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Chapter 7


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BIOL 1000
Julie Clark

Tanya Sivamanoaharan Thurs, Nov, 10/11 Biology- chap# 7 textbook notes PHOTOSYNTHESIS 7.1- Photosynthesis: An Overview -photosynthetic organisms are photoautotrophs and are primary producers of earth -they convert energy of sunlight into chem. energy and make simple inorganic molec. into complex organic molecules -photoautotrophs serve as direct or indirect food for consumers (animals, humans) -eventually the bodies of primary producers and consumers provide chem. energy for bacteria, fungi and other decomposers 7.1a- The 2 Parts of Photosynthesis -photosynthesis can be divided into 2 parts: -light-dependant reactionscapturing of sunlight by pigment molec. and using that energy to produce NADPH and ATP. This process requires an ETC chain that uses electrons donated from water 2H2O+ light energy 4H+ + 4e- + O2 -Calvin cycle electrons carried by NADPH and energy of ATP are used to convert carbon dioxide from inorganic to organic form, process called carbon fixation. This conversion is a reduction where electrons and protons are added to carbon dioxide. -3-carbon sugar are major product of Calvin cycle -3-carbon sugars can combine to make 6=carbon sugars (glucose) -the overall balanced equation of photosynthesis is: 6CO2 + 12H2O C6H1206 + 602 + 6H2O -sugar is main product of photosynthesis -all the organic molec. of plants are assembled as direct or indirect products of photosynthesis 7.1b- In Eukaryotes, Photosynthesis Takes Place in Chloroplasts -in eukaryotes, both light reactions and Calvin cycle take place within the chloroplast -the chloroplast has an inner membrane, outer membrane, and intermembrane space -the intermembrane space is an aqueous envir. called the stroma -within the stroma are the thylakoid membranes/thylakoids -space enclosed within the thylakoids is called thylakoid lumen -thylakoid membrane holds molecules that carry out photosynthesis (pigments, electron transfer electrons, and ATP synthase enzymes for ATP production -enzymes that catalyze Calvin cycle are found in stroma of chloroplast -cells lacking chloroplasts may still be photosynthetic -many photosynthetic prokaryotes also have thylakoid membranes formed by infoldings of plasma membrane 7.2- The Photosynthetic Apparatus -to trap light and make it into chem. energy requires photochemical apparatus -we will look at components of light reactions located on thylakoid membranes 7.2a- Electrons in Pigment Molecules Absorb Light Energy -2 import. points about light and pigment molecules: 1.-the absorption of photon by a pigment molecule excites electron, moving it from ground state to excited state. 2.-difference in energy level between ground state and excited state is equivalent to the energy of photon of light that was absorbed -following light absorption (photon) there are 3 things that can happen to the energy that is absorbed: 1.the electron returns to its ground state by emitting less energetic photon/releasing heat as energy 2.the electron returns to its ground state, and the energy released transfers to a neighbouring pigment molec., inductive resonance 3.the high-energy electron is accepted by an electron accepting molecule, primary acceptor 7.2b- Chlorophylls and Carotenoids Cooperate in Light Absorption -chlorophyllmajor photosynthetic pigments in plants (chlorophyll a and chlorophyll b) -carotenoidsanother major group of pigments involved in photosynthesis -during photosynthesis chlorophyll a becomes oxidized and donates electron to primary acceptor -carotenoids and chlorophyll b are accessory pigments b/c after light absorption they donate excitation energy by inductive resonance to molec. of chlorophyll a -pigment molec. do not absorb all wavelengths of light, pigment is the colour of wavelength that cannot be absorbed (green chlorophyll cannot absorb green light) -chlorophyll a absorbs a lot of red and blue and no green and yellow -photosynthesis is dependent on absorption of light by chlorophyll and carotenoids 7.2c- Photosynthetic Pigments Are Organized into Photosystems -pigment molec. are bound precisely to a # of diff. proteins within thylakoid membrane -these pigment proteins are organized into photosystems -each photosystem is composed of large antenna complex (light- harvesting complex) that surround a central reaction site -reaction centrecomprises a small # of proteins that bind a pair of specialized chlorophyll a molec. as well as the primary electron acceptor -there is photosystem 1 and photosystem 2 -the specialized chlorophyll a of photosystem 1 is P700nm
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