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evolution.berkeley.edu/.../images/chicxulub.gif The ability to capture sunlight energy and convert it to chemical energy. iStockphoto.com/hougaardmalan 6CO2 + 6H2O + light energy C6H12O6 + 6O2 carbon water dioxide sunlight glucose (sugar) oxygen Plants, algae, and some prokaryotes Are autotrophs (“self- feeders”) uni-bielefeld.de Are interconnected Water, CO2, sugar, and O2 are used or produced as byproducts in both processes Leaves Chloroplasts Flattened leaf shape exposes large surface area to catch sunlight Epidermis upper and lower leaf surfaces Cuticle waxy, waterproof outer surface reduces water evaporation Stomata Mesophyll adjustable pores allow for entry of air with CO2 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 Chloroplasts Stroma bounded by a double membrane composed of inner and outer membranes semi-fluid medium within the inner membrane Thylakoids disk-shaped sacs found within the stroma in stacks called grana 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. Fig. 7-5b, p. 111 2 sets of chemical reactions occur in the: 1. Thylakoid membranes 2. Stroma 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 sunlight O2 CO2 H2O CHLOROPLAST lightdependent reactions NADPH, ATP NADP+, ADP lightindependent 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. Fig. 7-5c, p. 111 Enzymes in stroma synthesize glucose and other organic molecules using the chemical energy stored in ATP and NADPH 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 Light Captured by Pigments Absorption of certain wavelengths light is “trapped” Reflection of certain wavelengths light bounces back Transmission of certain wavelengths light passes through 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 autumn-pictures.com 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 Photosystems within thylakoids Assemblies of proteins, chlorophyll, & accessory pigments Two Photosystems PSII (comes 1st) and PSI (comes 2nd) Each Photosystem is associated with a chain of electron carriers 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+) to second stage of reactions The Light-Dependent Reactions of Photosynthesis light energy photosystem II electron transfer chain light energy NADPH ATP ATP synthase ADP + Pi photosystem I NADP+ thylakoid compartment stroma A Light energy drives electrons out of photosystem II. C Electrons from photosystem II enter an electron transfer chain. B Photosystem II pulls replacement electrons from water molecules, which dissociate into oxygen and hydrogen ions (photolysis). The oxygen leaves the cell as O2. 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. E Light energy drives electrons out of photosystem I, which accepts replacement electrons from electron transfer chains. F Electrons from photosystem I move through a second electron transfer chain, then combine with NADP+ and H+. NADPH forms. G Hydrogen ions in the thylakoid compartment are propelled through the interior of ATP synthases by their gradient across the thylakoid membrane. H H+ flow causes the ATP synthases to attach phosphate to ADP, so ATP forms in the stroma. Fig. 7-8, p. 113 Electrons from PSII flow one-way into PS I PSII – produces ATP PSI – produces NADPH May be used by plant or released into atmosphere NADPH and ATP from light-dependent rxns used to power glucose synthesis Light not directly necessary for lightindependent rxns if ATP & NADPH available Light-independent rxns called the CalvinBenson Cycle or C3 Cycle 6 CO2 molecules used to synthesize 1 glucose (C6H12O6) CO2 is captured and linked to a sugar called ribulose bisphosphate (RuBP) ATP and NADPH from light dependent rxns used to power C3 reactions “Photo” “Synthesis” capture of light energy (light dependent rxns) glucose synthesis (light-independent rxns) Light dependent rxns produce ATP and NADPH which is used to drive light-independent rxns Depleted carriers (ADP and NADP+) return to lightdependent rxns for recharging The ideal leaf: Large surface area to intercept sunlight lowcarboneconomy.com biology-blog.com Very porous to allow for CO2 entry from air forestry.about.com sbs.utexas.edu Problem: Substantial leaf porosity leads to substantial water evaporation, causing dehydration stress on the plant Plants evolved waterproof coating and adjustable pores (stomata) for CO2 entry When stomata close, CO2 levels drop and O2 levels rise Photorespiration occurs Carbon fixing enzyme combines O2 instead of CO2 with RuBP Photorespiration: O2 is used up as CO2 is generated No useful cellular energy made No glucose produced Photorespiration is unproductive and wasteful Hot, dry weather causes stomata to stay closed O2 levels rise as CO2 levels fall inside leaf Photorespiration very common under such conditions Plants may die from lack of glucose synthesis weedtwister.com “C4 plants” have chloroplasts in bundle sheath cells and mesophyll cells Bundle sheath cells surround vascular bundles deep within mesophyll C3 plants lack bundle sheath cell chloroplasts C4 plants utilize the C4 pathway Two-stage carbon fixation pathway Takes CO2 to chloroplasts in bundle sheath cells C4 pathway uses up more energy than C3 pathway C3 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 C4 plants thrive when light is abundant but water is scarce (deserts and hot climates) Ex. : corn, sugarcane, sorghum, crabgrass, some thistles
