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 The ability to capture sunlight energy and convert it to chemical energy. iStockphoto.com/hougaardmalan

6CO 2 carbon dioxide + 6H 2 O water + light energy  sunlight C 6 H 12 O 6 glucose (sugar) + 6O 2 oxygen

 Plants, algae, and some prokaryotes  Are autotrophs (“self- feeders”) uni-bielefeld.de

 Are interconnected  Water, CO 2, sugar, and O 2 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  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

 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

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.

 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

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.

 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 nanometers of wavelength

 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

 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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 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 1 st ) and PSI (comes 2 nd )  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+)

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 +

 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 light- independent rxns if ATP & NADPH available  Light-independent rxns called the Calvin- Benson Cycle or C 3 Cycle

 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

 “ 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

 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

 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

 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

 Photorespiration :  O 2 is used up as CO 2 is generated  No useful cellular energy made  No glucose produced  Photorespiration is unproductive and wasteful

 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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 “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

 C 4 plants utilize the C 4 pathway  Two-stage carbon fixation pathway  Takes CO 2 to chloroplasts in bundle sheath cells

 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