Transcript C 4 plants
Photosynthesis Mader Ch. 8 Photosynthetic Organisms • Algae • Plants • Cyanobacteria Autotrophs EK 2A2e: Photosynthesis first evolved in prokaryotic organisms - these were responsible for an oxygen atmosphere - these pathways were the foundation for Please Note • Chemosynthetic organisms are also autotrophs but capture free energy from inorganic molecules in the environment – this does not require O2 Energy Flow – 1st Law of Thermodynamics • Photosynthetic Organisms capture free energy: – Absorb light energy – Transform light energy into stored chemical energy (ie. carbohydrate bonds, etc.) • All living organisms require stored chemical energy to live! • Heterotrophic Organisms capture free energy: – Can’t convert light energy – Consume carbohydrates as food – Can metabolize carbohydrates, lipids and prteins by hydrolysis as sources of free energy. • Both convert stored energy to ATP Chloroplasts • Specialized organelles that compartmentalize the process of photosynthesis. – gather the sun's free energy with light-absorbing molecules called pigments. – main pigment in plants is chlorophyll. • There are two main types of chlorophyll: – chlorophyll a – chlorophyll b Two main types of Chlorophyl • Chlorophyl a – absorbs light mostly in the blue-violet and red regions • Chlorophyl b – absorbs light in the blue and red regions • Why do leaves look green? Clorophyl a Chlorophyl b Carotene Light and Pigments • Light is a form of energy – any compound that absorbs light = absorbs energy • When chlorophyll absorbs light, much of the free energy is transferred directly to electrons in the chlorophyll molecule, raising the energy levels of these electrons. • These high-energy electrons are what make photosynthesis work. Copyright Pearson Prentice Hall Chloroplasts • Structure –Double outer membrane – compartmentalizes reactions/processes –Membrane-bound thylakoids • Organized in stacks – grana –Stroma – semi-fluid interior of the chloroplast Leaves and Photosynthesis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. cuticle upper epidermis Leaf cross section mesophyll lower epidermis CO2 O2 leaf vein outer membrane stoma inner membrane stroma stroma granum Chloroplast 37,000 thylakoid space thylakoid membrane Grana independent thylakoid in a granum overlapping thylakoid in a granum © Dr. George Chapman/Visuals Unlimited 9 Photosynthesis • The raw materials for photosynthesis are carbon dioxide and water Roots absorb water that moves up vascular tissue Carbon dioxide enters a leaf through small openings called stomata and diffuses into chloroplasts in mesophyll cells Overview of Photosynthesis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. thylakoid membrane 11 Overview of Photosynthesis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. solar energy thylakoid membrane 12 Overview of Photosynthesis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H2O solar energy ADP + NADP+ Light reactions thylakoid membrane 13 Overview of Photosynthesis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H2O solar energy ADP + NADP+ Light reactions NADPH ATP thylakoid membrane O2 14 Overview of Photosynthesis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CO2 H2O solar energy ADP + P NADP+ Light reactions Calvin cycle reactions NADPH ATP stroma thylakoid membrane O2 CH2O 15 Photosynthesis Equation 6CO2 + 6H2O Light C6H12O6 + 6O2 carbon dioxide + water Light sugars + oxygen • Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into high-energy sugars and oxygen. Light-Dependent Reaction The light-dependent reaction requires light. The light-dependent reactions produce oxygen gas and convert ADP and NADP+ into the energy carriers ATP and NADPH. Copyright Pearson Prentice Hall Light-Dependent Reactions takes place in the thylakoid of the chloroplasts. - use proteins embedded in the membrane Copyright Pearson Prentice Hall Photosynthesis begins when pigments in photosystem II absorb light energy. The light energy is absorbed by electrons, increasing their energy level. Photosystem II Copyright Pearson Prentice Hall These high-energy electrons are passed on to the electron transport chain. Photosystem II High-energy electron Electron carriers Copyright Pearson Prentice Hall Enzymes on the thylakoid membrane break water molecules into: Photosystem II 2H2O Photosystem II High-energy electron Electron carriersPearson Prentice Hall Copyright – hydrogen ions – oxygen atoms – energized electrons Photosystem II + O2 2H2O High-energy electron Electron carriers Copyright Pearson Prentice Hall The energized electrons from water replace the highenergy electrons that chlorophyll lost to the electron transport chain. Photosystem II + O2 2H2O High-energy electron Copyright Pearson Prentice Hall As plants remove electrons from water, oxygen is left behind and is released into the air. Photosystem II + O2 2H2O High-energy electron Copyright Pearson Prentice Hall The hydrogen ions left behind are released into the inside of the thylakoid. Photosystem II + O2 2H2O High-energy electron Copyright Pearson Prentice Hall Energy from the electrons passing through membrane proteins is used to transport H+ ions from the stroma into the inner thylakoid space. – Active Transport Photosystem II + O2 2H2O Copyright Pearson Prentice Hall High-energy electrons move through the Electron Transport Chain from photosystem II to photosystem I. Photosystem II + O2 2H2O Photosystem II Photosystem I Copyright Pearson Prentice Hall When light energy hits Photosystem I, energized electrons leave the system and are accepted by electron acceptors. + O2 2H2O Photosystem I Copyright Pearson Prentice Hall NADP+ then picks up these high-energy electrons, along with H+ ions, and becomes NADPH. + O2 2H2O 2 NADP+ 2 2 Copyright Pearson Prentice Hall NADPH + O2 2H2O 2 NADP+ 2 2 Copyright Pearson Prentice Hall NADPH When electrons are passed through the Electron Transport Chain, an electrochemical gradient (or proton gradient) of H+ ions across the thylakoid membrane is established. + O2 2H2O 2 NADP+ 2 2 Copyright Pearson Prentice Hall NADPH Potential Energy: The difference in charges across the membrane provides the energy to make ATP + O2 2H2O 2 NADP+ 2 2 Copyright Pearson Prentice Hall NADPH H+ ions cannot cross the membrane directly. ATP synthase + O2 2H2O 2 NADP+ 2 2 Copyright Pearson Prentice Hall NADPH The cell membrane contains the enzyme ATP Synthase that allows H+ ions to pass through it ATP synthase + O2 2H2O 2 NADP+ 2 2 Copyright Pearson Prentice Hall NADPH The movement of H+ ions as they pass through ATP Synthase powers the building of ATP from ADP and Pi ATP synthase + O2 2H2O 2 NADP+ 2 2 Copyright Pearson Prentice Hall NADPH This is called chemiosmosis ATP synthase + O2 2H2O ADP 2 NADP+ 2 2 Copyright Pearson Prentice Hall NADPH Because of this system, light-dependent electron transport produces not only high-energy electrons but ATP – both are sent to the CALVIN CYCLE ATP synthase + O2 2H2O ADP 2 NADP+ 2 2 Copyright Pearson Prentice Hall NADPH Review • The thylakoid space acts as a reservoir for hydrogen ions (H+) • Each time water is oxidized, two H+ remain in the thylakoid space • Transfer of electrons in the electron transport chain yields energy – Used to pump H+ across the thylakoid membrane – Protons move from stroma into the thylakoid space • Flow of H+ back across the thylakoid membrane – Energizes ATP synthase, which – Enzymatically produces ATP from ADP + Pi • This method of producing ATP is called chemiosmosis • Photosystem I and II are embedded in the thylakoid membrane and are connected by the transfer of e-’s Reactants of The Light Dependent Reaction (ETC) • Water • ADP and P • NADP+ and H+ Products of the Light-Dependent Reaction (ETC) • The electron transport chain produces: – Oxygen from the breakdown of water – NADPH from NADP+ and H+ (+energy from electrons) – ATP from ADP • Through the ATP synthase protein as H+ pass from one side of the thylakoid membrane to the other • The NADPH and ATP are used in the Calvin Cycle Connections: • Be able to explain how internal membranes and organelles contribute to cell function! Plants as Carbon Dioxide Fixers • A cyclical series of reactions – CALVIN CYCLE • Utilizes atmospheric carbon dioxide to produce carbohydrates • Known as C3 photosynthesis • CO2 undergoes carbon dioxide fixation which produces carbohydrates. – These carbohydrates can be used to make fatty acids/glycerol for oils, glucose, fructose, starch, cellulose, amino acids • This cycle is powered by the ATP and NADPH from the Light Dependent Reaction – ADP and NADP+ form which are recycled back to the LDR to pick up energy and electrons. 42 The Calvin Cycle Reactions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H2O CO2 solar energy ADP + P NADP + Calvin cycle Light reactions NADPH ATP Metabolites of the Calvin Cycle stroma O2 CH2O 3CO2 intermediate 3 C6 3 RuBP C5 3 ADP + 3 ribulose-1,5-bisphosphate 3PG 3-phosphoglycerate BPG 1,3-bisphosphoglycerate G3P glyceraldehyde-3-phosphate 6 3PG C3 CO2 fixation 6 ATP CO2 reduction Calvin cycle P RuBP 6 ADP + 6 P These ATP and NADPH molecules were produced by the light reactions. regeneration of RuBP These ATP molecules were produced by the light reactions. 6 BPG C3 3 ATP 6 NADPH 5 G3P C3 6 G3P C3 6 NADP+ net gain of one G3P Other organic molecules 43 Glucose Fate of G3P Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. G3P fatty acid synthesis glucose phosphate amino acid synthesis + fructose phosphate Sucrose (in leaves, fruits, and seeds) Starch (in roots and seeds) Cellulose (in trunks, roots, and branches) 44 © Herman Eisenbeiss/Photo Researchers, Inc. Other Types of Photosynthesis - Adaptations • In hot, dry climates – Stomata must close to avoid wilting – CO2 decreases and O2 increases – O2 starts combining with RuBP, leading to the production of CO2 – This is called photorespiration • C4 plants solve the problem of photorespiration – Fix CO2 to PEP (a C3 molecule) – The result is oxaloacetate, a C4 molecule – In hot & dry climates • C4 plants avoid photorespiration • Net productivity is about 2-3 times greater than C3 plants in hot/dry environments – In cool, moist environments, C4 plants can’t compete with C3 plants CO2 Fixation in C3 and C4 Plants Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CO2 RuBP Calvin cycle 3PG G3P mesophyll cell a. CO2 fixation in a C3 plant, wildflowers CO2 mesophyll C 4 cell bundle sheath cell CO2 Calvin cycle G3P b. CO2 fixation in a C4 plant, corn, Zea mays a: © Brand X Pictures/PunchStock RF; b: Courtesy USDA/Doug Wilson, photographer 46 Other Types of Photosynthesis • CAM Photosynthesis – Crassulacean-Acid Metabolism – CAM plants partition carbon fixation by time • During the night – CAM plants fix CO2 – Form C4 molecules, which are – Stored in large vacuoles • During daylight – NADPH and ATP are available – Stomata are closed for water conservation – C4 molecules release CO2 to Calvin cycle 47 CO2 Fixation in a CAM Plant Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. night CO2 C4 day CO2 Calvin cycle G3P CO2 fixation in a CAM plant, pineapple, Ananas comosus © S. Alden/PhotoLink/Getty Images. 48 Adaptations of Plants • Each method of photosynthesis has advantages and disadvantages – Depends on the climate • C4 plants most adapted to: – High light intensities – High temperatures – Limited rainfall • C3 plants better adapted to – Cold (below 25°C) – High moisture • CAM plants are better adapted to extreme aridity – CAM occurs in 23 families of flowering plants – Also found among nonflowering plants 49 Connection • EK 1C3a: Scientific evidence supports the idea that evolution has occurred in all species – The student is able to describe a model that represents evolution within a population.