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Chapter 7 Photosynthesis 1 7.1 Photosynthetic Organisms • All life on Earth depends on solar energy • Photosynthetic organisms (algae, plants, and cyanobacteria) transform solar energy into the chemical energy of carbohydrates Called autotrophs because they produce their own food. • Photosynthesis: A process that captures solar energy solar energy chemical energy Energy ends up stored in a carbohydrate • Photosynthesizers produce food energy Feed themselves as well as heterotrophs 2 Photosynthetic Organisms Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. mosses trees kelp Euglena garden plants cyanobacteria diatoms (Moss): © Steven P. Lynch; (Trees): © Digital Vision/PunchStock; (Kelp): © Chuck Davis/Stone/Getty Images; (Cyanobacteria): © Sherman Thomas/Visuals Unlimited; (Diatoms): © Ed Reschke/Peter Arnold; (Euglena): © T.E. Adams/Visuals Unlimited; (Sunflower): © Royalty-Free/Corbis 3 Photosynthetic Organisms • Photosynthesis takes place in the green portions of plants Cells containing chloroplasts are specialized to carry out 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 In stroma, CO2 is combined with H2O to form C6H12O6 (sugar) Energy supplied by light • Chlorophyll and other pigments absorb solar energy and energize electrons prior to reduction of CO2 to a carbohydrate 4 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 5 7.2 The Process of Photosynthesis • Light Reactions – take place on thylakoid membrane Energy-capturing reactions Chlorophyll absorbs solar energy This energizes electrons Electrons move down an electron transport chain • Pumps H+ into thylakoids • Used to make ATP out of ADP and NADPH out of NADP • Calvin Cycle Reactions (Dark Reactions) – take place in the stroma CO2 is reduced to a carbohydrate Use ATP and NADPH to produce carbohydrate 6 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 7 7.3 Plants as Solar Energy Converters • Pigments: Chemicals that absorb certain wavelengths of light Wavelengths that are not absorbed are reflected/transmitted • Absorption Spectrum Pigments found in chlorophyll absorb various portions of visible light Graph showing relative absorption of the various colors of the rainbow Chlorophyll is green because it absorbs much of the reds and blues of white light 8 Photosynthetic Pigments and Photosynthesis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Increasing wavelength chlorophyll a chlorophyll b carotenoids Gamma rays X rays UV Micro- Radio Infrared waves waves visible light 500 600 Wavelengths (nm) a. The electromagnetic spectrum includes visible light. 750 Relative Absorption Increasing energy 380 500 600 750 Wavelengths (nm) b. Absorption spectrum of photosynthetic pigments. 9 Plants as Solar Energy Converters • Noncyclic pathway Takes place in the thylakoid membrane Uses two photosystems, PS I and PS II PS II captures light energy Causes an electron to be ejected from the reaction center (chlorophyll a) • Electron travels down electron transport chain to PS I • Replaced with an electron from water, which is split to form O2 and H+ • This causes H+ to accumulate in thylakoid chambers • The H+ gradient is used to produce ATP PS I captures light energy and ejects an electron • The electron is transferred permanently to a molecule of NADP+ • Causes NADPH production 10 Noncyclic Electron Pathway Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H2 O CO2 solar energy ADP+ P NADP+ Light reactions sun Calvin cycle sun NADPH ATP thylakoid membrane energy level electron acceptor electron acceptor O CH2O e– e– e– e– NADP+ H+ ATP e– NADPH e– reaction center reaction center pigment complex pigment complex Photosystem I e– Photosystem II CO2 H2O CH2O Calvin cycle reactions 2H+ 1 – O2 2 11 Plants as Solar Energy Converters • PS II: Consists of a pigment complex and electron acceptors Receives electrons from the splitting of water Oxygen is released as a gas • Electron transport chain: Consists of cytochrome complexes and plastoquinone Carries electrons between PS II and PS I Also pumps H+ from the stroma into the thylakoid space • PS I: Has a pigment complex and electron acceptors Adjacent to the enzyme that reduces NADP+ to NADPH • ATP synthase complex: Has a channel for H+ flow H+ flow through the channel drives ATP synthase to join ADP and Pi 12 Organization of a Thylakoid Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H2O CO2 solar energy ADP + P NADP + Light reactions Calvin cycle reactions NADPH ATP thylakoid membrane thylakoid membrane thylakoid thylakoid space O2 CH2O granum photosystem II electron transport chain H+ stroma photosystem I H+ NADP reductase Pq ee- e- NADP+ NADPH e- eH+ H+ H2 O 2 H+ + 1 2 H+ H+ H+ H+ H+ H+ H+ O2 ATP synthase H+ H+ H+ H+ Thylakoid space H+ ATP H+ H+ chemiosmosis P Stroma + ADP 13 Plants as Solar Energy Converters • 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 14 Tropical Rain Forest Destruction and Climate Change • Tropical rain forests can exist where Temperatures are above 26º C Rainfall is heavy (100-200 cm) and regular • Most plants are woody; many vines and epiphytes; little or no undergrowth • Contribute greatly to CO2 uptake, slowing global warming Development has reduced them from 14% to 6% of Earth’s surface Deforestation accounts for 20-30% of atmospheric CO2, but also removes a CO2 sink Increasing temperatures also reduce productivity 15 Tropical Rain Forest Destruction and Climate Change Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mean Global Temperature Change (0c) 5.5 4.5 3.5 maximum likely increase most probable temperature increase for 2 × CO2 2.5 1.5 Minimum likely increase 0.5 –0.5 1860 1940 2020 Year 2060 2100 16 7.4 Plants as Carbon Dioxide Fixers • A cyclical series of reactions • Utilizes atmospheric carbon dioxide to produce carbohydrates • Known as C3 photosynthesis • Involves three stages: • Carbon dioxide fixation • Carbon dioxide reduction • RuBP regeneration 17 Plants as Carbon Dioxide Fixers • CO2 is attached to 5-carbon RuBP carboxylase Results in a 6-carbon molecule This splits into two 3-carbon molecules (3PG) Reaction is accelerated by RuBP carboxylase (Rubisco) • CO2 is now “fixed” because it is part of a carbohydrate 18 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 19 Glucose Plants as Carbon Dioxide Fixers • Importance of the Calvin Cycle: G3P (glyceraldehyde-3-phosphate) can be converted to many other molecules The hydrocarbon skeleton of G3P can form • Fatty acids and glycerol to make plant oils • Glucose phosphate (simple sugar) • Fructose (which with glucose = sucrose) • Starch and cellulose • Amino acids 20 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) 21 © Herman Eisenbeiss/Photo Researchers, Inc. 7.5 Other Types of Photosynthesis • 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 cool, moist environments, C4 plants can’t compete with C3 plants 22 Chloroplast Distribution in C4 vs. C3 Plants Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. C3 Plant C4 Plant mesophyll cells bundle sheath cell vein stoma bundle sheath cell vein stoma 23 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 24 Other Types of Photosynthesis • 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 25