Chapter 10 Overview Concept 10 1 photosynthetic bacteria Chloroplasts Photosynthesis converts solar energy into chemical energy Autotrophs sustain themselves without eating other organisms Photoautotrophs Use sunlight for energy Heterotrophs Obtain organic material from other organism Chloroplasts are structurally similar to and likely evolved from Chlorophyll Green pigment within chloroplasts Mesophyll Interior tissue of the leaf Location of chloroplast 30 40 Chloroplasts Thylakoids Connected sacs in the chloroplasts Grana Stacked columns of Thylakoids Stomata Microscopic pores where CO2 enters and O2 exits Stroma Dense interior fluid located in chloroplasts Photosynthesis as a redox process H2O is oxidized CO2 is reduced Endergonic Delta G Non Spontaneous Two Stages of Photosynthesis Light Reaction In the thylakoids Split H2O Release O2 Reduce NADP to NADPH Generate ATP from ADP by Photophosphorylation Calvin Cycle In the Stroma Uses ATP and NADPH Begins with Carbon fixation incorporating CO2 into organic molecules Concept 10 2 and NADPH Nature of Sunlight Chloroplasts are solar powered chemical factories Their Thylakoids transform light energy into the chemical energy of ATP Electromagnetic Radiation Light is a form of electromagnetic energy Wavelength The distance between crests of waves Visible Light Wavelengths that produce colors that we can see Photons Light also behaves as thought is consists of discrete particles Spectrophotometer Measures a pigment s ability to absorb various wavelengths Absorption Spectrum Graph plotting a pigment vs wavelength Chlorophyll a Violet blue and red light work best for photosynthesis main photosynthetic pigment Action Spectrum Profiles the relative effectiveness of different wavelengths of radiation in driving a process First demonstrated in 1883 by Theodor W Exposed different segment of a filamentous alga to different wavelengths excess O2 pigment Areas receiving wavelengths favorable to photosynthesis produced Used growth of aerobic bacteria clustered along the alga as a measure of O2 production Chlorophyll b Broaden the spectrum used for photosynthesis accessory Carotenoids Accessory pigments that absorb excessive light that would damage chlorophyll photoprotection Excitation of Chlorophyll by light causing fluorescence Photosystem When electrons go from unstable to a ground state photons are given off Consists of a reaction center complex protein couples surrounded by light harvesting complexes pigment molecules bound to proteins transfer the energy of photons to the reaction center Primary electron acceptor In the reaction center accepts excited electrons from chlorophyll a and is reduced as a result Two types of photosystems in the thylakoid membrane Photosystem II PS II functions first s reflect order of discovery and is best at absorbing a wavelength of 680 nm Reaction Center Chlorophyll is called P680 Photosystem I PS I Best at absorbing wavelength of 700 Reaction Center Chlorophyll is called P700 Linear Electron Flow 2 Possible routes Cyclic and Linear Linear Electron Flow Primary pathways involves both photosystems and produces ATP and NADPH using light energy A photo hits a pigment and its energy is passed among pigment molecules until it excites P680 electron acceptor We call it P680 An excited electron from P680 is transferred to the primary P680 is a strong oxidizing agent Enzymes split H2O and the electrons are transferred from the H atoms to Each electron falls down electron transport chain from the primary P680 thus reducing it to P680 electron acceptor of PS II to PS I the thylakoid membrane drives ATP synthesis electron to an electron acceptor Energy released by the fall drives the creation of a proton gradient across Diffusion of H Protons across the membrane from thylakoid to Stroma In PS I like PS II transferred light energy excites P700 which loses an P700 accepts an electron passed down from PS II via the ETC Each electron falls down an ETC form the primary electron acceptor of PS I to the protein ferredoxin Fd The electrons are transferred to NADP and reduce it to NADPH Removes Electrons of NADPH are available for the reactions of the Calvin cycle H from Stroma Cyclic Electron Flow is released in the Calvin cycle Uses ONLY photosystem I and produces ATP but NOT NADPH no oxygen Cyclic electron flow generates surplus ATP satisfying the higher demand Some organism such as purple sulfur bacteria have PS I but not PS II Comparison of Chemiosmosis in Chloroplasts and Mitochondria Chloroplasts and mitochondria generate ATP via Chemiosmosis In mitochondria protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial space In Chloroplasts protons are pumped into the thylakoid space and drive ATP ATP and NADPH are produced on the side facing the Stroma outside the thylakoid where the Calvin cycle takes place Reactions generate ATP and increase potential energy of electrons by moving them from water to NADPH Calvin Cycle like the citric acid cycle regenerates its staring material after Cycle builds sugar from smaller molecules by using ATP and the reducing Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde Concept 10 3 molecules enter and leave the cycle power of electrons carried by NADPH 3 phosphate G3P molecules of CO2 3 Phases of the Calvin Cycle For net synthesis of 1 G3P the cycle must take place 3 times fixing 3 limits photosynthesis up Photorespiration soybeans carbon compound 1 Carbon fixation Catalyzed by rubisco ribulose bisphosphate RuBP carboxylase the most abundant protein in the world 2 Reduction By NADPH 6 ATP and 6 NADPH used 3 Regeneration of CO2 acceptor RuBP 3 ATP used Concept 10 4 Dehydration is a problem for plants sometimes requiring trade offs with other metabolic processes especially photosynthesis On hot dry days plants close stomata which conserves water but also Closing of stomata reduces access to CO2 and causes oxygen to build Favor wasteful process photorespiration In most plants C3 Plants initial fixation of CO2 via Rubisco forms a 3 carbon compound 3 phophoglycerate Examples of important agricultural C3 plants Rice wheat and Rubisco adds oxygen instead of CO2 in the Calvin cycle producing a two Photorespiration consumes oxygen and organic fuel and releases carbon dioxide without producing ATP or sugar Photorespiration may be an evolutionary relic because RUBISCO first evolved at a time when the atmosphere had far
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