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Unformatted text preview: Chapter 8 PHOTOSYNTHESIS Photosynthesis : (synthesis from light) biochemical process by which plants capture energy from sunlight and store it as carbohydrates It is the very basis of life on Earth converts light energy into chemical energy (stored in bonds of organic molecules) carried out by photoautotrophs these include green plants, algae, and a few bacteria Photosynthesis occurs in chloroplasts in green plants Chloroplast structure: Double membraned organelle Thylakoids arranged in stacks (grana) Surrounded by stroma OVERVIEW OF TWO MAIN COMPONENTS OF PHOTOSYNTHESIS : Photosynthesis can be divided into two pathways: Light reactions : produce ATP and NADPH Light-independent reactions Carbon Fixation uses ATP, NADPH + H + and CO 2 to produce sugars Calvin cycle, C 4 & CAM The Interactions of Light and Pigments When a photon and a pigment molecule meet, one of three things happens: The photon may be: Scattered or reflected Absorbed by molecule, Or it may pass through ( transmitted ) Molecules that absorb wavelengths in visible range are called pigments Plants look green because chlorophyll absorbs green light less effectively than other colors in sunlight Plants have two main chlorophylls: a & b . These absorb blue & red wavelengths, near ends of visible spectrum Accessory pigments absorb photons between red & blue wavelengths transfer energy to chlorophylls Ex. carotenoids , such as b-carotene, phycobilins Broaden usable spectrum An excited state is unstable the molecule will return to ground state some absorbed energy is given off as heat rest given off as fluorescence Alternatively, pigment molecules may pass some of the absorbed energy to other pigment molecules Pigments are arranged in antenna systems , packed together into thylakoid membrane proteins Excitation energy is passed to pigments that absorb short wavelengths to those that absorb longer wavelengths Ends at reaction center pigment In plants, the pigment molecule in the reaction center is chlorophyll a Electrons of excited molecules are less tightly held by the nucleus more likely to be passed on in a redox reaction to an oxidizing agent (an electron acceptor) Chlorophyll becomes a reducing agent (loses electron), in the redox reaction PHOTOSYNTHETIC PIGMENTS Chlorophyll a the only chlorophyll to directly participate in the light reactions- absorbs primarily red and blue-violet wavelengths Chlorophyll b- absorb mainly orange-red and blue wavelengths Carotenoids- yellow and orange pigments (include xanthophylls)- absorb mainly in the blue to blue-green range Phycobilins- red and blue pigments- absorb in the yellow-green, yellow, to orange range Electrons flow through series of redox reactions via electron carriers in thylakoid membrane , a process termed electron transport Results in two energy rich products of the light reactions,...
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This note was uploaded on 12/03/2010 for the course BSC 2010 taught by Professor Bowes during the Spring '08 term at University of Florida.
- Spring '08