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C3 Photosynthesis
Chapter 10
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What you need to know!
• How photosystems convert solar energy to chemical energy.
• How linear electron flow in the light reactions results in the formation of ATP, NADPH, and O2.
• How chemiosmosis generates ATP in the light reactions.
• How the Calvin cycle uses the energy molecules of the light reactions to produce G3P
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Photosynthesis
• Radiation energy is transformed into chemical bond energy in two distinct stages:
1. Light reactions• Occur in the thylakoid membrane• Water donates electrons to NADP+ to make
NADPH
• Water is split, O2 is released
• Photophosphorylation turns ADP into ATP
2. Calvin cycle• Occurs in the stroma
• CO2 transformed into sugar
Net Rx: 6 CO2 + 6 H2O + Light C6H12O6 + 6O2
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Big Picture
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Light Reactions• Location: thylakoid membrane
• Needs: Light, H2O, NADP+, ADP, P
• Makes: NADPH, O2, ATP
• Includes: Linear (non-cyclical), cyclical, & chemiosmosis
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Linear (Non-cyclical) Light Rxs
• Photosystem II (P680) pigments absorb light (photons)
• A photon excites chlorophyll which kicks an electron e- out of the reaction center
• The excited e- is captured by the Electron Transport Chain (ETC) between P680 and Photosystem I (P700)
• The missing e- is replaced by splitting water (photolysis of water):
H2O O + 2e- + 2H+
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Linear Light Rxs
• The excited e- moves down the ETC • The e-’s energy (excited) is used to pump H+
into the thylakoid space (creating a concentration gradient)
• e- is deposited into P700• P700 pigments absorb light (photons)• A photon excites chlorophyll which kicks an
electron e- out of the reaction center• The e- is captured by another shorter ETC• At the end of the 2nd ETC the e- binds to
NADP+• 2 e- and NADP+ are combined with H+ to form
NADPH
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Linear Light Reactions
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Cyclical Light Reactions
• Some e-’s, when kicked out of P700 do not go down the 2nd shorter ETC
• Instead they fall back on the first ETC between P680 and P700
• This produces less NADPH and more H+ gradient
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Cyclical Light Reactions
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Chemiosmosis
• This process makes ATP by using the H+ concentration gradient
• H+ concentration gradient across the thylakoid membrane means: H+ inside the thylakoid is high, while H+ in the stroma is low– On a sunny day it is 1000x’s more acidic in the
thylakoid space (pH 5 in thylakoid, pH 8 in stroma)
• ATP Synthase in the membrane functions like a turbine: when H+s rush through ATP Synthase (down the electrochemical gradient) ATP Synthase turns and uses kinetic energy to phosphorylize ADP
ADP + P ATPaka: Photophophorylation
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Calvin Cycle
aka: light independent reactions• Location: stroma
• Needs: CO2, ATP, NADPH
• Makes: G3P, ADP, P, NADP+
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Calvin Cycle
• Multiple enzyme pathways that uses ATP and NADPH to reduce CO2 into C6H12O6 (glucose)
• One turn of the cycle reduces one CO2
• 3 distinct steps:
1. Carbon fixation
2. Reduction
3. Regeneration
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Carbon Fixation
• First enzyme of the cycle is Rubisco (Ribulose Bisphosphate Carboxylase) which binds 3 CO2 to an acceptor molecule RuBP
• Rubisco is the most famous and abundant enzyme on earth: no other organic molecule can chemically binding CO2
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Reduction
• Several enzymes later the 3 CO2 have been reduced to a C3 sugar called G3P (glyceraldehyde phosphate)– powered by 6 ATP and 6 NADPH
• G3P leaves the cycle– 2 G3P can combine to form glucose
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Regeneration
• RuBP needs to be regenerated– powered by 3 ATP
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3. Regeneration
1. Carbon Fixation
2. Reduction
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Water Balance
• If water is running low, plants will close their stomata to avoid transpiration
When stomata are closed• CO2 is not replenished• ADP and NADP+ are not replenished by
the Calvin Cycle• Light Reactions run out of ADP and NADP+• Energized e-’s fall back to the reaction
center of chlorophyll– This can emit light (plant fluorescence)
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Review
• Biology Crash Course• http://www.youtube.com/watch?v=wEPUfJ
n0s-M• Mr. Anderson (Bozeman)• http://www.youtube.com/watch?v=g78utcL
QrJ4