Photosynthesis 1. Light Reactions and Photosynthetic Phosphorylation. Lecture 31. Key Concepts. Overview of photosynthesis and carbon fixation

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1 Photosynthesis 1 Light Reactions and Photosynthetic Phosphorylation Lecture 31 Key Concepts Overview of photosynthesis and carbon fixation Chlorophyll molecules convert light energy to redox energy The Z scheme of photosynthetic electron transport Oxidative phosphorylation and photosynthesis are related processes How do photosynthetic reaction centers convert light energy into redox energy? In what ways are photosynthesis and oxidative phosphorylation similar? Different?

2 Together, the photosynthetic electron transport system and Calvin cycle (carbon fixation) convert sunlight energy into chemical energy (ATP, NADPH, triose phosphate), and in the process, oxidize H 2 O to form O 2. The photosynthetic electron transport system is often referred to as the light reactions of photosynthesis, whereas, the Calvin cycle has been called the dark reactions. However, the term "dark reactions" can be misleading because the Calvin cycle is most active in the light when ATP and NADPH levels are high. Oxygenic Photosynthesis H 2 O O 2 CO 2 CH 2 O Buchanon, et. al., Fig.12.2

3 Overview of Photosynthesis Light excitation of Photosystems I and II results in oxygen evolution from the splitting H 2 O, and the generation of chemical energy in the form of ATP and NADPH. Plants use this chemical energy (ATP and NADPH) to convert CO 2 into sugars via the Calvin cycle which takes place in the stroma. Net reaction of photosynthesis and carbon fixation H 2 O + CO 2 (light energy) (CH 2 O) + O 2 Although written as a balanced reaction, O 2 generation is the result of H 2 O oxidation, whereas, the CO 2 is used to synthesize carbohydrate (CH 2 O) It takes 2 H 2 O to make an O 2 and six CO 2 molecules are required for the synthesis of each molecule of glucose, therefore: 6 H 2 O + 6 CO 2 (light energy) C 6 H 12 O O 2 Gº' for this reaction is kj/mol! This is overcome by the energy potential stored in the products of photosynthetic electron transport, namely, ATP and NADPH.

4 General equation for photosynthesis: 2H 2 A (red. e- donor) + CO 2 (CH 2 O) + H 2 O + 2A (ox. e- donor) In oxygenic PS A= O Diverse photosynthetic organisms use electron donors other than H 2 O for photosynthesis: Green sulfur bacteria: 2H 2 S + CO 2 light (CH 2 O) + H 2 O + 2S Other PS bacteria: light 2 lactate + CO 2 (CH 2 O) + H 2 O + 2 pyruvate Joseph Priestly s Experiment (200 yrs ago!) Live plants and respiring animals could coexist in a closed system for a limited period of time as long as H 2 O and light were provided It was later shown that chloroplasts contain light gathering pigments called chlorophyll that work with proteins to convert light energy into chemical energy and in the process release O 2.

5 A modern Priestly experiment Biosphere 2 outside of Tucson, built in the 1990s by a private company called Space Biospheres Ventures. This experiment did not actually work very well because CO 2 levels built up inside the sealed environment and periodic CO 2 removal was required. Metabolic pathway questions related to the photosynthetic electron transport system and the Calvin cycle

6 1. What do the photosynthetic electron transport system and Calvin cycle accomplish for the cell? The photosynthetic electron transport system converts light energy into redox energy which is used to generate ATP by chemiosmosis and reduce NADP + to form NADPH. Calvin cycle enzymes use energy available from ATP and NADPH to reduce CO 2 to form glyceraldehyde-3-p, a three carbon carbohydrate used to synthesize glucose. Photosynthetic cells use the carbohydrate produced by the Calvin cycle as a chemical energy source for mitochondrial respiration in the dark. Photosynthetic organisms are autotrophs because they derive energy from light rather than from organic materials (as food). Questions: Do plants use oxidative phosphorylation? Do plants have mitochondria? 2. What are the overall net reactions of photosynthetic electron transport system and the Calvin cycle? Photosynthetic electron transport system (production of ATP and O 2 ): 2 H 2 O + 8 photons + 2 NADP + + ~3 ADP + ~3 Pi O NADPH + ~3 ATP Calvin cycle (six turns of cycle yields glucose): 6 CO NADPH + 18 ATP + 12 H 2 O Glucose + 12 NADP ADP + 18 Pi

7 3. What are the key enzymes in the photosynthetic electron transport system and the Calvin cycle? Protein components of the photosynthetic electron transport system three protein complexes are required for the oxidation of H 2 O and reduction of NADP + ; photosystem II (P 680 reaction center), cytochrome b 6 f (proton pump) and photosystem I (P 700 reaction center). Chloroplast ATP synthase enzyme responsible for the process of photophosphorylation which converts proton-motive force into net ATP synthesis; this enzyme is very similar to mitochondrial ATP synthase. Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) -is responsible for CO 2 fixation in the first step of the Calvin cycle. Rubisco activity is maximal in the light when stromal ph is ~8 and Mg 2+ levels are elevated due to proton pumping. 4. What are examples of the photosynthetic electron transport system and Calvin cycle in real life? DCMU (dichlorophenyl dimethylurea) is a broad spectrum herbicide that functions by blocking electron flow through photosystem II and is used to reduce weeds in non-crop areas. Another herbicide, paraquat, prevents reduction of NADP + by accepting electrons from intermediate reductants in photosystem I.

8 Chloroplast Structure Inner envelope Chloroplast Structure Outer envelope Stroma Granal thylakoids Stroma thylakoids Buchanon et al., Fig.12.1

9 Light energy is absorbed by numerous accessory pigments which can transfer the absorbed energy to nearby reaction centers containing specialized chlorphyll molecules. These accessory pigments function as light harvesting antenna. Chlorophyll (and Other Pigments) Grab hold of those photons!

10 Chlorophylls are the primary light gathering pigments. They have a heterocyclic ring system that constitutes an extended polyene structure, which typically has strong absorption in visible light. Buchanon et al., Fig.12.4 Chlorophyll absorption spectra Buchanon et al., Fig.12.6

11 Other Photosynthesis Pigments Buchanon et al., Fig.12.6 Expression of carotenoid biosynthetic genes in E. coli Buchanon et at., Fig.12.8

12 Light Energy: In photosynthesis, light energy is converted to chemical energy, therefore it is important to understand the energy content of light. Light energy is expressed as: Einsteins = moles of photons Light energy is wavelength dependent: E = hν h = Planck s constant (6.626 x J sec) ν = light frequency = c/λ (speed of light/wavelength) How is the energy of light related to light wavelength? Energy in the electromagnetic spectrum Buchanon et al., Fig :

13 1) Light energy is wavelength dependent: E = hn = c/λ h = Planck s constant (6.626 x J sec) n = light frequency = c/λ (speed of light/wavelength) 2) Standard Free Energy change of redox reactions: G = - nf E E = E acceptor - E donor 3) Free Energy of PMF: G= RT ln(c 2 /C 1 ) + ZF Ψ G = 2.3 RT ph + F Ψ Chlorophyll energy levels Buchanon et al., Fig.12.3

14 Organization of Photosynthetic pigments Light absorbing pigments are organized in functional arrays called Photosystems. Light harvesting or antenna pigment molecules are specialized to absorb light and transfer the energy to neighboring pigment molecules. Photochemical reaction center pigment molecules are specialized to transduce light energy into chemical energy. Several hundred light harvesting pigment molecules funnel light to one reaction center molecule. Steps in Photochemistry Or, How Reaction Centers Work 1) Light harvesting chlorophyll (LHC) electron is excited by light. 2) Energy of LHC electron is passed to successive LHC chlorophyll molecules by resonance energy transfer. 3) LHCs near reaction center (RC) transfer energy to RC chlorophyll. 4) Excited RC chlorophyll donates electron to an electron acceptor in the RC. 5) RC regains electron from electron donor creating charge separation in the RC.

15 Chl* = excited chlorophyll

16 In the chloroplast PSII reaction center, the electron acceptor is a molecule called pheophytin which becomes negatively charge as denoted by Pheo-. Importantly, the oxidized chlorophyll molecule (now positively charged, Chl + ) returns to the ground state by accepting an electron through a coupled redox reaction involving the oxidation of H 2 O. This process of O 2 evolution takes place in the manganese center present in the thylakoid membrane and is ultimately the source of electrons needed for the photosynthetic electron transport system. The Z Scheme of Photosynthetic Electron Transport Light energy captured by chlorophyll molecules actually involves the interplay of two reaction centers called photosystem I and photosystem II that are linked together by redox reactions. The reaction center complexes are functionally linked by an electron carrier protein called plastocyanin that shuttles electrons one at a time from PSII to PSI.

17 The Z Scheme of Photosynthetic Electron Transport The oxidation of 2 H 2 O requires 8 photons to transport 4 e - through the system, resulting in the accumulation of 12 H + in the thylakoid space and generation of 2 NADPH in the stroma. Z Scheme Described The photosynthetic electron transport system in plants consists of two linked electron circuits, each requiring an input of energy from light absorption at PSII and PSI reaction center complexes to initiate electron flow. The Z scheme energy diagram showing how photon absorption by the PS II reaction center complex results in electron flow from H 2 O to plastocyanin, providing energy to translocate H + across the thylakoid membrane. A second photon absorption event at PS I drives electron transport from plastocyanin to NADP +. Electron flow through protein-linked redox reactions involves numerous electron carriers, including Fe-S centers, the hydrophobic molecule plastoquinone (Q) which is reduced to form plastoquinol (QH 2 ) and analogous to ubiquinone/ubiquinol in the mitochondrial electron transport system. Plastocyanin has the same job in photosynthetic electron transport as does cytochrome c in mitochondrial electron transport.

18 Z Scheme of Photosynthetic Electron Transport Buchanon et al., Fig TIPS 7:

19 TIPS 7: Photosystem II (PSII) Photosystem II contains chlorophylls a and b and absorbs light at 680nm. This is a large protein complex that is located in the thylakoid membrane.

20 Schematic drawing of electron flow through PS II The absorption of light energy by PS II results in electron flow leading to generation of O 2 from the splitting of H 2 O and to the reduction of plastoquinone (Q) to form plastoquinol (QH 2 ). Water Oxidation at PSII 2H 2 O 2H + + 4e - + O 2 A single photon of 680 nm light does not have enough energy to break the bonds in water. Instead, the 4 e - are passed one at a time to oxidized P680 through a Tyr residue in the D1 subunit of PSII. 4 P Tyr. 4 P Tyr 4 Mn ions store the positive charge and coordinate the oxygen until oxidation of water is complete. 4 Tyr. + [Mn complex] 0 4Tyr + [Mn complex] 4+ [Mn complex] H 2 O [Mn complex] 0 + 2H + + O 2

21 Manganese cluster of the PS II Reaction Center from Synechocystis elongatus Nature 409:739 (2001) The electron flow resulting from the absorption of 4 photons leads to the reduction of two molecules of plastoquinone, and in the process, generates a net increase of 4 H + inside the thylakoid lumen.

22 The net reaction from oxidation of QH 2, and from proton pumping through the cytochrome b 6 f complex, is the addition of 8 H + to the thylakoid lumen. This brings to 12 the total number of H + molecules accumulated inside the thylakoid lumen for every O 2 generated in PS II. Photosystem I (PSI) The final stage of photosynthesis: the absorption of light energy by PS I is at a maximum of 700 nm. Again 4 photons are absorbed, but in this case, the energy is used to generate reduced ferredoxin, which is a powerful reductant. Structure of PS I complex showing Fe-S clusters

23 Electron flow diagram of PS I leading to generation of reduced ferredoxin Ferrodoxin-NADP + reductase converts NADP + to NADPH through a semiquinone intermediate involving FADH

24 We can put it all together to showing that ATP synthesis results from protons flowing OUT of the thylakoid lumen back into the stroma. Note that the chloroplast ATP synthase is structurally and functionally similar to the mitochondrial ATP synthase we have already described, with the exception that 4 H + are required for every ATP synthesized based on experiments showing that ~3 ATP are synthesized for every O 2 generated (12 H + transported/o 2 generated). This difference (3 H + /ATP in mitochondria versus 4 H + /ATP in chloroplasts) could be due to differences in the "gear ratio" of the complex, or uncertainties in the experimental measurements. ATP synthesis occurs from protons flowing OUT of the thylakoid lumen back into the stroma. Understand the compartmentalization here!

25 Comparison of Photosynthesis and Oxidative Phosphorylation These processes are related. You should understand how they are related. The light induced proton motive force generated during photosynthesis is used to generate ATP in the stroma as a result of protons moving out of the thylakoid lumen.

26 In contrast, protons pumped out of the mitochondrial matrix as a result of NADH oxidation by the electron transport system, flow back into the matrix to generate ATP inside the mitochondria.

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