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Photosynthesis Converting Light Energy to Chemical Energy Photosynthesis Is an Endergonic Reaction Energy Supplied O + O O H + O C O O OH 6 CO2 + 6 H2O H CH2OH O glucose (C6H12O6) + energy = + 6 O2 Photosynthesis Occurs in Chloroplasts of Leaf Mesophyll Cells Leaf Structure Upper Epidermis Mesophyll Cells Vein Stoma Lower Epidermis Chloroplast Structure Thylakoids (contain pigments) Stroma (interior matrix) Granum (stack of thylakoids) Inner & Outer Membranes Light, Chloroplast Pigments, & Photosynthesis Gamma Rays X-Rays UV Infrared Microwaves Radio Waves Visible Light 400 450 500 550 600 650 7000 Wavelength (nanometers) 750 Visible Light 450 500 550 600 Chlorophyll 650 700 Collectively 750 100 80 60 Carotenoids 20 Phycocyanin 400 450 500 550 600 650 Wavelength (nanometers) 40 700 0 750 % Light Absorption 400 Photosynthetic Pigments • Chlorophyll – major photosynthetic pigment – absorbs violet, blue, red wavelengths, reflects green • Accessory Pigments – Carotenoids • absorb blue and green, reflect yellow – Phycocyanins • absorb green and yellow, reflect blue Photosynthesis Involves Two Sets of Reactions Light-Dependent Reactions O2 H2O (in thylakoids) Depleted Carriers Energized Carriers (ADP, NADP+) Glucose (ATP, NADPH) Light-Independent Reactions (in stroma) CO2+H2O Light-Dependent Reactions • Energy from light is used to form – ATP: energy carrier – NADPH: electron carrier • Occur in photosystems located in thylakoid membranes – organized array of pigment molecules – each photosystem contains • light-harvesting complex, with reaction center • electron transport system Photosystems II and I Reach electron carriers Electrons absorb energy Absorbs light energy and transfers it to Electron Flow in Light-dependent Reactions Summary of Light-Dependent Reactions Reactants Water + energy + PO4 Products ½ O2 Protons (2H+) Electrons (2e- ) ATP + 2e- + H+ NADPH electron supplier ADP energy carrier NADP+ electron carrier Applying Your Knowledge 1. 2. 3. 4. Photosystem I Photosystem II Both PS I and PS II Neither PS I nor PS II A. Where is ATP produced? B. Which one contains an electron transport system? C. Where is NADPH produced? Chemiosmosis Produces ATP Chemiosmosis Produces ATP ATP is Produced by Chemiosmosis • Diffusion of hydrogen ions (H+ ) through a differentially permeable membrane, resulting in ATP production – energy from electron transport pumps H+ to interior of thylakoid – H+ diffuses to exterior through pores of ATP-synthesizing enzyme – movement of H+ provides energy for ATP synthesis Light-Independent Reactions (C3 Photosynthesis Cycle) • Produce Glucose (C6 H12O6) using CO2 as a carbon source • Occur in stroma of chloroplast • Use ATP and NADPH from LightDependent Reactions • Six turns of the cycle produce one glucose molecule 6-Carbon Compounds form briefly Cycle is named for 3-Carbon Compounds The C3 Cycle of Carbon Fixation C3 Photosynthesis Cycle Carbon Dioxide C X6 C C C C C 5-Carbon Reactants X 6 6 ADP RUBISCO 6 ATP Rearrangements to reform 5C reactants 3-Carbon X 10 C C C C C Glucose 6-Carbon Product C C C C C C 6-Carbon Intermediates X 6 C C C C C C 3-Carbon Intermediates X 12 12 ADP 12 NADP+ C 3-Carbon X 2 C C C 12 ATP 12 NADPH C C Modified 3-Carbon Intermediates X 12 C Summary of Light-Independent Reactions CH2OH O O OH Reactants Products 6 CO2 Glucose (C6 H12O6) ATP energy carrier NADPH ADP + PO4 + energy (in bonds of glucose) NADP+ + 2e- + H+ (used in C3 reactions) electron carrier A Summary of Photosynthesis Light-dependent reactions occur in thylakoids Lightindependent reactions (c3 cycle) occur in stroma Applying Your Knowledge 1. 2. 3. 4. A. B. C. D. Light-Dependent Reactions Light-Independent Reactions Neither choice 1 nor 2 Both choices 1 and 2 Which reactions produce glucose? 2 Which reactions use water? 4 Which reactions release oxygen? 1 Which reactions use energy from ATP? 2 Adaptations to Hot and Dry Conditions • Plants close leaf pores (stoma) to avoid losing water at high temperatures • Closed stoma prevent diffusion of CO2 into leaf and diffusion of O2 out of leaf • Excess O2 leads to photorespiration, interferes with photosynthesis • C4 Pathway is used to continue CO2 collection C3 Plants & the C3 Pathway C3 plants use the C3 pathway In a C3 plant, most chloroplasts are in mesophyll cells. C3 Plants & the C3 Pathway RUBISCO RUBISCO combines with O2 instead of CO2 Much photorespiration occurs under hot, dry conditions C4 Plants & the C4 Pathway C4 plants use the C4 pathway In a C4 plant, both mesophyll & bundlesheath cells contain chloroplasts. Mesophyll cell in a C4 plant CO2 is captured with a highly specific enzyme. 3C 4C C4 Plants & the C4 Pathway RUBISCO Much glucose synthesis occurs. Almost no photorespiration occurs under hot, dry conditions. Bundle-sheath cell in a C4 plant Interconnections: Photosynthesis & Respiration