photosynthesis chapter 10. n objectives f compare the overall reaction of photosynthesis with the...
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Photosynthesis
Chapter 10
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Objectives Compare the overall reaction of photosynthesis with
the overall reaction for respiration Describe where the processes of photosynthesis occur Describe the basic processes involved in
photosynthesis: water splitting to obtain electrons, redox reactions of the electron transport chains, electron and energy shuttling by means of ATP and NADPH, and the coupling of the light-dependent reactions and the Calvin cycle
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Explain how pigments capture light and excite electrons
Describe the structural and functional differences between photosystem I and photosystem II, and cyclic and noncyclic photophosphorylation
Outline the steps in the cyclic fixation of carbon in the Calvin cycle and where these occur
Contrast the C4 and CAM photosynthetic systems with the simpler C3 system, and how they are adaptions to hot, dry climates
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Introduction
Overall equation is reversal of cellular respiration 6CO2+12H2O+energy--->C6H12O6+6O2+6H2O
Increasingly probing studies provided knowledge about how photosynthesis works van Helmont-developed early ideas about where
plants obtain materials for growth showed that soil not sufficient concluded that water important
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Priestly-showed that plants restore “bad” air Ingenhousz-plants only restore air when
exposed to light
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Autotrophs are Producers
Autotroph-means self-feeding applies to any organism that makes own food
without eating, decomposing or absorbing other organisms or organic molecules
Photosynthetic autotrophs include plants, algae and photosynthetic bacteria
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Site of Photosynthesis
Photosynthesis occurs in chloroplasts in all photosynthetic organisms except monerans
Leaves (specifically, mesophyll cells) are primary site of photosynthesis
Light-absorbing pigment is chlorophyll located in protein complexes in internal
membranes of chloroplasts Sugars assembled in stroma
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Underlying Processes Oxygen produced by splitting water
demonstrated using 18O-labeled reactants plant given C18O2 does not release 18O2
plant given H218O does give off 18O2
Photosynthesis is redox process H2O oxidized--->1/2O2+2H++2e-
CO2 reduced to glucose by addition of e-’s and H+’s
compare with respiration where glucose oxidized and O2 reduced
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In photosynthesis, electrons travel “uphill” from water to glucose, adding light energy captured by chlorophyll
In respiration, electrons travel “downhill” from glucose to water, releasing energy to ATP
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Overview
Photosynthesis is a two-stage process light-dependant reactions
convert light energy to chemical energy, releases O2 as waste product
occurs in thylakoid membranes and produces energy shuttles ATP and NADPH
Calvin cycle cyclic series of steps that assemble organic
molecules from CO2
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occur in stroma and use energy and electrons from ATP and NADPH in carbon fixation
light not required but usually run during day as require shuttles from light-dependant reactions
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The Light Reactions
Driven by visible light light is electromagnetic radiation only small fraction of em radiation perceived
by organisms different wavelengths=different colors
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leaf absorbs some wavelengths (red-orange and blue-violet) and reflects others (green)
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in plants light absorbed by chlorophyll a, chlorophyll b and carotenoids
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only chlorophyll a directly involved in light reactions; other pigments act as “antenna” molecules to broaden range of energy absorbed
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The Photosystems
Light behaves like particles-photons When pigment absorbs photon, energy level
of one electron is raised to excited, unstable state if pigment is isolated from molecular
environment, excited electron loses energy as heat or light and returns to normal level chlorophyll fluoresces red
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In chloroplasts, 200-300 chlorophyll molecules grouped with proteins to form antenna assembly around two chlorophyll a molecules-reaction center chlorophylls excited electrons passed from antenna chlorophylls
to reaction center chlorophylls then to primary electron acceptor series of redox reactions
• final is oxidation of reaction center chlorophyll and reduction of primary electron acceptor
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Two photosystems (antenna assembly+primary electron acceptor) identified absorb at different wavelengths
photosystem I-absorbs maximally at 700nm (P700) photosystem II-absorbs maximally at 680nm (P680)
function together to carryout non-cyclic electron transport also known as non-cyclic photophosphorylation
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photosystem I can also carryout cyclic electron transport (cyclic photophosphorylation) thought to be the earliest form of photosynthesis
• present in many primitive photosynthetic bacteria
synthesizes only ATP
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Chemical Energy Generation
Electron transport chains generate ATP, NADPH and O2
kinetic energy of light absorbed and excites electrons
excited electrons passed along electron transport chain-series of redox reactions
released energy used to generate ATP, NADPH and O2
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production of NADPH requires 2 electrons supplied to PS I by PS II replaced in PS II by splitting water H2O ---> 1/2O2 + 2H+ + 2e-
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Chemiosmosis
Powers ATP synthesis H+ ions from splitting water and those pumped
across thylakoid membrane by electron transport chain form gradient across thylakoid membrane (inside to outside)
ATP synthase provides port for H+ to diffuse back into stroma releases energy and phosphorylates ADP to ATP similar process to ATP generation in mitochondria known as photophosphorylation
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Carbon Fixation
ATP and NADPH from light-dependant reactions power Calvin cycle net result of Calvin cycle is 3C molecules from
CO2 using energy and electrons in ATP and NADPH from light-dependant reactions
CO2 added to 5C intermediate ribulose-1,5-bisphosphate (RuBP) catalyzed by RuBP carboxylase/oxygenase (rubisco)
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Number of rearrangements occur in many steps, using energy in ATP and oxidation of NADPH last step in cycle regenerates RuBP all steps occur simultaneously but ultimately
regenerate starting reactants, hence cycle
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Three RuBP enter cycle for each 3C molecule released from chloroplast
Calvin cycle occurs in chloroplast stroma 3C molecules exported to cytoplasm
used to synthesize glucose and other organic molecules
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Plants that use only Calvin cycle to fix carbon called C3 plants first identifiable product of carbon fixation is
3C molecule
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Carbon-fixing Variations
C3 plants conserve water by closing stomata allows buildup of O2 in leaves
Rubisco fixes O2 rather than CO2
called photorespiration uses ATP and NADPH but makes no sugars
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C4 plants adapted to conserve water and prevent photorespiration CO2 incorporated into 4C molecule in mesophyll
cells diffuses into bundle sheath cells and released enters Calvin cycle in bundle sheath chloroplasts
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CAM (crassulacean acid metabolism) plants incorporate carbon during night stomata open at night, closed during day CO2 incorporated in 4C molecule and stored in
vacuole at night during day, 4C molecules exported into
cytoplasm and CO2 released
CO2 enters Calvin cycle
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C4 separate carbon incorporation and fixation spatially
CAM plants separate carbon incorporation and carbon fixation temporally