1 / 43

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.

takara
Télécharger la présentation

Photosynthesis

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

  30. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.

  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

More Related