Plant Metabolism - Photosynthesis PDF

Chapter 10 Plant Metabolism FIFTEENTH EDITION James E. Bidlack, Shelly H. Jansky © 2021 McGraw Hill. All rights reserved. Authorized only for instructor use in the classroom. No reproduction...

Chapter 10 Plant Metabolism FIFTEENTH EDITION James E. Bidlack, Shelly H. Jansky © 2021 McGraw Hill. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without Outline Introduction to Plant Metabolism Enzymes and Energy Transfer Oxidation-Reduction Reactions Photosynthesis The Essence of Photosynthesis Introduction to the Major Steps of Photosynthesis A Closer Look at Photosynthesis Other Significant Processes That Occur in Chloroplasts © Rob A. Johnston/Walkabout Wolf Photography /Getty Images Outline for Respiration, Additional Metabolic Pathways, and Assimilation and Digestion Respiration The Essence of Respiration Introduction to the Major Steps of Respiration Factors Affecting the Rate of Respiration A Closer Look at Respiration Additional Metabolic Pathways Assimilation and Digestion Introduction Photosynthesis - Converts light energy to a usable form Respiration - Releases stored energy Facilitates growth, development and reproduction Metabolism - Sum of all interrelated biochemical processes in living organisms Animals rely on green plants for oxygen, food, shelter and other products. Access the text alternative for slide images. © Ingram Publishing Enzymes and Energy Transfer Enzymes regulate metabolic activities. Anabolism - Forming chemical bonds to build molecules Photosynthesis reactions - Store energy by constructing carbohydrates by combining carbon dioxide and water Catabolism - Breaking chemical bonds Cellular respiration reactions - Release energy held in chemical bonds by breaking down carbohydrates, producing carbon dioxide and water Photosynthesis-respiration cycle involves transfer of energy via oxidation- reduction reactions. Oxidation-Reduction Reactions Oxidation - Loss of electron(s) Reduction - Gain of electron(s) Oxidation of one compound usually coupled with reduction of another compound, catalyzed by same enzyme or enzyme complex. Hydrogen atom is lost during oxidation and gained during reduction. Oxygen is usually final acceptor of electron. Photosynthesis The essence of photosynthesis: Energy for most cellular activity uses adenosine triphosphate (ATP). Plants make ATP using light as an energy source. Takes place in chloroplasts and other green parts of the organisms 6CO2+12H2O + light → C6H12O6+6O2+6H2O Many intermediate steps to process, and glucose is not immediate first product. Carbon Dioxide Carbon dioxide reaches chloroplasts in mesophyll cells by diffusing through stomata into leaf interior. Use of fossil fuels, deforestation, and other human activities add more carbon dioxide to atmosphere than is removed. Has potential to cause global increases in temperature May enhance photosynthesis Water Less than 1% of all water absorbed by plants used in photosynthesis. Most of remainder transpired or incorporated into plant materials. Water is source of electrons in photosynthesis and oxygen is released as by- product. If water is in short supply or light intensities too high, stomata close and thus reduce supply of carbon dioxide available for photosynthesis. Light About 40% of radiant energy received on earth is in form of visible light. Violet to blue and red- orange to red wavelengths are used more extensively. Green light is reflected. Visible light passed through prism Access the text alternative for slide images. Optimal Rates and Limiting Factors Plants vary considerably in light intensities needed for optimal photosynthetic rates. Temperature and amount of carbon dioxide can also be limiting. Access the text alternative for slide images. Effects of Changing Light and Temperature If light and temperatures too high - Ratio of carbon dioxide to oxygen inside leaves may change. Accelerates photorespiration, which uses oxygen and releases carbon dioxide May help some plants survive under adverse conditions If light intensity too high - Photooxidation occurs, which results in destruction of chlorophyll. If water in short supply or light intensities too high, stomata close and thus reduce supply of carbon dioxide available for photosynthesis. Chlorophyll There are several types of chlorophyll molecules. Magnesium end captures light. Lipid tail anchors into thylakoid membrane. Most plants contain chlorophyll a (blue-green color) and chlorophyll b (yellow-green color). Chlorophyll b transfers energy from light to chlorophyll a. Makes it possible for photosynthesis to occur over broader spectrum of light Chlorophyll a molecule Access the text alternative for slide images. Photosynthetic Pigments Other photosynthetic pigments include carotenoids (yellow and orange), phycobilins (blue or red, in cyanobacteria and red algae), and several other types of chlorophyll. About 250-400 pigment molecules grouped in light-harvesting complex = photosynthetic unit. Two types of photosynthetic units work together in light-dependent reactions. Two phases of photosynthesis: Light-dependent reactions Light-independent reactions Introduction to the Major Steps of Photosynthesis Light-dependent reactions: In thylakoid membranes of chloroplasts Water molecules split apart, releasing electrons and hydrogen ions; oxygen gas released. Electrons pass along electron transport system. ATP produced. NADP is reduced, forming NADPH (used in light-independent reactions). Light-Independent Reactions In stroma of chloroplasts Utilize ATP and NADPH to form sugars Calvin cycle Carbon dioxide combines with RuBP (ribulose bisphosphate) and then combined molecules are converted to sugars (glucose). Energy furnished from ATP and NADPH produced during light-dependent reactions. A Simplified Summary of Photosynthetic Reactions Access the text alternative for slide images. A Closer Look at Photosynthesis 1772: Joseph Priestley noted that photosynthesis “restored” oxygen 1779: Jan Ingen-Housz showed that air is only restored when the green parts of plants received sunlight. 1782: Jean Senebier discovered that photosynthesis requires CO2 1796: Ingen-Housz showed that carbon is a plant nutrient. 1804: Theodore de Saussure showed that water is required. Light-Dependent Reactions Reexamined Visible white light can be divided into different colors using a prism. Each pigment has its own distinctive pattern of light absorption = pigment’s absorption spectrum. Different colors = different wavelengths of light. Shorter wavelengths carry greater amounts of energy. Access the text alternative for slide images. Absorption of Light by Chlorophyll Chlorophylls absorb light in the violet to blue and red wavelengths. T.W. Engelmann demonstrated this in 1882 using Spirogyra. Access the text alternative for slide images. When Pigments Absorb Light When a pigment absorbs light, the energy levels of some of the pigment’s electrons are elevated. These electrons are said to be in an excited state. Energy from an excited electron is released when it drops back to its ground state. Fluorescence: energy is immediately released as light Phosphorescence: energy is emitted as light after a delay Energy may otherwise be converted to heat. In photosynthesis, energy is stored in chemical bonds. The Light-Dependent Reactions Reexamined Two types of photosynthetic units: photosystem I and photosystem II. Events of photosystem II come before those of photosystem I. 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 I and ll Photosystem I = chlorophyll a, small amount of chlorophyll b, carotenoid pigment, and P700 P700 = reaction-center molecule - Only one that actually can use light energy Remaining pigments = antenna pigments Gather and pass light energy to reaction center Iron-sulfur proteins - Primary electron acceptors, first to receive electrons from P700 Photosystem II = chlorophyll a, B-carotene, small amounts of chlorophyll b, and reaction-center molecule: P680 Pheophytin (Pheo) - Primary electron acceptor The Light-Dependent Reactions of Photosynthesis Access the text alternative for slide images. Photolysis Photolysis - Water-splitting, occurs in Photosystem II Light photons absorbed by P680, which boosts electrons to higher energy level. Electrons passed to acceptor molecule, pheophytin, then to PQ (plastoquinone), then along electron transport system to photosystem I. Electrons extracted from water replace electrons lost by P680. One molecule of oxygen, 4 protons and 4 electrons produced from two water molecules. Electron Flow and Photophosphorylation Electron transport system consists of cytochromes, other electron transfer molecules and plastocyanin. Photons move across thylakoid membrane by chemiosmosis. Phosphorylation - ATP is formed from ADP. Access the text alternative for slide images. Photosystem l Light absorbed by P700, which boosts electrons to higher energy level. Electrons passed to iron-sulfur acceptor molecule, Fd (ferredoxin), then to FAD (flavin adenine dinucleotide). NADP reduced to NADPH. Electrons removed from P700 replaced by electrons from photosystem II. Access the text alternative for slide images.  Chemiosmosis Net accumulation of protons in thylakoid lumen occurs from splitting of water molecules and electron transport. Proton gradient gives special proteins, ATPase, in thylakoid membrane potential to move protons form lumen to stroma. Movement of protons across membrane = source of energy for synthesis of ATP Access the text alternative for slide images. Calvin Cycle Six molecules of CO2 combine with six molecules of RuBP (ribulose 1,5- bisphosphate) with aid of rubisco. Eventually results in twelve 3-carbon molecules of 3PGA (3-phosphoglyceric acid). NADPH and ATP supply energy and electrons that reduce 3PGA to GA3P (glyceraldehyde 3-phosphate). Ten of the twelve GA3P molecules are restructured, using 6 ATP, into six 5-carbon RuBP molecules. Net gain of 2 GA3P, which can be converted to carbohydrates or used to make lipids and amino acids The Calvin Cycle Photo courtesy of Lawrence Berkeley Natlonal Laboratory Copyright Notice: © 2010 The Regents of the University of California Lawrence Berkeley National Laboratory Person/Principle Investigator Access the text alternative for slide images. Photorespiration Photorespiration - Competes with carbon-fixing role of photosynthesis Rubisco fixes oxygen instead of carbon dioxide. Allows C3 plants to survive under hot dry conditions Helps dissipate ATP and accumulated electrons, preventing photooxidative damage When stomata closed, oxygen accumulates and photorespiration more likely. Products are 2-carbon phosphoglycolic acid, which are processed in perioxisomes Forms CO2, and PGA that can reenter Calvin cycle. No ATP formed. The 4-Carbon Pathway Exhibited by sugarcane, corn, sorghum, and many other tropical grasses and arid region plants 4-Carbon pathway - Produces 4-carbon compound instead of 3-carbon PGA during initial steps of light-independent reactions Plants have Kranz anatomy. Mesophyll cells with smaller chloroplasts with well-developed grana Bundle sheath cells with large chloroplasts with numerous starch grains Access the text alternative for slide images. © Kingsley Stern Specifics of the 4-Carbon Pathway CO2 converted to organic acids in mesophyll cells. PEP (phosphoenolpyruvate) and CO2 combine, with aid of PEP carboxylase. oxaloacetic acid is produced instead of PGA CO2 concentration is high in bundle sheath, thus photorespiration minimized. CO2 is transported as organic acids to bundle sheath cells, is released and enters Calvin cycle. C4 Plants Besides Kranz anatomy, C4 plants also have these features: High concentrations of PEP carboxylase in the mesophyll cells PEP carboxylase converts CO2 to carbohydrate at lower CO2 concentrations than does rubisco. Optimum temperatures for C4 photosynthesis are much higher than those for C3 photosynthesis This allows C4 plants to thrive where C3 plants cannot. C4 Photosynthesis Pathway Access the text alternative for slide images. Cost of C4 Photosynthesis At low temperatures, C3 more efficient Costs 2 ATP for C4 photosynthesis Access the text alternative for slide images. CAM Photosynthesis Crassulacean Acid Metabolism Found in cacti, stonecrops, orchids, bromeliads, and other succulents Often do not have well-defined palisade mesophyll Chloroplasts resemble the mesophyll cell chloroplasts of C3 plants Organic acids accumulate at night (stomata open). Access the text alternative for slide images. CAM and C4 Plants CAM photosynthesis - Similar to C4 photosynthesis in that 4-carbon compounds produced during light- independent reactions, however: converted back to CO2 during day for use in Calvin cycle (stomata closed) Allows plants to function well under limited water supply, as well as high light intensity. Access the text alternative for slide images. Other Significant Processes that Occur in Chloroplast Reduction of sulfate to sulfide Sulfides used to make amino-acids Nitrates converted to ammonia Ammonia used to make amino-acids, for eg-glutamine which is stored in roots and specialized stems