Photosynthesis 1

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Information about Photosynthesis 1

Published on January 28, 2008

Author: Carla


Capturing Solar Energy: Photosynthesis :  Capturing Solar Energy: Photosynthesis Slide3:  (a) (b) internal leaf structure chloroplast in mesophyll cell mesophyll cells chloroplasts stoma vein channel interconnecting thylakoids (c) outer membrane inner membrane thylakoid stroma Photosynthesis :  Photosynthesis Complex series of chemical reactions involving a transition in forms of energy Photosynthesis:  Photosynthesis Uses light energy to make food and is a process by which some organisms can make organic compounds from simple inorganic compounds using energy from the sun Photosynthesis :  Photosynthesis Light energy captured and stored as chemical potential energy in the covalent bonds of carbohydrate molecules 6 CO2 + 6 H2O + light  C6H12O6 + 6 O2 Life depends on photosynthesis :  Life depends on photosynthesis A. Foundation of energy for most ecosystems B. Source of oxygen C. Key component of the carbon cycle The mechanism of photosynthesis :  The mechanism of photosynthesis Solar energy and light The electromagnetic spectrum Pigment molecules absorb some wavelengths of light and reflect others Chlorophyll—a green photosynthetic pigment associated with the thylakoid membranes of chloroplasts Slide10:  (a) Slide11:  (b) chlorophyll carotenoids phycocyanin Slide12:  (c) The mechanism of photosynthesis:  The mechanism of photosynthesis Chloroplasts are the sites of photosynthesis Have a membrane system within internal space (stroma) Arranged in disk-shaped sacks (thylakoids) The thylakoids contain light-harvesting photosynthetic pigments & enzymes Internal membranes define space (lumen) that is separate from the rest of the stroma Slide14:  electron transport system chloroplast thylakoids light- harvesting complex reaction center The mechanism of photosynthesis:  The mechanism of photosynthesis Photosynthesis occurs in two steps 1. Light-dependent reactions a. Provides the energy necessary to fix carbon b. Occurs in the thylakoid membranes c. Generates ATP d. Photolysis—light, electrons and water Slide16:  2 1 3 4 5 6 7 8 9 electron transport system energy to drive photosystem II photosystem I reaction center reaction center synthesis Light-Dependent Reactions:  Light-Dependent Reactions What happens during light reactions? Chlorophyll in plants (Photosystem I [PS I]) Absorbs blue & red light and reflects back the green light Blue & red light energy boosts electrons to higher energy levels Electrons are passed to electron transport molecule in the thylakoid Can only be passed when highly energized Light-Dependent Reactions:  Light-Dependent Reactions What happens during light reactions? During transport of electrons from PS II to PS I Some energy is harnessed to produce ATP Eventually, chlorophyll from PS II is oxidized Gets replacement electrons from water Light-Dependent Reactions:  Light-Dependent Reactions Energy of light has thus been captured in two forms: The synthesis of NADPH from NADP+ Proton gradient across the thylakoid membrane Cannot be used directly to make food Must first be converted to ATP by chloroplast ATP synthase The mechanism of photosynthesis:  The mechanism of photosynthesis Energy carriers ATP and NADPH transport energy from the light-dependent reactions to the light-independent reactions Slide21:  energy from sunlight Light-dependent reactions occur in thylakoids. Light-independent reactions (C3cycle) occur in stroma. The mechanism of photosynthesis:  The mechanism of photosynthesis 2. Light-independent reactions a. Uses energy of the light-dependent reaction to make sugar from CO2 b. Occurs in the stroma Light-Independent Reactions:  Light-Independent Reactions Steps in Light-Independent Reactions: CO2 joins with RuBP forming an unstable 6-C molecule Breaks into two 3-C PGA molecules This first step in Calvin-Benson cycle is catalyzed by enzyme Called ribulose biphosphate carboxylase (Rubisco) Slide24:  2 G3P available for synthesis of organic molecules. 3 RuBP regeneration uses energy and 10 G3P. 2 G3P synthesis uses energy. 1 Carbon fixation combines CO2 with RuBP. C4 plants utilize an alternate pathway to make sugars in dry environments :  C4 plants utilize an alternate pathway to make sugars in dry environments Closing stomata to conserve water results in photorespiration in C3 plants Slide26:  Much photorespiration occurs under hot, dry conditions. CO2 is captured with a highly specific enzyme. Almost no photorespiration occurs in hot, dry conditions. Much glucose synthesis occurs. mesophyll cell in C4 plant mesophyll cell in C3 plant bundle-sheath cell in C4 plant bundle- sheath cells C3 plants use the C3 pathway C4 plants use the C4 pathway In a C3 plant, most chloroplasts are in mesophyll cells. In a C4 plant, both mesophyll and bundle-sheath cells contain chloroplasts. (a) (b)

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