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Where It Starts – Photosynthesis Lecture 8 - Autumn 2007 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 (deep blue sea - blue light travels the deepest because it has the most energy) Pigments • Light-absorbing molecules • Absorb some wavelengths and transmit others • Color you see are the wavelengths not absorbed Variety of Pigments Chlorophylls a and b Carotenoids Etc. Chlorophylls Main pigments in most photoautotrophs Wavelength absorption (%) - Organisms which make their own food 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 - simple elegance illuminate a long strand of photosynthetic algae with a spectrum of light, and watch were the mobile bacteria accumulate! Light-Dependent Reactions 1. Pigments absorb light energy, give up ewhich enter electron transfer chains 2. Water molecules are split, ATP and NADH are formed, and oxygen is released (waste product - lucky for us!) 3. Pigments that gave up electrons get replacements Light-Independent Reactions • Synthesis part of photosynthesis • Can proceed in the dark • Take place in the stroma (special structures inside the chloroplast) • Calvin-Benson cycle Photosynthesis Equation - remember forever 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 (mostly) • Heterotrophs – Get carbon and energy by eating autotrophs (plants) and/or one another (cannibalism too) 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 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 some energy loss - heat and atomic motion 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 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 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)