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Unformatted text preview: Chapter 10 PHOTOSYNTHESIS Chapter From sunlight to Snickers Summary *Overview of photosynthesis *What is light? *Chloroplasts *Pigments *Capturing light energy *Light dependent reactions *Light independent reactions *Photorespiration *C4 pathway Why is Photosynthesis Important? *Autotrophs vs. heterotrophs *What do autotrophs provide heterotrophs with? *What do heterotrophs provide autotrophs with? Photosynthesis *Overall reaction: 6CO2 + 12 H2O + llight energy → ight Overall C6H12O6 + 6O2 + 6H2O *2 sets of reactions, fig 10.1 *Photo = light dependent reactions Photo light *Synthesis = light independent reactions Synthesis light *What is light? *Electromagnetic energy – many different energy levels = spectrum. *Visible light is a small part of the electromagnetic spectrum, fig 10.4. *Visible light appears white but contains many different wavelengths (energy levels) of light that we interpret as different colors. levels) *If an object absorbs all of the wavelengths (or colors) of light it appears black, if it reflects all of the colors it appears white. black, *If an object appears green, what color is its reflecting? Absorbing? *If an object appears orange, what color is it reflecting? Absorbing? *Chloroplasts – a review, fig 10.2. *Double membrane *Inside *Stroma – Stroma *Grana – Grana *Thylakoids – Thylakoids *Pigments in chloroplasts *Chlorophyll a and b; absorb blue and red light, reflect greens Chlorophyll absorb *Carotinoids: β carotene, xanthophylls, etc.; absorb blue and green light, 1 reflect yellow/orange/red. reflect *Chlorophyll b and the carotenoids are accessory pigments. Carotenoids also protect the plant against UV (sunburn). protect *How do pigments capture light energy? *Photons of light hit a pigment. *Electrons in pigments absorb the energy and move to a higher energy level (electron shell), fig 10.9. (electron *In a test tube the electrons eventually return to ground level and energy is released as heat and electromagnetic energy = fluorescence. fluorescence *How do pigments in plants transform light energy to ATP and NADPH? *Photosystems organize pigments, fig 10.11 *Antenna complex – accessory pigments absorb light energy, an electron is excited and transfers the energy to another pigment and another, etc. energy *Reaction center – a chlorophyll a molecule is the last to receive the energy and donates an energized electron to an electron acceptor. electron electron Light Dependent Reactions *Purple non-sulfur and purple sulfur bacteria have only 1 photosystem and produce ATP but no NADPH = cyclic photophosphorylation. cyclic *Plants, green algae and cyanobacteria have 2 photosystems and produce ATP and NADPH = non-cyclic photophosphrylation. non-cyclic *Photosystem II, fig 10.13 *Chlorophyll a in the reaction center receives energy from accessory Chlorophyll pigments and donates an “energized” electron to pheophytin (PP) pheophytin *Pheophytin transfers the electron to plastoquinone (PQ) Pheophytin plastoquinone *H2O is split and H+ donates an electron to chlorophyll a to replace the one lost. lost. *Plastoquinone delivers electron to the electron transport chain (cytochrome complex). complex). *Electron transport chain “pumps” H+ into the thylakoid. into *ATP synthase allows H+ to exit and uses the energy in the H+ to produce ATP = photophosphorylation. photophosphorylation *Photosystem I, fig 10.14 *Electron from photosystem II is transported by plastocyanin (PC) to Electron plastocyanin chlorophyll a in photosystem I. *Electron is reenergized by photons of light. *Energized electron is transferred to ferredoxin that donates it and a H+ to Energized ferredoxin NADP – NADPH is reducing power. reducing *Products *ATP *NADPH *O2 2 Light Independent Reactions – Calvin Cycle Light *Calvin Cycle – “reduction” of CO2 to produce sugars. *Takes place in stroma Takes stroma *3 Steps, fig 10.19 *Fixation: 3 CO2 combine with 3 ribulose bisphosphate (RuBP, 5 C) in Calvin CO Cycle to form 3 6-C molecules. * These molecules immediately split to form 6 3-C molecules. Enzyme used is rubisco. rubisco *Reduction: ATP and NADPH from light dependent reactions used to form 6 ATP G3P (glyceraldehyde 3 phosphate). G3P *Regeneration: 1 G3P exits the cycle and remaining 5 G3P are used to G3P regenerate the 3 RuBP used in the beginning of the cycle. regenerate *G3P used to produce glucose and fructose. See glycolysis! How many G3P are needed to produce 1 glucose or fructose molecule? are *Glucose + fructose = sucrose, a transport sugar, that is delivered to other parts of the plant. of *When glucose production is high, glucose is used to produce a storage carbohydrate, starch (stored in chloroplasts). carbohydrate, Photorespiration *The enzyme rubisco catalyzes CO2 fixation but also catalyzes oxidation of The rubisco RuBP, called photorespiration. photorespiration *When CO2 levels are high CO2 fixation dominates. *When O2 levels are high, oxidation dominates. *To live, plants must be able to fix CO2. *How do plants take in CO2? *Pores, stomata, on underside of leaves open and allow CO2 to enter, fig Pores, stomata 10.21. Opening and closing of stomata are controlled by guard cells. guard *When stomata are open, H2O iis lost from the leaves. s lost *During hot dry weather stomata close to prevent H2O loss and CO2 intake declines. declines. *As CO2 levels decline, photorespiration predominates and sugars are not produced. produced. *Plants that thrive in hot dry conditions have adapted – C4 pathway. NOTE: the Calvin cycle is a C3 pathway because it produces a 3-C molecule. A C4 Calvin pathway involves a 4-C molecule. pathway C4 Pathway *The C4 pathway is used to store CO2 when the stomata are open and donate CO2 to the C3 pathway when the stomata are closed. CO *2 Methods, fig 10.24 *C4 plants have the C4 pathway and C3 pathway in different cells. *CAM plants have the C4 and C3 pathway in the same cell. *C4 plants open stomata for short periods and store CO2 in C4 pathway. 3 *CAM plants open stomata at night and store CO2 in C4 pathway. CO2 is donated to C3 pathway during the day. donated 4 ...
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