Chapter 22 Photosynthesis PDF

Chapter 22 Photosynthesis © 2018 Cengage Learning. All Rights Reserved. Chapter Outline (22-1) Chloroplasts are the site of photosynthesis (22-2) Photosystems I and II and the light reactions of photosynthesis (22-...

Chapter 22 Photosynthesis © 2018 Cengage Learning. All Rights Reserved. Chapter Outline (22-1) Chloroplasts are the site of photosynthesis (22-2) Photosystems I and II and the light reactions of photosynthesis (22-3) Photosynthesis and ATP production (22-4) Evolutionary implications of photosynthesis with and without oxygen (22-5) Dark reactions of photosynthesis fix CO2 (22-6) CO2 fixation in tropical plants © 2018 Cengage Learning. All Rights Reserved. Net Equation of Photosynthesis Photosynthetic organisms convert CO2 and H2O to carbohydrates and molecular oxygen (O2) 6CO 2 + 6H 2 O → C6 H12 O6 + 6O 2 Represents two processes Light reactions Oxidation of H2O to produce O2 depends on solar energy, which is absorbed by chlorophyll Generate NADPH and ATP Dark reactions ATP and NADPH produced during light reactions provide energy for the fixation of CO2 to give sugars © 2018 Cengage Learning. All Rights Reserved. Site of Photosynthesis In prokaryotes, photosynthesis occurs in granules bound to the plasma membrane In eukaryotes, photosynthesis occurs in the chloroplast that: Has inner and outer membranes and an intermembrane space Consists of bodies called grana that consist of stacks of flattened membranes called thylakoid disks Trapping of light and production of O2 take place in thylakoid disks Consists of stroma, an organelle that is the site of dark reactions © 2018 Cengage Learning. All Rights Reserved. Figure 22.1 - Membrane Structures in Chloroplasts © 2018 Cengage Learning. All Rights Reserved. Figure 22.2 - Light-Dependent and Light- Independent Reactions of Photosynthesis © 2018 Cengage Learning. All Rights Reserved. Chlorophyll Absorbs light Types - Chlorophyll a and chlorophyll b Eukaryotes (green plants and green algae) contain both chlorophyll a and chlorophyll b Prokaryotes contain only chlorophyll a Photosynthetic bacteria other than cyanobacteria have bacteriochlorophylls Such organisms are anaerobic and have only one photosystem © 2018 Cengage Learning. All Rights Reserved. Structure of Chlorophyll Similar to that of the heme group of myoglobin, hemoglobin, and the cytochromes since it is based on the tetrapyrrole ring of porphyrins Metal ion bound to the tetrapyrrole ring is Mg (II) Cyclopentanone ring is fused to the tetrapyrrole ring Phytol group Long hydrophobic side chain that contains four isoprenoid units and binds to the thylakoid membrane via hydrophobic interactions © 2018 Cengage Learning. All Rights Reserved. Figure 22.3 - Molecular Structures of Chlorophyll a, Chlorophyll b, and Bacteriochlorophyll a © 2018 Cengage Learning. All Rights Reserved. Absorption Spectra of Chlorophyll Chlorophyll a and chlorophyll b absorb red (600–700 nm) and blue (400–500 nm) portions of the visible spectrum Accessory pigments Plant pigments other than chlorophyll that play a role in photosynthesis Absorb light and transfer energy to the chlorophylls Wavelength of light absorbed plays a critical role in the light reaction of photosynthesis Energy of light is inversely related to wavelength © 2018 Cengage Learning. All Rights Reserved. Figure 22.4 - Visible Spectra of Chlorophylls © 2018 Cengage Learning. All Rights Reserved. The Photosynthetic Unit All chlorophylls are bound to proteins, either in antenna complexes or in one of two kinds of photosystems Photosystems: Membrane-bound protein complexes that carry out light reactions Light-harvesting molecules pass their excitation energy to a pair of specialized chlorophyll molecules at a reaction center There are several hundred light-harvesting antenna chlorophylls for each unique chlorophyll at a reaction center © 2018 Cengage Learning. All Rights Reserved. Figure 22.6 - Schematic Diagram of a Photosynthetic Unit © 2018 Cengage Learning. All Rights Reserved. Chloroplasts Are the Site of Photosynthesis: Summary In eukaryotes, photosynthesis takes place in chloroplasts Light reactions take place in the thylakoid membrane Dark reactions of photosynthesis take place in the stroma between the thylakoid membrane and the inner membrane of the chloroplast Absorption of light by chlorophyll supplies the energy required for the reactions of photosynthesis © 2018 Cengage Learning. All Rights Reserved. Chloroplasts Are the Site of Photosynthesis: Summary (continued) All types of chlorophylls have a tetrapyrrole ring structure similar to that of the porphyrins of heme, but they also have differences that affect the wavelength of light they absorb This property allows more wavelengths of sunlight to be absorbed than would be the case with a single type of chlorophyll © 2018 Cengage Learning. All Rights Reserved. Overview of Light Reactions In the light reactions of photosynthesis: H2O is oxidized to O2 NADP+ is reduced to NADPH Photophosphorylation: Process in which reduction of NADP+ to NADPH is coupled to the phosphorylation of ADP to ATP H 2 O + NADP + → NADPH + H + + O 2 ADP + Pi → ATP © 2018 Cengage Learning. All Rights Reserved. Photosystems Involved in Light Reactions Light reactions consist of two parts that are accomplished by two distinct photosystems Photosystem I (PSI): Portion of the photosynthetic apparatus responsible for the production of NADPH Photosystem II (PSII): Portion of the photosynthetic apparatus responsible for the splitting of H2O to O2 Both photosystems: Carry out redox reactions Interact with each other indirectly via an electron transport chain that links them © 2018 Cengage Learning. All Rights Reserved. Photosystems Involved in Light Reactions (continued 1) Net electron transport reaction of the two photosystems taken together, except for the substitution of NADPH and NADH, is the reverse of mitochondrial electron transport NADP + + 2H + + 2e − → NADPH + H + H 2 O → 1 O 2 + 2H + + 2e − 2 NADP + + H 2 O → NADPH + H + + 1 O 2 2 Reaction is endergonic with a positive ΔG°′ = +220 kJ mol–1 = +52.6 kcal mol–1 Reaction is driven by light energy absorbed by the chlorophylls of the two photosystems © 2018 Cengage Learning. All Rights Reserved. Photosystems Involved in Light Reactions (continued 2) Reaction centers of the two photosystems provide different environments for the unique chlorophylls involved Unique chlorophyll of PSI is referred to as P700, and unique chlorophyll of PSII is referred to as P680 P is for pigment Subscripts represent the longest wavelength of absorbed light that initiates the reaction Path of electrons starts with the reactions in PSII rather than in PSI © 2018 Cengage Learning. All Rights Reserved. Photosystems Involved in Light Reactions (continued 3) Z scheme Absorption of light by Chl (P680) allows electrons to be passed to the electron transport chain that: Links PSII and PSI Generates an oxidizing agent strong enough to split water, producing O2 Absorption of light by Chl (P700) provides enough energy to allow the ultimate reduction of NADP+ to take place © 2018 Cengage Learning. All Rights Reserved. Figure 22.7 - Z Scheme of Photosynthesis © 2018 Cengage Learning. All Rights Reserved. Photosystem II Oxidation of H2O by PSII to produce O2 is the ultimate source of electrons in photosynthesis Electrons released by oxidation of H2O are first passed to P680, which is reduced Oxygen-evolving complex: Part of PSII that splits water to produce oxygen Passes through a series of five oxidation states (designated as S0 through S4) in the transfer of four electrons in the process of evolving O2 © 2018 Cengage Learning. All Rights Reserved. Figure 22.8 - PSII Reaction Center Passes through Five Different Oxidation States in the Course of Oxygen Evolution © 2018 Cengage Learning. All Rights Reserved. Photosystem II (continued 1) Immediate electron donor to Chl (P680) is a tyrosine residue of one of the protein components that does not contain manganese Several quinones serve as intermediate electron transfer agents Excited chlorophyll passes an electron to the primary electron acceptor, pheophytin (Pheo) Transfer of electrons is mediated by events that take place in the reaction center Plastoquinone (PQ), a substance similar to CoQ, is the next electron acceptor © 2018 Cengage Learning. All Rights Reserved. Photosystem II (continued 2) Electron transport chain that links PSI and PSII consists of: Pheophytin Plastoquinone Complex of plant cytochromes (b6–f complex) Copper-containing protein called plastocyanin (PC) Oxidized form of P700 © 2018 Cengage Learning. All Rights Reserved. PSI: Reduction of NADP+ Absorption of light by P700 leads to a series of electron transfer reactions of PSI P700* gives an electron to a molecule of chlorophyll a Transfer is mediated by processes that occur in the reaction center Bound ferredoxin passes its electron to a molecule of soluble ferredoxin Soluble ferredoxin reduces ferredoxin-NADP+ reductase, an FAD-containing enzyme, whose FAD portion reduces NADP+ to NADPH © 2018 Cengage Learning. All Rights Reserved. PSI: Reduction of NADP+ (continued) Chl* + Ferredoxin oxidized ⎯⎯⎯⎯⎯⎯ Ferredoxin-NADP → Chl + + Ferredoxin reduced reductase 2 Ferredoxin reduced + H + + NADP + → Ferredoxin oxidized + NADPH Net reaction for the two photosystems together is the flow of electrons from H2O to NADP+ 2H 2 O + 2NADP + → O 2 + 2NADPH + 2H + © 2018 Cengage Learning. All Rights Reserved. Figure 22.10 - Pathway of Cyclic Photophosphorylation by PSI © 2018 Cengage Learning. All Rights Reserved. Structure of Photosynthetic Reaction Centers in Rhodopseudomonas Viridis Reaction center contains a pair of bacteriochlorophyll molecules (P870) embedded in a protein complex Absorption of light by the pair raises one of their electrons to a higher energy level, and this electron is passed to a series of accessory pigments Electron is first passed to pheophytin, raising it to an excited energy level Electron is then passed to menaquinone (QA) and then to ubiquinone (QB) Electron passed to QB is replaced by an electron donated by a cytochrome, which acquires a positive charge in the process © 2018 Cengage Learning. All Rights Reserved. Figure 22.11 - Model of the Structure and Activity of the Rhodopseudomonas Viridis Reaction Center © 2018 Cengage Learning. All Rights Reserved. Figure 22.12 - Structures of Menaquinone and Ubiquinone © 2018 Cengage Learning. All Rights Reserved. Photosystems I and II: Summary Photosynthesis consists of two processes Light reactions and dark reactions Light reactions are electron transfer processes in which water is oxidized to produce oxygen and NADP+ is reduced to produce NADPH Path of electrons in the light reactions Transfer of electrons from water to the reaction-center chlorophyll of PSII Transfer of electrons from the excited-state chlorophyll of PSII to an electron transport chain consisting of accessory pigments and cytochromes, with energy provided by absorption of a photon of light © 2018 Cengage Learning. All Rights Reserved. Photosynthesis I and II: Summary (continued) Transfer of electrons from the excited-state chlorophyll of PSI to the ultimate electron NADP+, producing NADPH Processes of the electron transport chain resemble those of the mitochondrial electron transport chain Energy is provided by absorption of a photon of light © 2018 Cengage Learning. All Rights Reserved. Photosynthesis and ATP Production Proton gradient across the inner mitochondrial membrane drives the phosphorylation of ADP Chloroplasts can synthesize ATP from ADP and Pi in the dark if they are provided with a pH gradient © 2018 Cengage Learning. All Rights Reserved. ATP Production in Chloroplasts Several reactions contribute to the generation of a proton gradient in chloroplasts Oxidation of H2O releases H+ into the thylakoid space Electron transport from PSII and PSI helps create a proton gradient by involving PQ and cytochromes in the process PSI reduces NADP+ by using H+ in the stroma to produce NADPH Causes pH of the thylakoid space to be lower than that of the stroma ATP synthase in chloroplasts consists of two parts CF1 and CF0 © 2018 Cengage Learning. All Rights Reserved. ATP Production in Chloroplasts (continued) Components of the electron chain in chloroplasts are arranged asymmetrically in the thylakoid membrane Electron transport apparatus in the thylakoid membrane consists of several large membrane- bound complexes PSII PSI Cytochrome b6–f complex Flow of H+ back to the stroma through ATP synthase provides the energy for the synthesis of ATP from ADP and Pi © 2018 Cengage Learning. All Rights Reserved. Figure 22.14 - Mechanism of Photophosphorylation © 2018 Cengage Learning. All Rights Reserved. Evolutionary Implications of Photosynthesis with and without Oxygen Photosynthetic prokaryotes other than cyanobacteria have only one photosystem and do not produce oxygen Anaerobic photosynthesis Means for organisms to use solar energy to satisfy their needs for food and energy Not as efficient as photosynthesis linked to oxygen but it appears to be evolutionary © 2018 Cengage Learning. All Rights Reserved. Photosynthesis without O2 Production In heterotrophs, light energy absorbed by chlorophyll can be trapped in the forms of ATP and NADPH In autotrophs, the ultimate source of electrons is not H2O but some more easily oxidized substance Possible donors include H2S, H2S2O3, and succinic acid CO 2 + 2H 2S → ( CH 2O ) + 2S + H 2O H-acceptor H-donor Carbohydrate Possible H-acceptors include NO2– or NO3–, in which case NH3 is a product © 2018 Cengage Learning. All Rights Reserved. Figure 22.15 - Two Possible Electron Transfer Pathways in a Photosynthetic Anaerobe © 2018 Cengage Learning. All Rights Reserved. Evolutionary Implications of Photosynthesis with and without Oxygen: Summary When photosynthesis first evolved, it was most likely to have been carried out by organisms that used compounds other than water as the primary electron source Cyanobacteria were the first organisms to use water as the source of electrons thus giving rise to the present oxygen-containing atmosphere © 2018 Cengage Learning. All Rights Reserved. Dark Reactions of Photosynthesis Fix CO2 CO2 fixation takes place in the stroma Equation for the overall reaction is: 6CO 2 + 12NADPH + 18ATP ⎯⎯⎯ ⎯ Enzymes → C6 H12O6 + 12NADP + + 18ADP + 18Pi Actual reaction pathway has some features in common with glycolysis and some in common with the pentose phosphate pathway Overall reaction pathway is cyclic and is called the Calvin cycle after the scientist who first investigated it, Melvin Calvin, winner of the 1961 Nobel Prize in chemistry © 2018 Cengage Learning. All Rights Reserved. Figure 22.19 - The Calvin Cycle of Reactions © 2018 Cengage Learning. All Rights Reserved. The Calvin Cycle First reaction is the condensation of ribulose-1,5- bisphosphate with CO2 to form 2-carboxy-3- ketoribitol-1,5-bisphosphate, which quickly hydrolyzes to two molecules of 3-phosphoglycerate Catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) Represents the actual fixation process © 2018 Cengage Learning. All Rights Reserved. The Calvin Cycle (continued 1) Second reaction involves the reduction of 3- phosphoglycerate to form glyceraldehyde-3- phosphate Product can be used for the production of six-carbon sugars or can be used in the regeneration of ribulose- 1,5-bisphosphate Takes place in the same fashion as in gluconeogenesis, except that reactions in chloroplasts require NADPH rather than NADH for the reduction of 1,3-bisphosphoglycerate to glyceraldehyde-3- phosphate © 2018 Cengage Learning. All Rights Reserved. The Calvin Cycle (continued 2) Formation of glucose from glyceraldehyde-3- phosphate takes place in the same manner as in gluconeogenesis Conversion of glyceraldehyde-3-phosphate to dihydroxyacetone phosphate takes place easily Dihydroxyacetone phosphate in turn reacts with glyceraldehyde-3-phosphate, in a series of reactions, to give rise to fructose-6-phosphate and ultimately to glucose © 2018 Cengage Learning. All Rights Reserved. Steps in the Regeneration of Starting Material in the Calvin Cycle Preparation Step begins with the conversion of some of the glyceraldehyde-3-phosphate to dihydroxyacetone phosphate Reaction is catalyzed by triose phosphate isomerase Portions of both glyceraldehyde-3-phosphate and dihydroxyacetone phosphate are condensed to form fructose-1,6-bisphosphate Reaction is catalyzed by aldolase Fructose-1,6-bisphosphate is hydrolyzed to fructose-6- phosphate Reaction is catalyzed by fructose-1,6-bisphosphatase © 2018 Cengage Learning. All Rights Reserved. Steps in the Regeneration of Starting Material in the Calvin Cycle (continued 1) Reshuffling Many reshuffling reactions are like those of the pentose phosphate pathway Reactions are catalyzed in turn by transketolase, aldolase, and sedoheptulose bisphosphatase Isomerization Involves the conversion of both ribose-5-phosphate and xylulose-5-phosphate to ribulose-5-phosphate Reactions are catalyzed by ribose-5-phosphate isomerase and xylulose-5-phosphate epimerase, respectively © 2018 Cengage Learning. All Rights Reserved. Steps in the Regeneration of Starting Material in the Calvin Cycle (continued 2) Phosphorylation Ribulose-1,5-bisphosphate is regenerated by the phosphorylation of ribulose-5-phosphate Reaction requires ATP and is catalyzed by phosphoribulose kinase © 2018 Cengage Learning. All Rights Reserved. Regeneration of Starting Material in the Calvin Cycle: Net Reaction Net reaction for the path of carbon in photosynthesis is: 6CO 2 + 18ATP + 12NADPH + 12H + + 12H 2O → Glucose + 12NADP + + 18ADP + 18Pi ΔG°′ for the reduction of CO2 to glucose is +478 kJ for each mole of CO2 © 2018 Cengage Learning. All Rights Reserved. Table 22.1 - The Calvin Cycle Series of Reactions © 2018 Cengage Learning. All Rights Reserved. Dark Reactions of Photosynthesis Fix CO2: Summary In the dark reactions of photosynthesis, the fixation of CO2 takes place when the key intermediate ribulose- 1,5-bisphosphate reacts with CO2 to produce two molecules of 3-phosphoglycerate Reaction is catalyzed by rubisco Remainder of the dark reaction is the regeneration of ribulose-1,5-bisphosphate via the Calvin cycle © 2018 Cengage Learning. All Rights Reserved. C4 Pathway Alternative way to fix CO2 found in tropical plants Involves four-carbon compounds Also called the Hatch–Slack pathway Steps CO2 enters the leaf through pores in the mesophyll cells and reacts with phosphoenolpyruvate to produce oxaloacetate and Pi Oxaloacetate is reduced to malate, and NADPH is oxidized Malate is transported to the bundle-sheath cells where it is decarboxylated to give pyruvate and CO2 © 2018 Cengage Learning. All Rights Reserved. C4 Pathway (continued) CO2 reacts with ribulose-1,5-bisphosphate to enter the Calvin cycle Pyruvate is transported back to the mesophyll cells, where it is phosphorylated to phosphoenolpyruvate When pyruvate is phosphorylated, ATP is hydrolyzed to AMP and PPi © 2018 Cengage Learning. All Rights Reserved. Figure 22.23 - The C4 Pathway © 2018 Cengage Learning. All Rights Reserved. Figure 22.24 - Characteristic Reactions of the C4 Pathway © 2018 Cengage Learning. All Rights Reserved. Photorespiration Process by which plants oxidize carbohydrates aerobically in the light Salvage pathway that saves some of the carbon that would be lost because of the oxygenase activity of rubisco Reaction features Principal substrate oxidized is glycolate, which arises from the oxidative breakdown of ribulose-1,5- bisphosphate Product of the oxidation reaction is glyoxylate © 2018 Cengage Learning. All Rights Reserved. Figure 22.25 - Characteristic Reactions of Photorespiration © 2018 Cengage Learning. All Rights Reserved.