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Published byBertina Simpson Modified over 8 years ago
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Photosynthesis: The Details
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Photosynthesis Divided into two steps: 1.The Light Reactions Noncyclic electron flow 2.The Calvin Cycle Cyclic electron flow
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The Light Reactions Photosystems are embedded in the thylakoid membrane. They contain chlorophyll and accessory pigments that are associated with proteins. A photosystem consists of an antenna complex and a reaction centre.
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The Light Reactions The antenna complex absorbs a photon and transfers energy to the reaction centre. The reaction centre contains chlorophyll a, whose electrons absorb energy and begin photosynthesis.
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The Light Reactions Photosystem II (P680) Two photons strike photosystem II and excite 2 electrons from chlorophyll P680. The excited electrons are captured by a primary electron acceptor and are transferred to plastoquinone (PQ) and the ETC.
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The Light Reactions Photosystem II (P680) In the ETC, the 2 electrons pass through a proton pump (Q cycle). The Q cycle transports 4 protons from the stroma into the thylakoid lumen to create a proton gradient.
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The Light Reactions Photosystem II (P680) The electrochemical gradient drives the photophosphorylation of ADP to ATP. 1 ATP forms for every 4 protons that pass through ATPase from the thylakoid lumen into the stroma.
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The Light Reactions Photosystem II (P680) Z protein splits water into 2 protons, 2 electrons and 1 oxygen atom. –The electrons replace those lost from chlorophyll P680. –The protons remain in the thylakoid lumen to add to the proton gradient. –Oxygen leaves as a byproduct.
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The Light Reactions Photosystem I (P700) Two photons strike photosystem I and excite 2 electrons from chlorophyll P700 (replaced by electrons from P680). These electrons pass through another ETC. The enzyme NADP reductase uses the 2 electrons and a proton from the stroma to reduce 1 NADP + to 1 NADPH.
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The Calvin Cycle Occurs in the stroma of chloroplasts. Cyclical reactions similar to the Krebs Cycle. Divided into three phases: 1.Carbon Fixation 2.Reduction Reactions 3.Regeneration of RuBP
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The Calvin Cycle Phase 1: Carbon Fixation 3 CO 2 are added to RuBP to form 3 unstable 6-carbon intermediates. The intermediates split into six 3-carbon molecules called PGA. These reactions are catalyzed by rubisco.
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The Calvin Cycle Phase 2: Reduction Reactions 6 PGAs are phosphorylated by 6 ATPs to form 6 molecules of 1, 3-BPG. 6 NADPH molecules reduce the six 1,3- BPG to 6 G3P or PGAL. One molecule of G3P exits the cycle as a final product.
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The Calvin Cycle Phase 3: Regeneration of RuBP 3 ATP are used to rearrange the remaining 5 G3P into 3 molecules of RuBP. The cycle continues with the RuBP fixing more CO 2.
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To Produce One G3P… 3 RuBP + 3 CO 2 + 9 ATP + 6 NADPH + 5 H 2 O 9 ADP + 8 P i + 6 NADP + + G3P + 3 RuBP
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