Chapter 7 Capturing Solar Energy: Photosynthesis What is Photosynthesis? Answer: The capture of sunlight energy and the subsequent storage of that energy in the chemical bonds (e.g., glucose) Chemical Reaction: 6 CO 2 Carbon dioxide + 6 H 2 O Water + Light Energy = C 6 H 12 O 6 Glucose + 6 O 2 Oxygen Performed by autotrophs ( self-feeders): Plants (Eukaryotes) Algae (Eukaryotes) Bacteria (Prokaryotes) 1
Adaptations for Photosynthesis: 1) Leaf: Flattened shape (large surface area) Thin (light can penetrate entire leaf) Cuticle: Waxy covering that prevents water loss Stomata: Adjustable openings that regulate CO 2 uptake and O 2 release Stoma Mesophyll Cells: Contain majority of chloroplast Vascular Bundles (Veins): Supply water / minerals; carry away sugars Figure 7.2 / 7.3 Audesirk 2 & Byers Adaptations for Photosynthesis: 2) Chloroplast: Double outer membrane Stroma: Semi-fluid medium (Light Independent Reactions) Thylakoids: Disk-shaped sacks (Light Dependent Reactions) Mesophyll cell containing chloroplasts Figure 4.18 Audesirk 2 & Byers 2
Figure 7.4 Audesirk 2 & Byers Overview of Photosynthesis: Light Dependent Reactions: The Conversion of Light Energy to Chemical Energy Step 1: Light is captured by pigments in chloroplasts Photon: Packet of light energy When photon hits leaf, the light is either: 1) Absorbed 2) Reflected (bounced back) 3) Transmitted (passes through) Chlorophyll & accessory pigments absorb specific wavelengths of light Why do many leaves turn yellow in fall? Chlorophyll a & b: Absorb blue and red light Carotenoids: Absorb blue and green light Figure 7.5 Audesirk 2 & Byers 3
Light Dependent Reactions: The Conversion of Light Energy to Chemical Energy Step 2: Light energy transferred to energy carrier molecules Photosystems: Metabolic pathways located in thylakoid membranes A) Light harvesting complex (chlorophyll / carotenoids) B) Electron transport chain (ETC) (protein cluster) Utilize light energy to reenergize energy transport molecules Photosystem II: 1) Light excites in light-harvesting complex 2) ETC accepts excited (chemiosmosis) 3) ETC uses energy to synthesis Photosystem I: 1) Light excites in light-harvesting complex 2) ETC accepts excited 3) ETC captures energy to form Light-harvesting Complex Electron Transport Chain Light-harvesting Complex Electron Transport Chain Light Dependent Reactions: The Conversion of Light Energy to Chemical Energy Step 2: Light energy transferred to energy carrier molecules Photosystem II and Photosystem I work simultaneously Photosystem II: Photosystem I: Light-harvesting Complex Electron Transport Chain Light-harvesting Complex Electron Transport Chain H 2 O O 2 Electrons lost in Photosystem I are replaced by electrons from Photosystem II Electrons lost in Photosystem II are replaced by splitting water to form O 2 4
Light Dependant Reactions: Figure 7.5 Audesirk 2 & Byers Light Independent Reactions: Energy is Stored in Glucose Molecules (Light Dependent Reactions) CO 2 + H 2 O C 6 H 12 O 6 Carbon Dioxide Water Glucose Energy from & are necessary to drive process Occurs in stroma of the chloroplast 5
Light Independent Reactions: Energy is Stored in Glucose Molecules (C 3 Cycle) Calvin-Benson Cycle: Set of reactions which capture carbon dioxide CO 2 RuBP H 2 O PGA G3P Requirements: 1) CO 2 (from air) 2) Ribulose Bisphosphate (RuBP) 3) Multiple enzymes 4) Energy ( & ) Sequence of Events: 1) Carbon Fixation: CO 2 combines with ribulose bisphosphate to form phosphoglyceric acid (PGA) 2) PGA is converted to glyceraldehyde-3-phosphate (G3P) Requires energy Some G3P spontaneously converted into glucose (1 glucose from 6 CO 2 ) 3) G3P converted back to RuBP (requires energy) Glucose Fate of Glucose: 1) Broken down for energy 2) Stored as starch (energy storage) 3) Converted to cellulose (structure) Figure 7.10 Audesirk 2 & Byers 6
Photosynthesis Review: Figure 7.11 Audesirk 2 & Byers Plants must balance between obtaining CO 2 and H 2 O loss Cool, wet conditions stomata open (= plenty of CO 2 ) Hot, dry conditions stomata closed (= low CO 2 ) Photosynthesis inefficient (photorespiration occurs) Figure 7.12 Audesirk 2 & Byers 7
How Do Plants Flourish in Arid Conditions? Answer: They use the C 4 pathway for photosynthesis Corn CO 2 initially captured as oxaloacetate (mesophyll cells) Oxaloacetate releases CO 2 to bundle-sheath cells where photosynthesis continues as normal ( [CO 2 ]) Figure 7.12 Audesirk 2 & Byers 8