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Where It Starts – Photosynthesis Chapter 6 Sunlight as an Energy Source • Photosynthesis runs on a fraction of the electromagnetic spectrum, or the full range of energy radiating from the sun Visible Light • Wavelengths humans perceive as different colors • Violet (380 nm) to red (750 nm) • Longer wavelengths, lower energy Electromagnetic Spectrum Shortest wavelength Longest wavelength Gamma rays X-rays UV radiation Visible light Infrared radiation Microwaves Radio waves Photons • Packets of light energy • Each type of photon has fixed amount of energy • Photons having most energy travel as shortest wavelength (blue-green light) Pigments • Light-absorbing molecules • Absorb some wavelengths and transmit others • Color you see are the wavelengths not absorbed Pigment Structure • Light-catching part of molecule often has alternating single and double bonds • These bonds contain electrons that are capable of being moved to higher energy levels by absorbing light Variety of Pigments Chlorophylls a and b Carotenoids Xanthophylls Phycobilins Anthocyanins Chlorophylls Wavelength absorption (%) Main pigments in most photoautotrophs chlorophyll a chlorophyll b Wavelength (nanometers) Carotenoids • Found in all photoautotrophs • Absorb blue-violet and blue-green that chlorophylls miss • Reflect red, yellow, orange wavelengths • Two types – Carotenes - pure hydrocarbons – Xanthophylls - contain oxygen Xanthophylls Yellow, brown, purple, or blue accessory pigments Phycobilins & Anthocyanins Red to purple pigments • Phycobilins – Found in red algae and cyanobacteria • Anthocyanins – Give many flowers their colors T.E. Englemann’s Experiment Background • Certain bacterial cells will move toward places where oxygen concentration is high • Photosynthesis produces oxygen T.E. Englemann’s Experiment Hypothesis • Movement of bacteria can be used to determine optimal light wavelengths for photosynthesis T.E. Englemann’s Experiment Method • Algal strand placed on microscope slide and illuminated by light of varying wavelengths • Oxygen-requiring bacteria placed on same slide T.E. Englemann’s Experiment Results Bacteria congregated where red and violet wavelengths illuminated alga Conclusion Bacteria moved to where algal cells released more oxygen – areas illuminated by the most effective light for photosynthesis T.E. Englemann’s Experiment Light-Dependent Reactions • Pigments absorb light energy, give up ewhich enter electron transfer chains • Water molecules are split, ATP and NADH are formed, and oxygen is released • Pigments that gave up electrons get replacements Light-Independent Reactions • Synthesis part of photosynthesis • Can proceed in the dark • Take place in the stroma • Calvin-Benson cycle Photosynthesis Equation Chloroplasts Organelles of photosynthesis leaf’s upper surface photosynthetic cells central vacuole chloroplast one photosynthetic cell inside the leaf vein stoma (gap) in lower epidermis section from the leaf, showing its internal organization Inside the Chloroplast • Two outer membranes enclose a semifluid interior, the stroma • Thylakoid membrane inside the stroma two outer membranes thylakoid membrane system chloroplasts see next slide stroma Inside the Chloroplast • Photosystems are embedded in thylakoids, containing 200 to 300 pigments and other molecules that trap sun’s energy • Two types of photosystems: I and II light harvesting complex electron transfer chain PHOTOSYSTEM II thylakoid membrane PHOTOSYSTEM I thylakoid compartment Carbon and Energy Sources • Photoautotrophs – Carbon source is carbon dioxide – Energy source is sunlight • Heterotrophs – Get carbon and energy by eating autotrophs or one another Photoautotrophs • Capture sunlight energy and use it to carry out photosynthesis – Plants – Some bacteria – Many protistans Linked Processes Photosynthesis • Energy-storing pathway Aerobic Respiration • Energy-releasing pathway • Releases oxygen • Requires oxygen • Requires carbon dioxide • Releases carbon dioxide Two Stages of Photosynthesis sunlight energy CO2 (carbon dioxide) H2 O (water) ATP lightdependent reactions ADP + Pi lightindependent reactions NADPH NADPH+ glucose O2 H2O (metabolic water) Excitation of Electrons • Excitation occurs only when the quantity of energy in an incoming photon matches the amount of energy necessary to boost the electrons of that specific pigment • Amount of energy needed varies among pigment molecules Pigments in Photosynthesis • Bacteria – Pigments in plasma membranes • Plants – Pigments embedded in thylakoid membrane system – Pigments and proteins organized into photosystems – Photosystems located next to electron transfer chains Photosystem Function: Harvester Pigments • Most pigments in photosystem are harvester pigments • When excited by light energy, these pigments transfer energy to adjacent pigment molecules • Each transfer involves energy loss Photosystem Function: Reaction Center • Energy is reduced to level that can be captured by molecule of chlorophyll a • This molecule (P700 or P680) is the reaction center of a photosystem • Reaction center accepts energy and donates electron to acceptor molecule Electron Transfer Chains • Adjacent to photosystem • Acceptor molecule donates electrons from reaction center • As electrons flow through chain, energy they release is used to produce ATP and, in some cases, NADPH Cyclic Electron Flow • Electrons – are donated by P700 in photosystem I to acceptor molecule – flow through electron transfer chain and back to P700 • Electron flow drives ATP formation • No NADPH is formed Noncyclic Electron Flow • Two-step pathway for light absorption and electron excitation • Uses two photosystems: type I and type II • Produces ATP and NADPH • Involves photolysis - splitting of water ATP and NADPH Formation LIGHTHARVESTING COMPLEX photon PHOTOSYSTEM II sunlight PHOTOSYSTEM I a light-harvesting complex has a ring of pigment molecules NADPH NADPH + H+ H+ H+ H+ A photosystem is surrounded by densely packed light harvesting complexes. H+ H+ H+ H+ H+ H+ H+ H+ thylakoid compartment thylakoid membrane ADP + Pi ATP stroma ATP Formation • When water is split during photolysis, hydrogen ions are released into thylakoid compartment • More hydrogen ions are pumped into the thylakoid compartment when the electron transfer chain operates ATP Formation • Electrical and H+ concentration gradient exists between thylakoid compartment and stroma • H+ flows down gradients into stroma through ATP synthesis • Flow of ions drives formation of ATP Energy Transfers PHOTOSYSTEM I p700* H+ Higher energy photon e- p700 Cyclic Pathway of ATP Formation Energy Transfers PHOTOSYSTEM I NADPH p700* PHOTOSYSTEM II NADH+ p680* photon e- p700 p680 2H2O 4H+ + O2 Noncyclic Pathway of ATP and NADPH Formation Calvin-Benson Cycle • Overall reactants • Overall products – Carbon dioxide – Glucose – ATP – ADP – NADPH – NADP+ Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated Calvin-Benson Cycle 6CO2 ATP 6 RuBP 12 PGA 12 6 ADP Calvin-Benson cycle ATP 12 ADP + 12 Pi 12 NADPH 4 Pi 12 NADP+ 10 PGAL 12 PGAL 1 Pi 1 glucose-6-1-phosphate Building Glucose • PGA accepts – phosphate from ATP – hydrogen and electrons from NADPH • PGAL (phosphoglyceraldehyde) forms • When 12 PGAL have formed – 10 are used to regenerate RuBP – 2 combine to form phosphorylated glucose Using the Products of Photosynthesis • Phosphorylated glucose is the building block for: – Sucrose • The most easily transported plant carbohydrate – Starch • The most common storage form The C3 Pathway • In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA • Because the first intermediate has three carbons, the pathway is called the C3 pathway stomata closed, no CO2 uptake RuBP PGA CalvinBenson cycle sugar Photorespiration in C3 Plants • On hot, dry days stomata close • Inside leaf – Oxygen levels rise – Carbon dioxide levels drop • Rubisco attaches RuBP to oxygen instead of carbon dioxide • Only one PGAL forms instead of two C4 Plants • Carbon dioxide is fixed twice – In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate – Oxaloacetate is transferred to bundlesheath cells – Carbon dioxide is released and fixed again in Calvin-Benson cycle C4 Plants stomata closed, no CO2 uptake C4 oxaloacetate cycle mesophyll cell CO2 RuBP PGA CalvinBenson cycle sugar bundle-sheath cell CAM Plants • Carbon is fixed twice (in same cells) • Night – Carbon dioxide is fixed to form organic acids CO2 uptake at night only C4 cycle runs at night • Day – Carbon dioxide is released and fixed in Calvin-Benson cycle CalvinBenson cycle sugar runs during day Summary of Photosynthesis sunlight LightDependent Reactions 12H2O 6O2 ADP + Pi ATP 6CO2 6 RuBP LightIndependent Reactions NADPH CalvinBenson cycle NADP+ 12 PGAL 6H2O phosphorylated glucose end products (e.g., sucrose, starch, cellulose)