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evolution.berkeley.edu/.../images/chicxulub.gif
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The ability to capture sunlight energy and convert it to chemical energy. iStockphoto.com/hougaardmalan
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6CO 2 carbon dioxide + 6H 2 O water + light energy sunlight C 6 H 12 O 6 glucose (sugar) + 6O 2 oxygen
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Plants, algae, and some prokaryotes Are autotrophs (“self- feeders”) uni-bielefeld.de
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Are interconnected Water, CO 2, sugar, and O 2 are used or produced as byproducts in both processes
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Leaves Chloroplasts
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Flattened leaf shape exposes large surface area to catch sunlight Epidermis upper and lower leaf surfaces Cuticle waxy, waterproof outer surface reduces water evaporation
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Stomata adjustable pores allow for entry of air with CO 2 Mesophyll inner cell layers that contain majority of chloroplasts Vascular bundles (veins) supply water and minerals to the leaf while carrying sugars away from the leaf
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Chloroplasts bounded by a double membrane composed of inner and outer membranes Stroma semi-fluid medium within the inner membrane Thylakoids disk-shaped sacs found within the stroma in stacks called grana
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Fig. 7-5b, p. 111 two outer membranes of chloroplast stroma part of thylakoid membrane system: thylakoid compartment, cutaway view B Chloroplast structure. No matter how highly folded, its thylakoid membrane system forms a single, continuous compartment in the stroma.
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2 sets of chemical reactions occur in the: 1. Thylakoid membranes 2. Stroma
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Pigment molecules (e.g. chlorophyll) of the thylakoids capture sunlight energy Sunlight energy is converted to the energy carrier molecules ATP and NADPH Oxygen is released as a by-product
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Fig. 7-5c, p. 111 sunlight O2O2 H2OH2O CO 2 CHLOROPLAST light- dependent reactions NADPH, ATP NADP +, ADP light- independent reactions sugars CYTOPLASM C In chloroplasts, ATP and NADPH form in the light-dependent stage of photosynthesis, which occurs at the thylakoid membrane. The second stage, which produces sugars and other carbohydrates, proceeds in the stroma.
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Enzymes in stroma synthesize glucose and other organic molecules using the chemical energy stored in ATP and NADPH
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Sun radiates electromagnetic energy Photons (basic unit of light) packets of energy with different energy levels short-wavelength photons are very energetic longer-wavelength photons have lower energies Visible light is radiation falling between 400-750 nanometers of wavelength
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Absorption of certain wavelengths light is “trapped” Reflection of certain wavelengths light bounces back Transmission of certain wavelengths light passes through Light Captured by Pigments
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Absorbed light drives biological processes when it is converted to chemical energy Pigments absorb visible light Common pigments : Chlorophyll a and b absorb violet, blue, and red light but reflect green light (hence they appear green) Carotenoids absorb blue and green light but reflect yellow, orange, or red (hence they appear yellow-orange) Are accessory pigments
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autumn-pictures.com
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Chlorophyll breaks down before carotenoids in dying autumn leaves revealing yellow colors Red fall colors (anthocyanin pigments) are synthesized by some autumn leaves, producing red colors
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Photosystems within thylakoids Assemblies of proteins, chlorophyll, & accessory pigments Two Photosystems PSII (comes 1 st ) and PSI (comes 2 nd ) Each Photosystem is associated with a chain of electron carriers
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Steps of the light reactions: 1. Accessory pigments in Photosystems absorb light and pass energy to reaction centers containing chlorophyll 2. Reaction centers receive energized electrons… 3. Energized electrons then passed down a series of electron carrier molecules ( Electron Transport Chain ) 4. Energy released from passed electrons used to synthesize ATP from ADP and phosphate 5. Energized electrons also used to make NADPH from (NADP+) + (H+)
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Fig. 7-8, p. 113 to second stage of reactions The Light-Dependent Reactions of Photosynthesis ATP synthase light energy NADPH ATP ADP + P i photosystem II electron transfer chain photosystem I thylakoid compartment stroma A Light energy drives electrons out of photosystem II. C Electrons from photosystem II enter an electron transfer chain. E Light energy drives electrons out of photosystem I, which accepts replacement electrons from electron transfer chains. G Hydrogen ions in the thylakoid compartment are propelled through the interior of ATP synthases by their gradient across the thylakoid membrane. B Photosystem II pulls replacement electrons from water molecules, which dissociate into oxygen and hydrogen ions (photolysis). The oxygen leaves the cell as O 2. D Energy lost by the electrons as they move through the chain causes H + to be pumped from the stroma into the thylakoid compartment. An H + gradient forms across the membrane. F Electrons from photosystem I move through a second electron transfer chain, then combine with NADP + and H +. NADPH forms. H H + flow causes the ATP synthases to attach phosphate to ADP, so ATP forms in the stroma. NADP +
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Electrons from PSII flow one-way into PS I PSII – produces ATP PSI – produces NADPH
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May be used by plant or released into atmosphere
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NADPH and ATP from light-dependent rxns used to power glucose synthesis Light not directly necessary for light- independent rxns if ATP & NADPH available Light-independent rxns called the Calvin- Benson Cycle or C 3 Cycle
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6 CO 2 molecules used to synthesize 1 glucose (C 6 H 12 O 6 ) CO 2 is captured and linked to a sugar called ribulose bisphosphate (RuBP) ATP and NADPH from light dependent rxns used to power C 3 reactions
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“ Photo ” capture of light energy (light dependent rxns) “ Synthesis ” glucose synthesis (light- in dependent rxns) Light dependent rxns produce ATP and NADPH which is used to drive light-independent rxns Depleted carriers (ADP and NADP + ) return to light- dependent rxns for recharging
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The ideal leaf : Large surface area to intercept sunlight Very porous to allow for CO 2 entry from air lowcarboneconomy.com biology-blog.com forestry.about.com sbs.utexas.edu
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Problem : Substantial leaf porosity leads to substantial water evaporation, causing dehydration stress on the plant Plants evolved waterproof coating and adjustable pores (stomata) for CO 2 entry
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When stomata close, CO 2 levels drop and O 2 levels rise Photorespiration occurs Carbon fixing enzyme combines O 2 instead of CO 2 with RuBP
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Photorespiration : O 2 is used up as CO 2 is generated No useful cellular energy made No glucose produced Photorespiration is unproductive and wasteful
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Hot, dry weather causes stomata to stay closed O 2 levels rise as CO 2 levels fall inside leaf Photorespiration very common under such conditions Plants may die from lack of glucose synthesis
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weedtwister.com
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“C 4 plants” have chloroplasts in bundle sheath cells and mesophyll cells Bundle sheath cells surround vascular bundles deep within mesophyll C 3 plants lack bundle sheath cell chloroplasts
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C 4 plants utilize the C 4 pathway Two-stage carbon fixation pathway Takes CO 2 to chloroplasts in bundle sheath cells
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C 4 pathway uses up more energy than C 3 pathway C 3 plants thrive where water is abundant or if light levels are low (cool, wet, and cloudy climates) Ex. : most trees, wheat, oats, rice, Kentucky bluegrass C 4 plants thrive when light is abundant but water is scarce (deserts and hot climates) Ex. : corn, sugarcane, sorghum, crabgrass, some thistles
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