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Chapter 10 Photosynthesis PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero 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 – Are the organelles in which photosynthesis occurs – Contain thylakoids and grana Mesophyll Chloroplast 5 µm Outer membrane Stroma Granum Thylakoid Thylakoid space Intermembrane 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 LEO says GER Lose electrons = oxidized Gain electrons = reduced 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 • Wavelength – Is the distance between the crests of waves – Determines the type of electromagnetic energy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The electromagnetic spectrum – Is the entire range of electromagnetic energy, or radiation 10–5 nm 10–3 nm Gamma rays X-rays UV 1m 106 nm 106 nm 103 nm 1 nm Infrared Microwaves 103 m Radio waves Visible light 380 450 500 550 Shorter wavelength Figure 10.6 Higher energy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 600 650 700 Longer wavelength Lower energy 750 nm – Reflect light, which include the colors we see Light Reflected Light Chloroplast Absorbed light Granum Transmitted light Figure 10.7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An absorption spectrum – Is a graph plotting light absorption versus wavelength Refracting Chlorophyll prism solution White light 2 Photoelectric tube Galvanometer 3 1 0 100 4 Slit moves to Green pass light light of selected wavelength The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. 0 Figure 10.8 Blue light Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 100 The low transmittance (high absorption) reading chlorophyll absorbs most blue light. • 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 • The action spectrum of a pigment Rate of photosynthesis (measured by O2 release) – Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis (b) 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 O2 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. CONCLUSION Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis. 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 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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