photosynthesis--carbon fixation.docx

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Integrative Biology
Course Code
IB 103

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Energy Part 1, Lecture 3: Photosynthesis: Carbon Fixation Pgs 32-34, 76-80, 558-561 Background Information (pages 32-34) Sugars, starches, and cellulose are examples of carbohydrates - Carbs=organic compounds like sugars, starch and cellulose - Plants use carbs as fuel molecules, for nucleic acids,and structure of cells - Carbon, hydrogen, and oxygen atoms in ratio 1C:2H;1O - 3 kinds of carbs 1. Monosaccharides (simple sugars) - 3-6 carbons - Ex: glucose; fructose are fuel molecules that cells break down to perform activities 2. Disaccharides (2 monosaccharides) - Ex: sucrose (C12H12O11) (table sugar) - Transported in plant’s vascular system - Condensation rxn= 2 molecules joined and a molecule of water is removed (formation of sucrose from glucose and fructose) 3. Polysaccharides—carbs with many sugar units - Long chain of monosacchharides - Ex: starches—principal storage compound in plants; thousands of glucose units in a chain, which are the main storage of carbs in plants - Plants build energy by storing starch - Ex: cellulose—major part of cell wall; most abundant carb in world; part of plant fiber; aids in bowel function and adds bulk to the materials being digested Carbon Fixation(pages 76-80) Most plants use the Calvin cycle to fix carbon - Occur in stroma - 3 phases: 1. CO2 uptake - Rubisco(enzyme) combines co2 annd RuBP, splitting the RuBP into 3-carbon molecules (2 of them ) called phosphoglycerate (PGA) - Calvin cycle also called C3 pathway 2. Carbon reduction - PGA molecules converted to G3P (glyceraldehyde-3-phosphate) using NADPH and ATP - Ever 6 turns of calvin cycle, 2 of the 12 G3P molecules leave cycle to be used in making carbs - G3P molecules joined in pairs to make glucose or fructose; sometimes glucose then combines to form sucrose;then sugarcane is made; glucose used to make cellulose and starch 3. RuBP regeneration (ribulose biphosphate005 carbon sugar activated by addition of a phosphate group) - 10 G3Ps remain;30 carbon atoms in all - Carbons rearranged into 6 5-carbon molecules in a compound called ribulose phosphate (RP) - ATP adds a second phosphate, and this is converted to RuBP—where cycle began Summary: inputs for carbon fixation= CO2, NADPH, ATP End: carb molecules Photorespiration reduces the efficiency of the C3 pathway pg 78 - To conserve water, some plants close stomata - Does not produce biologically useful energy; reduces photosynthetic efficiency because CO2 competes with O2 for the rxn with RuBP
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