8.3: PHOTOSYNTHESIS

Understandings• Light-dependent reactions take place in the intermembrane space of the thylakoids.• Light-independent reactions take place in the stroma.• Reduced NADP and ATP are produced in the light-dependent reactions.• Absorption of light by photosystems generates excited electrons.• Photolysis of water generates electrons for use in the light-dependent reactions.• Transfer of excited electrons occurs between carriers in thylakoid membranes. • Excited electrons from photosystem II are used to contribute to/generate a proton gradient.• ATP synthase in thylakoids generates ATP using the proton gradient.• Excited electrons from Photosystem I are used to reduce NADP.• In the light independent reactions a carboxylase catalyzes the carboxylation of RuBP.• Glycerate 3-phosphate (GP) is reduced to triose phosphate (TP) using reduced NADP and ATP.• TP is used to regenerate RuBP and produce carbohydrates.• RuBP is reformed using ATP.• The structure of the chloroplast is adapted to its function in photosynthesis.Applications and Skills• A: Calvin’s experiment to elucidate the carboxylation of RuBP.• S: Annotation of a diagram to indicate the adaptations of a chloroplast to its functions.

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LIGHT DEPENDENT REACTION• Photosystems are regions of pigments that are organized on the membranes of the thylakoids. • Each one includes:• Chlorophyll a molecules• Accessory pigments• A protein matrix•The reaction center of the photosystem includes:• pair of chlorophyll a molecules• matrix of protein• Primary electron acceptor

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LIGHT DEPENDENT REACTION•1: Photon is absorbed in Photosystem (PS) II; gets transferred to other pigments until it reaches a chlorophyll a molecule in the reaction center. Photon energy excites a chlorophyll a e- to a higher energy state.•2: e- is captured by the primary acceptor.•3: Water is split to produce e-’s, H+’s, and O. The e-’s are supplied one by one to the chlorophyll a molecules of the reaction center.

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LIGHT DEPENDENT REACTION•4: e-’s pass from primary acceptor down an ETC. • 1st: plastoquinone (PQ)•Middle: cytochrome complex

•5: Energy lost from e-’s drives chemiosmosis to bring about phosphorylation of ADPATP•6: Photon is absorbed by a pigment in PSI; transferred through several accessory pigments until received by chlorophyll a molecule. e- gets transferred to primary acceptor. De-energized e- from PSII fills the void.

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LIGHT DEPENDENT REACTION•7: e- passed down 2nd ETC that involves the carrier ferredoxin. •8: NADP reductase catalyzes the transfer of 2 e-’s from ferredoxin to NADP+ to make NADPH.

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LIGHT DEPENDENT REACTION• NADPH and ATP are the final products of the light-dependent reaction.• They supply the energy needed for the light independent reaction to occur.• ATP production in photosynthesis= photophosphorylation b/c the energy comes from light.• Production of ATP happens between PSII and PSI

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LIGHT INDEPENDENT REACTION• Occurs in the stroma.• Glucose is the product.• Calvin cycle

LIGHT INDEPENDENT REACTION• Ribulose biphosphate (RuBP) (a 5 C molecule) binds to a CO2 molecule (carbon fixation). This is catalyzed by rubisco.• Result: 6 C unstable compound.• 6 C compound two 3 C compounds (glycerate 3-phosphate aka GP).• GP acted on by ATP and NADPH to form triose phosphate (TP), another 3 C molecule.

LIGHT INDEPENDENT REACTION• Some TP’s leave the cycle to become sugar phosphates that may become more complex carbs.• Most continue the cycle to become RuBP (the original compound). • ATP is used to make RuBP from TP.

LIGHT DEPENDENT VS LIGHT INDEPENDENTLight-Dependent

- Occurs in the thylakoids - Uses light energy to form ATP and NADPH.

- Splits water in photolysis to provide replacement e-’s and H+, and to release oxygen to the environment.

- Includes two ETC’s and Photosystems II and I.

Light-Independent - Occurs in the stroma. - Uses ATP and NADPH to form triose phosphate.

- Returns ADP, inorganic phosphate, and NADP to the light-dependent reaction.

- Involves the Calvin cycle.

Label the structures of the chloroplast. Explain how the structures are adapted to their functions in photosynthesis.

- Explain the light-dependent reactions of photosynthesis. - Explain the light-independent reactions of photosynthesis. - Compare and contrast the light-dependent reactions of photosynthesis with the electron transport chain and oxidative phosphorylation.

8 marks each.

HOMEWORKVocab• Plastids• Chloroplast• Leucoplasts• Chromoplasts• Autotroph• Heterotroph• Photons• Thylakoids• Grana/granum• Pigments• Carotenoids• Photosysyem ( I and II)• Photophosphorylation• Non-cyclical photophosphorylation• Photolysis• Chemiosmosis

Other• Summarize Nature of Science on 375• TOK: 375• Exercise 9-13 on 377