Chapter 8 (Fall2010)

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University of Toronto Scarborough
Biological Sciences
Mary Olaveson

BIOA01FALL2010 CHAPTER 8 Photosynthesis 81 PhotosynthesisMetabolic process by which the energy of sunlight is captured and used to convert carbon dioxide and water into carbohydrate sugars and oxygen gasLight reactions driven by light energy Converts light energy into chemical energy in the form of ATP and a reduced electron carrier NADPHHLight independent reactions do not use light directly but use ATP NADPH H and carbon dioxide to produce sugarsCalvin cycle C4 photosynthesis and crassulacean acid metabolism 82 Light into Chemical EnergyLight is a form of electromagnetic radiation and it comes in packets of energy called photonsIt can behave as a particle or as a waveThe shorter the wavelength the greater the energy of the photonsabsorbed by a receptive moleculeWhen a photon meets a molecule it may bounce off scatter or reflect may pass through transmitted or it may be absorbeddisappearsThe molecule then becomes excited because of the new gained energyPigments molecules that absorb wavelenghts region of spectrum visible to humansAction spectrum rate of photosynthesis vs wavelengthAbsorption spectrum absorption by pigment vs wavelengthPigment molecules chlorophyll carotenoids phycobilins Chlorophylls chlorophyll a and chlorophyll b predominatecomplex ring structure with magnesium in the middle and a hydrocarbon tail attached at a peripheral location so the molecule can attach to integral proteins in the thylakoid membrane of the cholorplastAccessory pigments absorb photons intermediate in energy between the red and the blue wavelentghts and then transfer a portion of that energy to the chlorophylls Carotenoids such as beta carotene which absorb photons in the blue and bluegreen wavelengths and appear deep yellow Phycobillins which are found in re algae and cyanobacteria and absorb several yellowgreen yellow and orange wavelengthsThe pigments in photosynthetic organisms are arranged into energy absorbing antenna systemspigments are packed together and attached to thylakoid membrane proteins in such a way that the excitation energy from an absorbed photon can be passed along from one pigment molecule in the system to anotherenergy moves from pigments that absorb shorter wavelengths to longer wavelengths lower energy and ends up in the one migment molecule in the antenna system that absorbs the longest wavelength which is the reaction center of the antenna systemChlorophyll has 2 roles it absorbs light energy and transforms it ino chemical energy in the form of electrons and it transfers those electrons to other moleculesCholorophyll molecule in the reaction center is used as reducing agent to reduce a stable electron acceptorThen electron transport occurs where the final acceptor is NADP nicotinamide adenine dinucleotide phosphate which gets reduced
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