Chapter 8 Plant Metabolism Lecture Outline Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Outline Introduction Enzymes and Energy Transfer Photosynthesis 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.

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.

Enzymes and Energy Transfer 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 Energy for most cellular activity involves 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.

Photosynthesis 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

Photosynthesis 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.

Visible light passed through prism Photosynthesis 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 Leaves commonly absorb about 80% of visible light reaching them. Light intensity varies with time of day, season, altitude, latitude, and atmospheric composition.

Photosynthesis Plants vary considerably in light intensities needed for optimal photosynthetic rates. Temperature and amount of carbon dioxide can also be limiting.

Photosynthesis 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 a molecule Photosynthesis 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

Photosynthesis 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

Photosynthesis 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).

Photosynthesis Major Steps of Photosynthesis 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.

Photosynthesis A Closer Look: Light-Dependent Reactions Each pigment has its own distinctive pattern of light absorption = pigment’s absorption spectrum. When pigments absorb light, energy levels of electrons are raised. Energy from an excited electron is released when it drops back to its ground state. In photosynthesis, energy is stored in chemical bonds.

Photosynthesis A Closer Look: Light-Dependent Reactions 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.

Photosynthesis A Closer Look: Light-Dependent Reactions 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

Photosynthesis A Closer Look: Light-Dependent Reactions

Photosynthesis A Closer Look: Light-Dependent Reactions Photolysis - Water-splitting, 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.

Photosynthesis A Closer Look: Light-Dependent Reactions 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.

Photosynthesis A Closer Look: Light-Dependent Reactions Photosystem I 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.

Photosynthesis A Closer Look: Light-Dependent Reactions 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

Photosynthesis A Closer Look: Light-Independent Reactions 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

Photosynthesis A Closer Look: Light-Independent Reactions 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.

Photosynthesis A Closer Look: Light-Independent Reactions 4-Carbon pathway - Produces 4-carbon compound instead of 3-carbon PGA during initial steps of light-independent reactions C4 plants - Tropical grasses and plants of arid regions Plants have Kranz anatomy. Mesophyll cells with smaller chloroplasts with well- developed grana Bundle sheath cells with large chloroplasts with numerous starch grains

Photosynthesis A Closer Look: Light-Independent Reactions 4-Carbon pathway CO2 converted to organic acids in mesophyll cells. PEP (phosphoenolpyruvate) and CO2 combine, with aid of PEP carboxylase. Form 4-carbon, oxaloacetic acid, instead of PGA PEP carboxylase converts CO2 to carbohydrate at lower CO2 concentrations than does rubisco. Not sensitive to O2, no photorespiration

Photosynthesis A Closer Look: Light-Independent Reactions 4-Carbon pathway CO2 is transported as organic acids to bundle sheath cells, is released and enters Calvin cycle. CO2 concentration high in bundle sheath, thus photorespiration minimized. C4 plants photosynthesize at higher temperatures than C3 plants. At low temperatures, C3 more efficient . Costs 2 ATP for C4 photosynthesis.

Photosynthesis A Closer Look: Light-Independent Reactions CAM photosynthesis - Similar to C4 photosynthesis in that 4-carbon compounds produced during light- independent reactions, however: Organic acids accumulate at night (stomata open). 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.

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

Respiration Respiration is release of energy from glucose molecules that are broken down to individual carbon dioxide molecules. Initiated in cytoplasm and completed in mitochondria Aerobic respiration cannot be completed without oxygen. C6H12O6 + 6O2 6CO2 + 6H2O + energy

Respiration Anaerobic respiration and fermentation - carried on in absence of O2 Release less energy than that released during aerobic respiration Fermentation equations: C6H12O6 2C2H5OH + 2CO2 + ATP C6H12O6 2C3H6O3 + ATP

Respiration Major Steps of Respiration Glycolysis - First phase In cytoplasm No O2 required. Glucose converted to GA3P (glyceraldehyde 3-phosphate). 2 ATP molecules gained. Citric acid (Krebs) cycle - Second stage In fluid matrix of cristae in mitochondria High energy electrons and hydrogen removed as cycle proceeds. NADH, FADH2 , and small amount of ATP produced. CO2 produced as by-product.

Respiration Major Steps of Respiration Electron transport - Third stage In inner membrane of mitochondria NADH and FADH2 donate electrons to electron transport system. Produces ATP, CO2 and water

Respiration A Closer Look Glycolysis Steps: Phosphorylation - Glucose becomes fructose carrying two phosphates. Sugar cleavage - Fructose split into two 3-carbon fragments: GA3P (glyceraldehyde 3-phosphate). Pyruvic acid formation - Hydrogen, energy and water removed, leaving pyruvic acid. Prior to entering citric acid cycle, pyruvic acid loses CO2 and is converted to acetyl CoA. If O2 not available, anaerobic respiration or fermentation occurs. Hydrogen released during glycolysis transferred back to pyruvic acid, creating ethyl alcohol or lactic acid.

Respiration A Closer Look Citric acid (Krebs) cycle Acetyl CoA first combined with oxaloacetic acid, producing citric acid. Each cycle uses 2 acetyl CoA, releases 3 CO2 and regenerates oxaloacetic acid. O.A. + acetyl CoA + ADP+P+3NAD + FAD  O.A. + CoA+ATP+3NADH+H+ + FADH2+2CO2 High energy electrons and hydrogen removed, producing NADH, FADH2 and ATP.

Respiration A Closer Look Electron transport and oxidative phosphorylation Energy from NADH and FADH2 released as hydrogen and electrons are passed along electron transport system. Protons build up outside mitochondrial matrix, establishing electrochemical gradient. Chemiosmosis couples transport of protons into matrix with oxidative phosphorylation: formation of ATP. O2 acts as ultimate electron acceptor, producing water as it combines with hydrogen. Produces a net gain of 36 ATP and 6 molecules of CO2 and water

Respiration

Factors Affecting the Rate of Respiration Temperature Increase from 20o C to 30o C, respiration rates double. Water Medium in which enzymatic reactions take place Low water content - Respiration rate reduced. Oxygen Reduction in oxygen - Respiration and growth rates decline.

Additional Metabolic Pathways Other processes contribute to growth development, reproduction and survival. Compounds produced include: sugar phosphates nucleotides, nucleic acids, amino acids, proteins, chlorophylls, cytochromes, carotenoids, fatty acids, oils, and waxes. Secondary metabolism - Metabolic processes not required for normal growth and development Enable plants to survive and persist under special conditions Colors, aromas, poisons - Give competitive edge Codeine, Nicotine, Lignin, Salicin, Camphor, Menthol, Rubber

Assimilation and Digestion Assimilation - Conversion of organic matter produced in photosynthesis to build protoplasm and cell walls Sugars transformed into lipids, proteins, or other carbohydrates, such as sucrose, starch and cellulose. Digestion - Conversion of starch and other insoluble carbohydrates to soluble forms Nearly always hydrolysis process

Review Introduction Enzymes and Energy Transfer Photosynthesis Respiration Additional Metabolic Pathways Assimilation and Digestion