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SC BIOL 425 - BIOL 425 Reading Notes Ch 7

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BIOL 425/001CHAPTER 7: Photosynthesis, Light, and LifePhotosynthesis: A Historical Perspective- Photosynthesis is the route by which virtually all energy enters our biosphere- More than 250 billion metric tons/year of sugar are produced worldwide by photosynthetic organisms- Aristotle and other Greeks observed that life processes of animals were dependent on the food they ate; they thought that plants derived food from the soil- Jan Baptista van Helmont (1577-1644) carried out an experiment to prove thatsoil was not the only important factor in photosynthesiso Grew a small willow tree, only adding water ot the poto The plant increased in weight by 74.4 kilogramso The soil had only decreased in weight by 57 gramso Thus, he concluded that all substance of the plant was produced from the water and not from the soil (still wrong, broad conclusion)- Priestley showed that plants restore air by putting a mint plant in a room with little to no oxygen, and then lighting a candle 27 days later (fire requires O2)o Determined that plans cleanse the airo We now know that plants take up CO2 and give off O2- Jan Ingenhousz determined that Priestley’s experiment only works in sunlit rooms with green plantso Ingenhousz suggested that CO2 is split in photosynthesis to yield carbon and oxygen, with oxygen being released as gaso Also realized that the proportion of C, H, and O were found to be CH2Oo Determined the rxn: CO2 + H2O  (CH2O) + O2o Thought that the carbohydrate came from a carbon attaching to a water molecule – not the case- C.B> van Niel determined that the water, not the carbon dioxide, was the source of the oxygen for photosynthesis- Robin Hill (1937) showed that, when exposed to light, isolated chloroplasts were able to produce O2 in the absence of CO2 – called the Hill reactiono The light-driven release of O2 in the absence of Co2 occurred only when the chloroplasts were illuminated and provide with an artificial electron acceptor- Samuel Ruben and Martin Kamen traced an oxygen isotope in water through to the O2- F. F. Blackman showed that light is required for only part of the photosynthesisprocessThe Nature of Light- The electromagnetic spectrum – a continuous spectrum of radiation- All radiation travels in waves at varying wavelengths- The shorter the wavelength, the greater the energy- The wave model alone is not accurateo The wave model predicts that the brighter the light, the greater the force with which the electrons will be dislodged from a metalo Whether or not light can eject the electrons actualy depends on the wavelength- Thus, Einstein proposed the particle model of the lighto Light is composed of photonso The energy of a photon is inversely proportional to it’s wavelength- The two models are regarded as complementaryThe Role of Pigments- A substance that absorbs light is a pigment- Black – absorb all wavelengths of visible light- Most reflect the wavelengths that they do not absorb- The light absorption pattern of a pigment is known as the absorption spectrumof that substance- Chlorophyll absorbs blue, violet, and red, but not green- An action spectrum demonstrates the relative effectiveness of different wavelengths of light for a specific light requiring process- When pigments absorb light, electrons are boosted to an excited state- As the electrons return to their ground levels, the energy released has 3 possible fateso Energy is lost as a lower energy photon – fluorescence and heato The energy, but not the electron, may be transferred to another chlorophyll molecule, exciting that chlorophyll’s electrons – this is called resonance energy transfero The electron itself may be transferred to a neighboring molecule that ispart of an electron transport chain – this leaves an “electron hole” in the excited chlorophyll molecule- The last two are productive reactions for photosynthesis- The main photosynthetic pigmentso Chlorophyll a occurs in all photosynthetic eukaryotes and in cyanobacteria Essential for the oxygen-generating photosynthesis carried out by those organisms ¾ of total chlorophyll in most plantso Plants, green algae, and euglenoid algae also produce chlorophyll b, which has a slightly different absorption spectrum than chlorophyll a An accessory pigment – not directly involved in photosynthetic energy transduction but serves to broaden the range of light that can be used in photosynthesis The energy absorbed by chlorophyll b is ultimately transferred to chlorophyll a, which then converts it to chemical energyo Chlorophyll c takes the place of chlorophyll b in some groups of algaeo Other bacteria use bacteriochlorophyll (in purple bacteria) or chlorobium chlorophyll (green sulfur bacteria) – cannot extract electrons from water and thus do not evolve oxygeno Carotenoids Red, orange, or yellow lipid-soluble pigments found in all chloroplasts and in cyanobacteria Embedded in the thylakoid membranes like chlorophylls Carotenes and xanthophylls are normally present in chloroplastso Phycobilins are found in cyanobacteria and in the chloroplasts of red algae; these are water solubleThe Reactions of Photosynthesis- Two types: light reactions and carbon-fixation reactions- Light Reactions – light is used to form ATP and to reduce NADP+ to NADPH, water molecules are split, O2 is liberated, and the electrons released convert NADP+ to NADPH- Carbon-fixation Reactions = energy of ATP links CO2 covalently to an organic molecule, and then reducing power of NAPH reduces the newly fixed carbon atoms to a simple sugar- Light Reactionso The chlorophyll and other pigment molecules are embedded in the thylakoids in photosystems Each photosystem contains 250-400 pigment molecules and consists of two closely linked components: an antenna complex and a reaction center Antenna Complex: pigment molecules that gather light and “funnel” it to the reaction center Reaction Center: a complex of proteins and chlorophyll molecules that enable light energy to be converted to chemical energyo All of the pigments within a photosystem can capture photons, but onlyone pair of special chlorophyll a molecules, found in the core of the reaction center, can actually use the energy in the photochemical reaction Antenna pigments are the pigments that are part of the light-gathering network located in the antenna complex; also contains carotenoidso Light energy absorbed by a pigment molecule is transferred from one pigment molecule


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