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NAME: Photosynthetic Organisms A. Photosynthesis Transforms B. Organic molecules built by photosynthesis provide both the and . C. Plants use the raw materials: and D. carry out photosynthesis E. and other pigments involved in absorption of solar energy reside within of chloroplasts Plants as Solar Energy Converters A. Solar Radiation - Only of solar radiation that hits the earth’s atmosphere reaches surface; . B. Photosynthetic Pigments - Pigments found in chlorophyll absorb various portions of visible light; absorption spectrum. 1. Two major photosynthetic pigments are and . 2. Both chlorophylls absorb wavelengths best. 3. Very little light is absorbed; ; this is why leaves appear . 4. are yellow-orange pigments which absorb light in regions. 5. When chlorophyll breaks down in fall, the pigments in leaves show through. C. Absorption and action spectrum - A amount of light that passes through a sample of pigments. measures the 1) As different wavelengths are passed through, some are . 2) Graph of percent of light absorbed at each wavelength is . 3) Photosynthesis produces oxygen; . 4) Oxygen production and, therefore, photosynthetic activity is measured for plants under each specific wavelength; plotted on a graph, this produces an . 5) Since the action spectrum resembles , this indicates that chlorophylls contribute to photosynthesis. D. Photosynthetic Reaction 1. In 1930 C. B. van Niel showed that and not from CO2. 2. The net equation reads: given off by photosynthesis . E. Two Sets of Reactions in Photosynthesis 1. Light reactions cannot take place unless light is present. They are b. Chlorophyl within thylakoid membranes absorbs solar energy and . c. Energized electrons the electron transport system; energy is captures and used for . d. Energized electrons are also taken up by becoming . 2. Calvin Cycle Reactions a. These reactions take place in the ; can occur in either the b. These are synthesis reactions that use and to reduce . Light Reactions A. Two Pathways 1. Two electron pathways operate in the thylakoid membrane: the pathway and the pathway. 2. Both pathways produce but only the also produces . 3. ATP production during photosynthesis is sometimes called ; therefore these pathways are also known as cyclic and noncyclic . B. Noncyclic Electron Pathway (*SPLITS WATER, PRODUCES NADPH & ATP) 1. This pathway occurs in the thylakoid membranes and requires participation of two light-gathering units: photosystem I (PS I) and photosystem II (PS II). 2. A photosystem is a photosynthetic unit comprised of a pigment complex and electron acceptor; solar energy is absorbed and high-energy electrons are generated. 3. Each photosystem has a pigment complex composed of green chlorophyll a and chlorophyll b molecules and orange and yellow accessory pigments (e.g., carotenoid pigments). 4. Absorbed energy is passed from one pigment molecule to another until concentrated in reaction-center chlorophyll a. 5. Electrons in reaction-center chlorophyll a become excited; they escape to electron-acceptor molecule. 6. The noncyclic pathway begins with PSII; electrons move from H2O through PS II to PS I and then on to NADP+. 7. The PS II pigment complex absorbs solar energy; high-energy electrons (e) leave the reaction-center chlorophyll a molecule. 8. PS II takes replacement electrons from H2O, which splits, releasing O2 and H+ ions: 9. Oxygen is released as oxygen gas (O2). 10. The H+ ions temporarily stay within the thylakoid space and contribute to a H+ ion gradient. 11. As H+ flow down electrochemical gradient through ATP synthase complexes, chemiosmosis occurs. 12. Low-energy electrons leaving the electron transport system enter PS I. 13. When the PS I pigment complex absorbs solar energy, high-energy electrons leave reaction-center chlorophyll a and are captured by an electron acceptor. 14. The electron acceptor passes them on to NADP+. 15. NADP+ takes on an H+ to become NADPH: NADP+ + 2 e- + H+ NADPH. 16. NADPH and ATP produced by noncyclic flow electrons in thylakoid membrane are used by enzymes in stroma during light-independent reactions. C. Cyclic Electron Pathway 1. The cyclic electron pathway begins when the PS I antenna complex absorbs solar energy. 2. High-energy electrons leave PS I reaction-center chlorophyll a molecule. 3. Before they return, the electrons enter and travel down an electron transport system. a. Electrons pass from a higher to a lower energy level. b. Energy released is stored in form of a hydrogen (H+) gradient. c. When hydrogen ions flow down their electrochemical gradient through ATP synthase complexes, ATP production occurs. d. Because the electrons return to PSI rather than move on to NADP+, this is why it is called cyclic and also why no NADPH is produced. D. ATP Production ( ) 1. The thylakoid space acts as a reservoir for H+ ions; each time is split, two remain. 2. Electrons move carrier-to-carrier, giving up energy used to pump H+ from the into the 3. Flow of H+ from high to low concentration across thylakoid membrane provides energy to produce from by using an ATP synthase enzyme ons The Calvin Cycle Reactions (also called the Light Independent or Dark Reactions) A. Overview 1. The Calvin Cycle is a series of reactions producing . 2. The cycle is named for Melvin Calvin who used a radioactive isotope of carbon to trace the reactions. 3. The Calvin Cycle includes: carbon dioxide , carbon dioxide and of RuBP. B. Fixation of Carbon Dioxide 1. CO2 fixation is the to an organic compound called . 2. RuBP (ribulose bisphosphate) is a that combines with carbon dioxide. 3. The enzyme RuBP carboxylase ( ) speeds this reaction; this enzyme comprises of the protein content of chloroplasts, probably since it is a slow enzyme. C. Reduction of Carbon Dioxide 1. With reduction of carbon dioxide, a (3-phosphoglycerate[C3]) molecule forms. 2. Each of two PGA molecules undergoes reduction to in two steps. 3. Light-dependent reactions provide (electrons) and (energy) to reduce to . D. Regeneration of RuBP 1. Every turns of Calvin cycle, molecules of PGAL are used to re-form three molecules of . 2. Every three turns of Calvin cycle, there is net gain of E. The Importance of the Calvin Cycle 1. , the product of the Calvin Cycle can be converted into all sorts of other molecules. 2. is one result of PGAL metabolism; it is a common energy molecule. 3. Glucose phosphate is combined with fructose to form sucrose used by plants. 4. Glucose phosphate is the starting pint for synthesis of starch and cellulose. 5. The hydrocarbon skeleton of PGAL is used to form fatty acids and glycerol; the addition of nitrogen forms various amino acids. Other Pathways of Photosynthesis Environmental factors that affect photosynthesis are: : color of light - only red and blue light is really effective in the light reactions because the photosynthetic pigments tend to maximally absorb these colors of light. They reflect more green, orange and yellow wavelengths - the amount of light in terms of brightness plants in full sun will usually photosynthesize more than plants in the shade. – amount of daylight Water availability 1. The is the MOST Common Pathway for Carbon Fixation. Plant Species that fix Carbon through the Calvin Cycle are known as . 2. Other Plant Species Fix Carbon through alternative pathways and then release it to enter the Calvin Cycle. These alternative pathways are generally found in plants that evolved in Climates. 4. Under such conditions, plants can rapidly lose water to the air. Most of the water loss from plants occurs through small pores on the undersurface of the leaves called STOMATA. Plants obtain carbon dioxide for photosynthesis from the air. Plants must balance their need to open their Stomata to receive carbon dioxide and release oxygen with their need to close their Stomata to prevent water loss. A stoma is bordered by TWO Kidney Shaped GUARD CELLS, Guard Cells are modified cells that Regulate Gas and Water Exchange. 5. When a plant's Stomata are partly CLOSED, the level of CO2 FALLS (Used in Calvin Cycle), and the Level of O2 RISES (as Light reactions Split Water Molecules). 7. A LOW CO2 and HIGH O2 Level inhibits Carbon Fixing by the Calvin Cycle. Plants with alternative pathways of carbon fixing have evolved ways to deal with this problem. 8. C4 PLANTS - Allows certain plants to fix CO2 into FOUR-Carbon Compounds. During the Hottest part of the day, C4 plants have their Stomata Partially Closed. C4 plants include corn, sugar cane and crabgrass. Such plants lose only about half as much water as C3 plants when producing the same amount of Carbohydrate. 9. - Cactus, pineapples have different adaptations to Hot, Dry Climates. They Fix Carbon through a pathway called CAM. Plants that use the CAM Pathway their Stomata at and during the , the opposite of what other plants do. Because CAM Plants have their Stomata open at night, they grow very Slowly, But they lose LESS Water than C3 or C4 Plants.