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"Without this flow of energy from the sun, channeled largely through the eukaryotic cells, the pace of life on this planet would swiftly diminish and then,

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Presentation on theme: ""Without this flow of energy from the sun, channeled largely through the eukaryotic cells, the pace of life on this planet would swiftly diminish and then,"— Presentation transcript:

1 "Without this flow of energy from the sun, channeled largely through the eukaryotic cells, the pace of life on this planet would swiftly diminish and then, following the inexorable second law of thermodynamics would virtually cease altogether." Raven, P. H. et al I. INTRODUCTION CHAPTER 7 PHOTOSYNTHESIS 14 pts

2 PHOTOSYNTHESIS OVERVIEW II.CHAPTER OUTLINE

3 Photosynthesis - summary CARBON DIOXIDE + WATER + LIGHT  CARBOHYDRATE + OXYGEN 6 CO H 2 O  C 6 H 12 O O 2 + H 2 O This reaction captures energy from sunlight Light energy is used to reduce carbon (removes hydrogen from water and adds it to carbon) Energy is stored in plant tissues as starch, sugars and in cellulose (wood, coal, gas, oil) Stored energy can be released by burning, or by metabolism

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5 IMPORTANCE OF PHOTOSYNTHESIS II.CHAPTER OUTLINE

6 AUTOTROPHS (Can make own food, i.e., can manufacture organic compounds from inorganic compounds) HETEROTROPHS (Eat others to get organic compounds) PHOTOSYNTHETIC ORGANISMS ARE AT THE BASE OF THE FOOD CHAIN

7 Photosynthesis is the source of Oxygen in the atmosphere §Earth's atmosphere is a layer of gases surrounding the planet and retained by the Earth's gravity. l 78% nitrogen l 20.95% oxygen l 0.93% argon l 0.038% carbon dioxide (up from 0.028%) l a variable amount (average around 1%) of water vapor §Oxygen is reactive with inorganic compounds and does not stay in the atmosphere. Thus, our current atmosphere is a balance of oxygen produced by plants and loss to geological processes

8 THE NATURE OF LIGHT II.CHAPTER OUTLINE

9 THE VISIBLE SPECTRUM CAN BE SEEN WHEN WHITE LIGHT IS PASSED THROUGH A PRISM. THE SPECTRUM IS SEEN AS DIFFERENT COLORS OF LIGHT Spectrum

10 A “SPECTRUM” IS A RANGE OF WAVELENGTHS OF ELECTROMAGNETIC RADIATION One wavelength

11 Ultraviolet light, X-rays and gamma rays have too much energy for biological systems. They break bonds, knock electrons out of orbit and generally cause cell damage. We are protected from UV by the ozone layer Infrared light is heat and has too low an energy level to be used for photosynthesis

12 Pigments are molecules that absorb light. We see them as being the color of the light that is reflected In order to trap light, light must be absorbed. (If light is reflected, it has no value) Pigments - absorption of light

13 PIGMENTS II.CHAPTER OUTLINE

14 T. W. Engelmann (1883) Designed an elegant experiment with spirogyra, an algae with a single long coiled chloroplast. He combined the algae with an oxygen-requiring bacterium and exposed the algae to light coming through a prism. He showed that the bacteria clustered in the blue and red areas suggesting that these areas were producing oxygen. Engelmann 1883

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16 T. W. Englemann’s experiment 1882 Spirogyra

17 Chlorophyll a Chlorophyll b Chlorophyll c (replaces Chlorophyll b in some algae) Carotenoids Carotene Xanthophylls Phycobilins (photosynthetic bacteria) Phycoerythrin Phycocyanin PIGMENTS OF PLANTS

18 Chlorophyll a

19 Carotenoids Absorb maximally between 460 and 550 nm, absorb in the blue range and therefore appear red, orange and yellow Carotenoids

20 Phycobilins The third major class of pigments, the phycobilins, are found in the cyanobacteria and red algae only

21 REACTIONS OF PHOTOSYNTHESIS A. LIGHT vs DARK REACTIONS II.CHAPTER OUTLINE

22 F. F. Blackman (1905) English Plant Physiologist: Concluded that there were two reactions, one of which was light dependent (photochemical reactions) and one of which was temperature dependent (biochemical reactions or dark reactions)

23 Stroma Grana (stacked granum thylakoids) Stroma thylakoids Plant cell wall Chloroplast double membrane Light reactions take place in the grana thylakoids Light reactions

24 Closeup of grana and stroma Stack of granum thylakoids Stroma A stack of folded membranes is referred to as a granum. The stroma is a watery matrix Light reactions

25 Dark or biochemical rxns

26 REACTIONS OF PHOTOSYNTHESIS B. LIGHT REACTION II.CHAPTER OUTLINE

27 Light-trapping pigments float in the thylakoid membranes

28 The pigment aggregates have been given the name antenna complex because it acts as an antenna to trap light and transfer the absorbed energy Antenna complex = Chlorophyll a, chlorophyll b and carotenoids

29 e- When light has the correct energy level, it is absorbed by an electron. The electron must move to a new orbital whose energy level corresponds to the electron’s new energy load Photon Absorption of light

30 e- The excited state is unstable. The atom can become stable if the electron drops back into lower orbit. When this happens the excess energy may be released as visible light (fluorescence) or heat (infared light) Heat or light Absorption of light

31 Alternately, the electron can move to a more stable orbital on an entirely different atom. This electron transfer is the critical step in photosynthesis Absorption of light

32 Reaction center: Within the antenna complex, there is a special pair of chlorophyll A molecules and associated proteins called a reaction center Pigment complexes

33 Function of the reaction center: accepts electrons transferred from the antenna complex and boosts them into higher orbit where they can be transferred to another electron acceptor molecule Pigment complexes

34 There are two different types of reaction center/antenna complexes in the thylakoid membranes (light reaction). The reaction center of Photosystem I absorbs maximally at 700 nm. The reaction center is called P700 The reaction center of Photosystem II absorbs maximally at 680 nm. The reaction center is called P680

35 Energy depiction of Photosynthesis, the “Z” scheme

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37 Photophosphorylation = LIGHT driven production of ATP 1. Electrons are passed along a series of electron carriers When electrons reach transmembrane pumps, protons are pumped into the lumen of the thylakoid membrane This proton gradient drives synthesis of ATP into the stroma (cytoplasm) of the chloroplast 2.Similar in principle to oxidative phosphorylation in mitochondria Photophosphorylation

38 Requires light Independent of temperature (an indication that enzymes are not involved) Light is used to make ATP and to reduce electron carrier molecules Light reactions - summary

39 Produce 3 ATPs Reduce 2 molecules of NADP+ to NADPH Oxygen is produced from splitting water Summary of light reaction Eight photons used in the light (photochemical) reaction

40 CYCLIC ELECTRON FLOW In photosynthetic prokaryotes, only part of the reaction is present (photosystem 1). This is the older system. Green plants have added a second system to increase energy capture. No water is split No NADPH is made a. Cyclic phosphorylation alone may have existed for a billion years in bacteria who used sulfur as an electron donor but modern plants use water as a source of electrons and that required a major change because the energy in P700 isn’t high enough to remove an electron from water. b. Plants overcame this problem by grafting on a second, more powerful photosystem which could harvest shorter, higher energy wavelengths of light (Z scheme)

41 from ATP synthase

42 REACTIONS OF PHOTOSYNTHESIS B. DARK REACTIONS II.CHAPTER OUTLINE

43 PHOTOSYNTHESIS IS COMPOSED OF TWO SETS OF REACTIONS LIGHT DEPENDENT = PHOTOCHEMICAL TEMPERATURE INDEPENDENT LIGHT DEPENDENT DARK REACTIONS = CARBON FIXATION RXNS = BIOCHEMICAL RXNS TEMPERATURE DEPENDENT LIGHT INDEPENDENT

44 Berkeley scientist, Melvin Calvin (1950) was the first to elucidate the steps in the dark reaction of photosynthesis

45 Dark dependent stage Occurs in the light but does not require light Temperature dependent or sensitive reactions. Reactions increase up to 30C then decrease. This temperature profile is an indication that enzymes are involved Energy products of the light reaction are used to reduce carbon from CO 2 to a sugar (carbon fixation)

46 RUBISCO Ribulose-1,5-bisphosphate or RuBP 8 large subunits (gene in chloroplast) 8 small subunits (gene in nucleus) Most abundant protein on earth

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48 PGAL is an intermediate in respiration during the oxidation of glucose to pyruvate Sugar

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50 C3 PLANTS PLANTS WHICH USE ONLY THE CALVIN CYCLE TO FIX CO 2 INTO SUGARS ARE CALLED C3 PLANTS (BECAUSE A THREE-CARBON MOLECULE IS THE FIRST STABLE MOLECULE IN THE CYCLE) MOST PLANT SPECIES ARE C3 PLANTS CEREALS PEANUTS COTTON SOYBEANS MOST TREES MOST LAWN GRASSES

51 REACTIONS OF PHOTOSYNTHESIS Photorespiration II.CHAPTER OUTLINE

52 When CO 2 levels in the leaf get too low, RUBISCO starts binding oxygen instead of CO 2 Happens when stomates close to prevent water loss. (This prevents CO 2 diffusion into the leaf) Forces another pathway O 2 + RuBP  PGA + phosphoglycolic acid Reactions called photorespiration because they occur in the light, use oxygen and release CO 2 Unlike normal respiration, it produces no ATP It is a wasteful salvage pathway PHOTOSYNTHESIS IS NOT PERFECT: PHOTORESPIRATION

53 RUBISCO RUBISCO binds CO 2 and O 2 in the same site O2O2 CO 2 If oxygen is bound, the Calvin cycle cannot take place Instead, photorespiration takes place

54 wasted

55 REACTIONS OF PHOTOSYNTHESIS The C4 pathway II.CHAPTER OUTLINE

56 Some plants have developed mechanisms to bypass the RUBISCO carboxylase/oxygenase problem These plants are called C4 plants because they fix carbon into a 4-carbon sugar Plants out-compete C3 plants in hot dry weather only. C4 PLANTS

57 C4 carbon fixation has evolved on up to 40 independent occasions in different groups of plants, making it an example of convergent evolution. Plants which use C4 metabolism include sugarcane, maize, sorghum, finger millet, amaranth, and switchgrass. C4 plants arose around 25 to 32 million years ago convergent evolution

58 Calvin cycleMalate C4 plants have a specific kind of leaf anatomy. Carbon is fixed into malate in mesophyll cells. The Calvin cycle occurs in bundle sheath cells that surround the vascular system (few thylakoid grana and protected from oxygen). Thus, the two processes are separated in space.

59 Bundle sheath cell Chloroplasts lack grana Mesophyll cell with granna

60 Calvin cycle-note few grana Malate Plasmodesmata

61 REACTIONS OF PHOTOSYNTHESIS CAM Plants

62 Crassulacean acid metabolism Separates CO2 intake and the Calvin cycle by time rather than by space Crassula ovata – Jade Plant

63 Crassulacean acid metabolism

64 The majority of plants possessing Crassulacean Acid Metabolism are either epiphytes (e.g. orchids, bromeliads) or succulent xerophytes (e.g. cacti, cactoid Euphorbias). 16,000 species (7% of plants). Most are angiosperms but includes some gymnosperms, ferns and quillworts

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66 REACTIONS OF PHOTOSYNTHESIS Carbon Cycle II.CHAPTER OUTLINE

67 Global Warming

68 Another important carbon “sink” is the arctic tundra which is now decomposing and releasing CO 2 as the arctic thaws.

69 Carbon Dioxide in the atmosphere GREENHOUSE EFFECT – HUMANS ARE RELEASING STORED CARBON WHICH INCREASES ATMOSPHERIC CO 2 The present level is higher than at any time during the last 800 thousand years, [6] and likely higher than in the past 20 million years. [7] [6] [7] 6.Amos, Jonathan ( ). "Deep ice tells long climate story". BBC News. Retrieved "Deep ice tells long climate story" Retrieved Climate Change 2001: The Scientific Basis

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71 TRENDS TOTAL GHG EMMISIONS INCREASED 14.2% SINCE 1990 DOMINANT GAS EMITTED IS CO 2, MOSTLY FROM FOSSIL FUEL COMBUSTION (oxidation of sugars produced by fossil plants)

72 Summer NOAA CO2 tracker Winter NOAA CO2 tracker Red=high CO 2 emissions Blue = low CO 2 emissions

73 END


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