Light Dependent Reactions IB Topic 8.2.3: Explain the light dependent reactions.
Photosynthesis: Light Independent Reactions. Overview of Light Independent Reactions: The Calvin...
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Transcript of Photosynthesis: Light Independent Reactions. Overview of Light Independent Reactions: The Calvin...
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Photosynthesis:Light Independent Reactions
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Overview of Light Independent Reactions: The Calvin Cycle
(the synthesis part of photosynthesis)
• Occurs in the stroma• Uses ATP as an energy source• Uses NADPH as reducing power• Goal: make glyceraldehyde-3-phosphate (used to
make glucose and other organic compounds)
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The Calvin Cycle
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Phase 1: Carbon fixation
1. Three molecules of ribulose bisphosphate (RuBP), a 5 carbon compound, binds to three incoming CO2 molecules in a process called carbon fixation.
2. Fixation is catalyzed by a enzyme called RuBP carboxylase (rubisco)
3. The result is three unstable 6-carbon molecules which immediately break apart into six 3-carbon compounds called 3-phosphoglycerate
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Phase 2: Reduction1. The 3-phosphoglycerate then receives a
phosphate group from ATP which changes it to six molecules of 1,3 bisphosphoglycerate (this makes 6 ATP now return back to 6 ADP)
2. The 1,3 bisphosphoglycerate is reduced by gaining the electrons from NADPH (made in light dependent reactions) to six molecules of glyceraldehyde-3-phosphate, G3P (a phosphate group is also released during reaction, energy from the breaking of the bond used to make G3P)
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One G3P molecule is used to make
glucose, other 5 are recycled back to
continue the Calvin Cycle
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Phase 3: Regeneration of CO2 acceptor (RuBP)
1. The 5 molecules of G3P that were not an output of the cycle are rearranged into three molecules of the initial molecule, RuBP (energy for rearrangement fueled by ATP)
2. Now, RuBP is recycled and ready to once again accept incoming CO2…the cycle repeats
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Output from the Calvin cycle:• It takes 3 molecules of CO2 to make 1
molecule of usable G3P…and therefore it takes 6 molecules of CO2 to make one molecule of glucose– Note: if 3 CO2 are entering per turn of the cycle, it
takes 2 turns to make 1 glucose • For each turn of cycle, 9 molecules of ATP and
6 molecules of NADPH are used
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Light dependent reaction Light independent reactions
Occurs on thylakoid membrane
Occurs in the stroma
Uses light energy to form ATP , NADPH, O2
Uses ATP, NADPH, and CO2 to form G3P
Splits water in photolysis to provide replacement electrons and H+ ions and to release O2 into the atmosphere
Returns ADP, phosphate, and NADP to the light dependent reactions
Includes two electron transport chains and photosystems I and II
Involves the 3 phases of the Calvin cycle
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Calvin Cycle Simulationhttp://www.science.smith.edu/departments/Biology/Bio231/calvin.html