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Transcript
Plant Metabolism Chapter 10 Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display Outline • • Photosynthesis Major Steps of Photosynthesis Light-Dependent Reactions Light-Independent Reactions C4 Photosynthesis CAM Photosynthesis Respiration Glycolysis Electron Transport Chain Enzymes and Energy Transfer • • Enzymes regulate most metabolic activities. Anabolism - Storing Energy. - Photosynthesis reactions Catabolism - Consuming Stored Energy. - Respiration reactions Oxidation-Reduction Reactions Oxidation - Loss of electron(s). Reduction - Gain of electron(s) - Usually coupled Photosynthesis • Energy for most cellular activity involves adenosine triphosphate (ATP). Plants make ATP using light as an energy source. - Take place in cholorpolasts and other green parts of the organisms. 6CO2+12H2O + light C6H12O6+6O2+6H2O Carbon Dioxide • Carbon dioxide (0.035% of air) reaches cholorplasts in the mesophyll cells by diffusing through the stomata into the leaf interior. If not replenished, CO2 would be used up in 22 years. Use of fossil fuels, deforestation, and other human activities have added excess carbon dioxide to the atmosphere. - May enhance photosynthesis. Plants may counter-balance by developing fewer stomata. Water • • Less than 1% of all the water absorbed by plants is used in photosynthesis. Most of the remainder is transpired or incorporated into plant materials. If water is in short supply, stomata usually close and thus reduce the supply of carbon dioxide available for photosynthesis. Light • About 40% of the radiant energy received on earth is in the form of visible light. Leaves commonly absorb about 80% of the visible light reaching them. - Light intensity varies with time of day, season, altitude, latitude, and atmospheric composition. Considerable variation in the light intensities necessary for optimal photosynthetic rates. Light Wavelengths Effects of Light and Temperature on Photosynthesis Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display Chlorophyll • Several different types of chlorophyll. Most plants contain both chlorophyll a (blue-green) and chlorophyll b (yellowgreen). - Other pigments include carotenoids (yellow and orange) phycobilins (blue or red), and several other types of chlorophyll. - About 250-400 pigment molecules group as a photosynthetic unit. Grass green in color: C55H72MgN4O5 Chlorophyll b: bluish green in color : C55H70MgN4O6 3a:1b Major Steps of Photosynthesis • Light Dependent Reactions Water molecules split apart. Electrons passed along electron transport. ATP produced. NADPH produced. Major Steps of Photosynthesis • Light Independent Reactions Calvin Cycle - Carbon dioxide combined with RuBP and then combined molecules are converted to sugars (Glucose). Energy furnished by ATP and NADPH from Light-Dependent Reactions. Light Dependent Reactions - In Depth • Each pigment has its own distinctive pattern of light absorption. When pigment absorbs light : electrons are excited. When this occurs, energy may be immediately emmitted as light :flourescence or emmitted after a delay : phosphorescence or may be converted to heat. Light Dependent Reactions - In Depth • Two types of photosynthetic units present in most chloroplasts make up photosystems. Photosystems I and II - Both can produce ATP. - Only organisms with both photosystem I and photosystem II can produce NADPH and oxygen as a consequence of electron flow. Photosystems Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display Mitchell Theory or Chemiosmosis Light Independent Reactions - In Depth • Calvin Cycle Six molecules of CO2 combine with six molecules of RuBP with the aid of rubisco. Resulting complexes split into twelve 3PGA molecules. NADPH and ATP supply energy and electrons that reduce 3PGA to 12 GA3P. Ten of the twelve GA3P molecules are restructured into six RuBP molecules. The Calvin Cycle Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display Photorespiration • Stomata usually close on hot, dry days. Closed stomata prevent carbon dioxide from entering the leaf. Then CO2 decrease as relative O2 increase. - When carbon dioxide levels drop below about 50 parts per million, photorespiration is initiated. Rubisco fixes oxygen instead of carbon dioxide. Light Independent Reactions - In Depth • 4-Carbon Pathway Sugar cane, corn, sorghum and at least 1000 species of plants have Kranz Anatomy. Two forms of chloroplasts: - Large chloroplast with few to no grana in the bundle sheath cells surrounding the veins. - Smaller chloroplasts with well-developed grana in the mesophyll cells. Corn (Zea Mays) Cross-Section Fig. 10.2a 4-Carbon Pathway • Plants with Kranz Anatomy produce oxaloacetic acid (4carbon compound). Phosphoenolpyruvat e (PEP) and carbon dioxide combined in mesophyll cells with the aid of PEP carboxylase. - Provides a major reduction in photorespiration. CAM Photosynthesis • • Seen in plants of about 30 families including cacti orchids etc. Similar to C4 photosynthesis in that 4-carbon compounds are produced during the lightindependent reactions. However, in CAM, the organic acids accumulate at night and break down during the day, releasing carbon dioxide. - Allows plants to function well under limited water supplies, as well as high light intensity. CAM Photosynthesis Fig. 10.22 Comparision C3 C4 CAM Leaf Anatomy No bundle sheath Krantz Anatomy No palliside cells, large vauoles Carboxylation Enzymes RuBisco PEP +RuBisco Drakness PEP +Light RuBisco Transpiration ratio 450-950 (gm H2O/ gm dry weight increase) 250-350 50-55 Optimum P.S. Temperature 15-25 oC 30-40 oC 35 oC CO2 Compensation point 30-70 ppm 0-10 ppm 0-5 ppm in dark Dry matter prod. 22 tons/ha/year 39 tons/ha/year Low; highly variable C4 vs. CAM • C4 Stores CO2 in 4 carbon acid Separates CO2 fixation from sugar synthesis in SPACE(Bundle sheath vs mesophyll space) Adaptations to TROPICAL environments • CAM Stores CO2 in 4 carbon acid Separates CO2 fixation from sugar synthesis in TIME Adaptations to ARID environments Other Processes in Chloroplasts 1) Sulphates : reduced to sulphides these then used for biosynthesis of S-containing amino acids • • 2) Nitrites converted to ammonia. This is then converted to amino groups which are integral part of several important amino acids such as glutamine and aspartic acid. Respiration • Respiration is essentially the release of energy from glucose molecules that are broken down to individual carbon dioxide molecules. C6H12O6 + 6O2 6CO2 + 6H2O + energy Respiration • Fermentation C6H12O6 2C2H5OH + 2CO2 + ATP C6H12O6 2C3H6O3 + ATP Factors Affecting the Rate of Respiration • • • Temperature Water Oxygen Major Steps of Respiration • Glycolysis Glucose molecule becomes a fructose molecule carrying two phosphates. Fructose molecule is split into two GA3P molecules. Some hydrogen, energy, and water are removed, leaving pyruvic acid. Major Steps of Respiration • Aerobic Respiration Citric Acid (Krebs) Cycle O.A. + acetyl CoA + ADP+P+3NAD + FAD O.A. + CoA+ATP+3NADH+H+ + FADH2+2CO2 • Electron Transport Oxidative Phosphorylation Chemiosmosis Assimilation and Digestion • • Assimilation is the process of using organic matter produced through photosynthesis to build protoplasm and cell walls. Digestion is the conversion of starch and other insoluble carbohydrates to soluble forms. Nearly always hydrolysis. Endosperm starch grains of rice Starch Soluble forms DIGESTION Conversion of PSic products to structural forms such as protoplasm or cell wall is called ASSIMILATION Ps •Stores energy in sugar •Uses carbon dioxide and water •Increases weight •Occurs only in light •Occurs only in cells containing chlorphyll •Produces oxygen in green organisms •Produces ATP with light energy Rs • Releases energy from sugar molecules • Releases carbon dioxide and water • Decreases weight • Occurs in either light or darkness • Occurs in all living cells • Utilizes oxygen (aerobic respiration) • Produces ATP with energy released from sugar Review • • Photosynthesis Major Steps of Photosynthesis Light-Dependent Reactions Light-Independent Reactions C4 Photosynthesis CAM Photosynthesis Respiration Glycolysis Electron Transport Chain Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display