteria • Photosystem II (P680) • Can generate an oxidation potential high enough to oxidize water • Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH

  21. The connection • Photosystem I transfers electrons ultimately to NADP+, producing NADPH • Electrons lost from photosystem I are replaced by electrons from photosystem II • Photosystem II oxidizes water to replace the electrons transferred to photosystem I • 2 photosystems connected by cytochrome/ b6-f complex

  22. Noncyclicphotophosphorylation • Plants use photosystems II and I in series to produce both ATP and NADPH • Path of electrons not a circle • Photosystems replenished with electrons obtained by splitting water • Z diagram

  23. Photosystem II • Resembles the reaction center of purple bacteria • Core of 10 transmembrane protein subunits with electron transfer components and two P680 chlorophyll molecules • Reaction center differs from purple bacteria in that it also contains four manganese atoms • Essential for the oxidation of water • b6-f complex • Proton pump embedded in thylakoid membrane

  24. Photosystem I • Reaction center consists of a core transmembrane complex consisting of 12 to 14 protein subunits with two bound P700 chlorophyll molecules • Photosystem I accepts an electron from plastocyanin into the “hole” created by the exit of a light-energized electron • Passes electrons to NADP+ to form NADPH

  25. Chemiosmosis • Electrochemical gradient can be used to synthesize ATP • Chloroplast has ATP synthase enzymes in the thylakoid membrane • Allows protons back into stroma • Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions

  26. Production of additional ATP • Noncyclic photophosphorylation generates • NADPH • ATP • Building organic molecules takes more energy than that alone • Cyclic photophosphorylation used to produce additional ATP • Short-circuit photosystem I to make a larger proton gradient to make more ATP

  27. Carbon Fixation – Calvin Cycle • To build carbohydrates cells use • Energy • ATP from light-dependent reactions • Cyclic and noncyclic photophosphorylation • Drives endergonic reaction • Reduction potential • NADPH from photosystem I • Source of protons and energetic electrons

  28. Calvin cycle • Named after Melvin Calvin (1911–1997) • Also called C3 photosynthesis • Key step is attachment of CO2 to RuBP to form PGA • Uses enzyme ribulosebisphosphatecarboxylase /oxygenaseor rubisco

  29. 3 phases • Carbon fixation • RuBP + CO2 → PGA • Reduction • PGA is reduced to G3P • Regeneration of RuBP • PGA is used to regenerate RuBP • 3 turns incorporate enough carbon to produce a new G3P • 6 turns incorporate enough carbon for 1 glucose

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  31. Output of Calvin cycle • Glucose is not a direct product of the Calvin cycle • G3P is a 3 carbon sugar • Used to form sucrose • Major transport sugar in plants • Disaccharide made of fructose and glucose • Used to make starch • Insoluble glucose polymer • Stored for later use

  32. Energy cycle • Photosynthesis uses the products of respiration as starting substrates • Respiration uses the products of photosynthesis as starting substrates • Production of glucose from G3P even uses part of the ancient glycolytic pathway, run in reverse • Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria

  33. Photorespiration • Rubisco has 2 enzymatic activities • Carboxylation • Addition of CO2 to RuBP • Favored under normal conditions • Photorespiration • Oxidation of RuBP by the addition of O2 • Favored when stoma are closed in hot conditions • Creates low-CO2 and high-O2 • CO2 and O2 compete for the active site on RuBP

  34. Types of photosynthesis • C3 • Plants that fix carbon using only C3 photosynthesis (the Calvin cycle) • C4 and CAM • Add CO2 to PEP to form 4 carbon molecule • Use PEP carboxylase • Greater affinity for CO2, no oxidase activity • C4 – spatial solution • CAM – temporal solution

  35. C4 plants • Corn, sugarcane, sorghum, and a number of other grasses • Initially fix carbon using PEP carboxylase in mesophyll cells • Produces oxaloacetate, converted to malate, transported to bundle-sheath cells • Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2 • Carbon fixation then by rubisco and the Calvin cycle

  36. The cost of doing business this way • C4 pathway, although it overcomes the problems of photorespiration, does have a cost • To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone • C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone

  37. CAM plants • Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups • Stomata open during the night and close during the day • Reverse of that in most plants • Fix CO2 using PEP carboxylase during the night and store in vacuole

  38. CAM plants in action • When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO2 • High levels of CO2 drive the Calvin cycle and minimize photorespiration

  39. Compare C4 and CAM • Both use both C3 and C4 pathways • C4– two pathways occur in different cells • CAM – C4 pathway at night and the C3 pathway during the day

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