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I. INTRODUCTION

CHAPTER 7 PHOTOSYNTHESIS. I. INTRODUCTION.

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I. INTRODUCTION

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  1. CHAPTER 7 PHOTOSYNTHESIS I. INTRODUCTION "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. 1992. 14 pts

  2. CHAPTER OUTLINE PHOTOSYNTHESIS OVERVIEW

  3. Photosynthesis - summary • CARBON DIOXIDE + WATER + LIGHT  • CARBOHYDRATE + OXYGEN • 6 CO2 + 6 H2O  C6H12O6 + 6 O2 + H2O • This reaction captures energy from sunlight • Light energy is used to reducecarbon (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

  4. CHAPTER OUTLINE IMPORTANCE OF PHOTOSYNTHESIS

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

  6. 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. • 78% nitrogen • 20.95% oxygen • 0.93% argon • 0.038% carbon dioxide (up from 0.028%) • a variable amount (average around 1%) of water vapor • Oxygen is reactive with inorganic compounds and does notstay in the atmosphere. Thus, our current atmosphere is a balance of oxygen produced by plants and loss to geological processes

  7. CHAPTER OUTLINE THE NATURE OF LIGHT

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

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

  10. 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

  11. Pigments - absorption of light • 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)

  12. CHAPTER OUTLINE PIGMENTS

  13. Engelmann 1883 • 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.

  14. T. W. Englemann’s experiment 1882 Spirogyra

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

  16. Chlorophyll a

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

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

  19. CHAPTER OUTLINE REACTIONS OF PHOTOSYNTHESIS A. LIGHT vs DARK REACTIONS

  20. 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)

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

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

  23. Dark or biochemical rxns

  24. CHAPTER OUTLINE REACTIONS OF PHOTOSYNTHESIS B. LIGHT REACTION

  25. Light-trapping pigments float in the thylakoid membranes

  26. 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

  27. 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 Absorption of light Photon e-

  28. 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) Absorption of light e- Heat or light

  29. 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

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

  31. 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

  32. 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

  33. Energy depiction of Photosynthesis, the “Z” scheme

  34. Photophosphorylation • 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 • Similar in principle to • oxidative phosphorylation • in mitochondria

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

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

  37. 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)

  38. from ATP synthase

  39. CHAPTER OUTLINE REACTIONS OF PHOTOSYNTHESIS B. DARK REACTIONS

  40. 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

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

  42. 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 CO2 to a sugar (carbon fixation)

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

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

  45. C3 PLANTS • PLANTS WHICH USE ONLY THE CALVIN CYCLE TO FIX CO2 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

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