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• Overview: The Process That Feeds the Biosphere • Photosynthesis – Is the process that converts solar energy into chemical energy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Plants and other autotrophs – Are the producers of the biosphere • Plants are photoautotrophs – They use the energy of sunlight to make organic molecules from water and carbon dioxide Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Photosynthesis – Occurs in plants, algae, certain other protists, and some prokaryotes These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed not only themselves, but the entire living world. (a) On land, plants are the predominant producers of food. In aquatic environments, photosynthetic organisms include (b) multicellular algae, such as this kelp; (c) some unicellular protists, such as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur (a) Plants bacteria, which produce sulfur (spherical globules) (c, d, e: LMs). (c) Unicellular protist 10 m (e) Pruple sulfur bacteria Figure 10.2 (b) Multicellular algae Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (d) Cyanobacteria 40 m 1.5 m • Heterotrophs – Obtain their organic material from other organisms – Are the consumers of the biosphere Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 10.1: Photosynthesis converts light energy to the chemical energy of food Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chloroplasts: The Sites of Photosynthesis in Plants • The leaves of plants – Are the major sites of photosynthesis Leaf cross section Vein Mesophyll Stomata Figure 10.3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CO2 O2 • Chloroplasts – – Mesophyll Are the organelles in which photosynth esis occurs Chloroplast 5 µm Contain thylakoids and grana Outer membrane Thylakoid Stroma Granum Intermembrane space Thylakoid space Inner membrane 1 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Tracking Atoms Through Photosynthesis: Scientific Inquiry • Photosynthesis is summarized as 6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthesis as a Redox Process • Photosynthesis is a redox process – Water is oxidized, carbon dioxide is reduced – This is the reverse flow of electrons as in cellular respiration Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Two Stages of Photosynthesis: A Preview • Photosynthesis consists of two processes – The light reactions – The Calvin cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The light reactions – Occur in the grana – Split water, release oxygen, produce ATP, and form NADPH • The ATP is produced by photophosphorylation – generating ATP using chemiosmosis to power the addition of phosphate to ADP – The third way of generating ATP we’ve seen Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The Calvin cycle – Occurs in the stroma – Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An overview of photosynthesis H2O CO2 Light NADP ADP + P LIGHT REACTIONS CALVIN CYCLE ATP NADPH Chloroplast Figure 10.5 O2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings [CH2O] (sugar) • Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The absorption spectra of chloroplast pigments – Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The absorption spectra of three types of pigments in chloroplasts Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below. EXPERIMENT RESULTS Absorption of light by chloroplast pigments Chlorophyll a Chlorophyll b Carotenoids Wavelength of light (nm) (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Figure 10.9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Chlorophyll a – Is the main photosynthetic pigment • Chlorophyll b in chlorophyll a CHO in chlorophyll b CH2 CH – CH3 Is an accessory pigment C H3C C H C C C C C N C N C Mg N C C C H C N C H3C CH3 H CH2 H H C C C O C H C CH3 CH3 Porphyrin ring: Light-absorbing “head” of molecule note magnesium atom at center C O O CH2 C C CH2 C O O CH3 CH2 Figure 10.10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown • Other accessory pigments – Absorb different wavelengths of light and pass the energy to chlorophyll a Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Excitation of Chlorophyll by Light • When a pigment absorbs light – It goes from a ground state to an excited state, which is unstable e– Excited state Heat Photon (fluorescence) Photon Chlorophyll molecule Figure 10.11 A Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ground state A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes • A photosystem Thylakoid Photosystem Photon STROMA Light-harvesting complexes Thylakoid membrane – Is composed of a reaction center surrounded by a number of lightharvesting complexes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Primary election acceptor e– Transfer of energy Figure 10.12 Reaction center Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) • The light-harvesting complexes – Consist of pigment molecules bound to particular proteins – Act as an antenna for the reaction center – Funnel the energy of photons of light to the reaction center Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Reaction center – – Is a protein complex, including two special chlorophyll molecules and the primary electron acceptor Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings First step of light reactions • When a reaction-center chlorophyll molecule absorbs energy – One of its electrons gets bumped up to a primary electron acceptor – This is a redox rxn Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The thylakoid membrane – Is populated by two types of photosystems, I and II Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Noncyclic Electron Flow • Noncyclic electron flow – Is the primary pathway of energy transformation in the light reactions – Produces ATP and NADPH which will provide chemical energy and reducing power to the sugar making Calvin cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Produces NADPH, ATP, and oxygen H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH O2 [CH2O] (sugar) Primary acceptor Primary acceptor Fd 2 2 H+ + O2 Pq e H2O e NADP+ NADP+ + 2 H+ reductase 3 NADPH PC e– 5 + H+ P700 P680 Light 6 ATP Figure 10.13 8 e– Cytochrome complex e– Light 1 7 4 Photosystem II (PS II) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosystem-I (PS I) Cyclic Electron Flow • Under certain conditions – Photoexcited electrons take an alternative path – This is because the Calvin cycle needs more ATP than NADPH. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In cyclic electron flow – Only photosystem I is used – Only ATP is produced Primary acceptor Primary acceptor Fd Fd Pq NADP+ reductase Cytochrome complex NADPH Pc Figure 10.15 Photosystem II ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings NADP+ Photosystem I A Comparison of Chemiosmosis in Chloroplasts and Mitochondria • Chloroplasts and mitochondria – Generate ATP by the same basic mechanism: chemiosmosis – But use different sources of energy to accomplish this Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In both organelles – Redox reactions of electron transport chains generate a H+ gradient across a membrane • ATP synthase – Uses this proton-motive force to make ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The light reactions and chemiosmosis: the organization of the thylakoid membrane H2O CO2 LIGHT NADP+ ADP LIGHT REACTOR CALVIN CYCLE ATP NADPH STROMA (Low H+ concentration) O2 [CH2O] (sugar) Cytochrome Photosystem II complex Photosystem I NADP+ reductase Light 2 H+ Fd 3 NADP+ + 2H+ NADPH + H+ Pq Pc 2 H2O THYLAKOID SPACE 1 (High H+ concentration) 1⁄ 2 O2 +2 H+ 2 H+ To Calvin cycle STROMA (Low H+ concentration) Thylakoid membrane ATP synthase ADP ATP P Figure 10.17 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings H+ • Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar • The Calvin cycle – Is similar to the citric acid cycle – Occurs in the stroma Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The Calvin cycle has three phases – Carbon fixation – Reduction – Regeneration of the CO2 acceptor Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The Calvin cycle Light H2 O CO2 Input 3 (Entering one CO2 at a time) NADP+ ADP CALVIN CYCLE LIGHT REACTION ATP Phase 1: Carbon fixation NADPH O2 Rubisco [CH2O] (sugar) 3 P 3 P P Short-lived intermediate P Ribulose bisphosphate (RuBP) P 6 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 ADP 3 ATP Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 P P 1,3-Bisphoglycerate 6 NADPH 6 NADPH+ 6 P P 5 (G3P) 6 P Glyceraldehyde-3-phosphate (G3P) P 1 Figure 10.18 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings G3P (a sugar) Output Glucose and other organic compounds Phase 2: Reduction • Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • On hot, dry days, plants close their stomata – Conserving water but limiting access to CO2 – Causing oxygen to build up Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photorespiration: An Evolutionary Relic? • In photorespiration – O2 substitutes for CO2 in the active site of the enzyme rubisco – The photosynthetic rate is reduced – No sugar is made, ATP is used, not made – Hold over from earlier time Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings C4 Plants • C4 plants minimize the cost of photorespiration – By incorporating CO2 into four carbon compounds in mesophyll cells – These four carbon compounds • Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • C4 leaf anatomy and the C4 pathway Mesophyll cell Mesophyll cell Photosynthetic cells of C4 plant leaf CO CO 2 2 PEP carboxylase Bundlesheath cell PEP (3 C) ADP Oxaloacetate (4 C) Vein (vascular tissue) Malate (4 C) ATP C4 leaf anatomy BundleSheath cell Pyruate (3 C) CO2 Stoma CALVIN CYCLE Sugar Vascular tissue Figure 10.19 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CAM Plants • CAM plants – Open their stomata at night, incorporating CO2 into organic acids – During the day, the stomata close – And the CO2 is released from the organic acids for use in the Calvin cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The CAM pathway is similar to the C4 pathway Pineapple Sugarcane C4 Mesophyll Cell Organic acid Bundlesheath cell (a) Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different Figure 10.20 types of cells. CAM CO2 CALVIN CYCLE CO2 1 CO2 incorporated Organic acid into four-carbon organic acids (carbon fixation) 2 Organic acids release CO2 to Calvin cycle Sugar Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CALVIN CYCLE Sugar Night Day (b) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times. The Importance of Photosynthesis: A Review • A review of photosynthesis Light reaction Calvin cycle H2O CO2 Light NADP+ ADP +P1 RuBP 3-Phosphoglycerate Photosystem II Electron transport chain Photosystem I ATP NADPH G3P Starch (storage) Amino acids Fatty acids Chloroplast Figure 10.21 O2 Light reactions: • Are carried out by molecules in the thylakoid membranes • Convert light energy to the chemical energy of ATP and NADPH • Split H2O and release O2 to the atmosphere Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sucrose (export) Calvin cycle reactions: • Take place in the stroma • Use ATP and NADPH to convert CO2 to the sugar G3P • Return ADP, inorganic phosphate, and NADP+ to the light reactions • Organic compounds produced by photosynthesis – Provide the energy and building material for ecosystems Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings