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Chapter 6 Photosynthesis Overview: The Process That Feeds the Biosphere Photosynthesis Is the process that converts solar energy into chemical energy 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 Photosynthesis 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). Occurs in plants, algae, certain other protists, and some prokaryotes (c) Unicellular protist 10 m (e) Pruple sulfur bacteria (b) Multicellular algae (d) Cyanobacteria 40 m 1.5 m Heterotrophs Obtain their organic material from other organisms Are the consumers of the biosphere Concept 10.1: Photosynthesis converts light energy to the chemical energy of food Chloroplasts: The Sites of Photosynthesis in Plants The leaves of plants Are the major sites of photosynthesis Leaf cross section Vein Mesophyll Stomata CO2 O2 Chloroplasts Are the organelles in which photosynthesis occurs Contain thylakoids and grana Mesophyll Chloroplast 5 µm Outer membrane Thylakoid Thylakoid StromaGranum space Intermembrane space Inner membrane 1 µm Tracking Atoms Through Photosynthesis: Scientific Inquiry Photosynthesis is summarized as 6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O The Splitting of Water Chloroplasts split water into Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules Reactants: Products: 12 H2O 6 CO2 C6H12O 6 6 H2O 6 O2 Photosynthesis as a Redox Process Photosynthesis is a redox process Water is oxidized, carbon dioxide is reduced The Two Stages of Photosynthesis: A Preview Photosynthesis consists of two processes The light reactions The Calvin cycle The light reactions Occur in the grana Split water, release oxygen, produce ATP, and form NADPH The Calvin cycle Occurs in the stroma Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power An overview of photosynthesis H2O CO2 Light NADP ADP + P LIGHT REACTIONS CALVIN CYCLE ATP NADPH Chloroplast O2 [CH2O] (sugar) The light reactions convert solar energy to the chemical energy of ATP and NADPH The Nature of Sunlight Light Is a form of electromagnetic energy, which travels in waves Wavelength Is the distance between the crests of waves Determines the type of electromagnetic energy The electromagnetic spectrum Is the entire range of electromagnetic energy, or radiation 10–5 nm 10–3 nm Gamma X-rays rays UV 1m 106 nm 106 nm 103 nm 1 nm Infrared Microwaves 103 m Radio waves Visible light 380 450 500 Shorter wavelength Higher energy 550 600 650 700 750 nm Longer wavelength Lower energy The visible light spectrum Includes the colors of light we can see Includes the wavelengths that drive photosynthesis Photosynthetic Pigments: The Light Receptors Pigments Are substances that absorb visible light Reflect light, which include the colors we see Light Reflected Light Chloroplast Absorbed light Granum Transmitted light The spectrophotometer Is a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength An absorption spectrum Is a graph plotting light absorption versus wavelength Refracting prism White light Chlorophyll solution Photoelectric tube Galvanometer 2 3 1 4 Slit moves to pass light of selected wavelength Green light 0 The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. 0 Blue light 100 100 The low transmittance (high absorption) reading chlorophyll absorbs most blue light. The absorption spectra of chloroplast pigments Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis The absorption spectra of three types of pigments in chloroplasts EXPERIME NT Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below. Absorption of light by chloroplast pigments RESULTS 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. The action spectrum of a pigment Rate of photosynthesis (measured by O2 release) (b) Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids. The action spectrum for photosynthesis Was first demonstrated by Theodor W. Engelmann Aerobic bacteria Filament of alga 500 600 700 400 (c) Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O 2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b. Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis. Chlorophyll a Is the main photosynthetic pigment Chlorophyll b Is an accessory pigment H3C C CH2 CH C C H C H3C C C C H CH2 CH2 C O CH2 C N CH3 CHO H C CH3 C N C C C C N N C C C C C C H H C C O O O O CH3 Mg CH2 H CH3 CH3 in chlorophyll a in chlorophyll b Porphyrin ring: Light-absorbing “head” of molecule note magnesium atom at center 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 Excitation of Chlorophyll by Light When a pigment absorbs light It goes from a ground state to an Excited excited state,e which state is unstable – Heat Photon (fluorescence) Photon Figure 10.11 A Chlorophyll molecule Ground state If an isolated solution of chlorophyll is illuminated Figure 10.11 B It will fluoresce, giving off light and heat Associated with Light-Harvesting Complexes A photosystem Is composed of a reaction center surrounded by a number of light-harvesting complexes Thylakoid Photosystem Photon Light-harvesting complexes Thylakoid membrane STROMA 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 Funnel the energy of photons of light to the reaction center When a reaction-center chlorophyll molecule absorbs energy One of its electrons gets bumped up to a primary electron acceptor The thylakoid membrane Is populated by two types of photosystems, I and II Noncyclic Electron Flow Noncyclic electron flow Is the primary pathway of energy transformation in the light reactions H2O CO2 Light NADP+ ADP Produces NADPH, ATP, and oxygen CALVIN CYCLE LIGHT REACTIONS ATP NADPH O2 [CH2O] (sugar) Primary acceptor Primary acceptor Fd 2 e H2O 2 H+ + O2 Pq 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) Photosystem-I (PS I) e– A mechanical analogy for the light e reactions e ATP – – NADPH e– e– e– Mill makes ATP e– Figure 10.14 Photosystem II Photosystem I Cyclic Electron Flow Under certain conditions Photoexcited electrons take an alternative path In cyclic electron flow Only photosystem I is used Only ATP is producedPrimary Primary acceptor acceptor Fd Fd Pq NADP+ reductase Cytochrome complex NADPH Pc Figure 10.15 Photosystem II ATP 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 The spatial organization of Key chemiosmosis Higher [H ] + Lower [H+] Chloroplast Differs Mitochondrion in chloroplasts and mitochondria CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE Intermembrance space Membrance Matrix Figure 10.16 H+ Diffusion Electron transport chain ATP Synthase ADP+ Thylakoid space Stroma P H+ ATP 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 H2O CO2 The light reactions and chemiosmosis: the organization of the thylakoid membrane 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 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 The Calvin cycle has three phases Carbon fixation Reduction Regeneration of the CO2 acceptor Light H2 O CO2 NADP+ ADP The Calvin cycle CALVIN CYCLE LIGHT REACTION Input 3 (Entering one CO2 at a time) 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 6 NADPH 6 NADPH+ 6 P P 5 (G3P) 6 P Glyceraldehyde-3-phosphate (G3P) P 1 Figure 10.18 P 1,3-Bisphoglycerate 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 On hot, dry days, plants close their stomata Conserving water but limiting access to CO2 Causing oxygen to build up Photorespiration: An Evolutionary Relic? In photorespiration O2 substitutes for CO2 in the active site of the enzyme rubisco The photosynthetic rate is reduced 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 Mesophyll cell Mesophyll cell Photosynthetic cells of C4 plant leaf CO CO 2 2 PEP carboxylase Bundlesheath cell C4 leaf anatomy and the C4 pathway 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 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 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. CALVIN CYCLE Sugar CAM CO2 CO2 1 CO2 incorporated Organic acid into four-carbon organic acids (carbon fixation) 2 Organic acids release CO2 to Calvin cycle 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 Light reaction Calvin cycle H2O CO2 ALightreview of photosynthesis 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 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