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Chapter 7 Photosynthesis: Using Light to Make Food PowerPoint Lectures Campbell Biology: Concepts & Connections, Eighth Edition REECE • TAYLOR • SIMON • DICKEY • HOGAN © 2015 Pearson Education, Inc. Lecture by Edward J. Zalisko Introduction • Poison ivy contains urushiol, a chemical that may cause itchy and oozing blisters that can last for weeks. • Like all plants, poison ivy produces energy for its growth by photosynthesis, the process that converts light energy to the chemical energy of sugar. © 2015 Pearson Education, Inc. Introduction • Photosynthesis • removes CO2 from the atmosphere and • stores it in plant matter. • The burning of sugar in the cellular respiration of almost all organisms releases CO2 back to the environment. © 2015 Pearson Education, Inc. Figure 7.0-1 © 2015 Pearson Education, Inc. Figure 7.0-2 Chapter 7: Big Ideas An Introduction to Photosynthesis The Light Reactions: Converting Solar Energy to Chemical Energy The Calvin Cycle: Reducing CO2 to Sugar The Global Significance of Photosynthesis © 2015 Pearson Education, Inc. AN INTRODUCTION TO PHOTOSYNTHESIS © 2015 Pearson Education, Inc. 7.1 Photosynthesis fuels the biosphere • Plants are autotrophs, which • sustain themselves, • do not usually consume organic molecules derived from other organisms, and • make their own food through the process of photosynthesis, in which they convert CO2 and H2O to sugars and other organic molecules. © 2015 Pearson Education, Inc. 7.1 Photosynthesis fuels the biosphere • Photoautotrophs use the energy of light to produce organic molecules. • Chemoautotrophs are prokaryotes that use inorganic chemicals as their energy source. • Heterotrophs are consumers that feed on plants or animals or decompose organic material. © 2015 Pearson Education, Inc. 7.1 Photosynthesis fuels the biosphere • Photoautotrophs • • • • feed us, clothe us (think cotton), house us (think wood), and provide energy for warmth, light, transport, and manufacturing. © 2015 Pearson Education, Inc. Figure 7.1 © 2015 Pearson Education, Inc. Figure 7.1a © 2015 Pearson Education, Inc. Figure 7.1b © 2015 Pearson Education, Inc. Figure 7.1c © 2015 Pearson Education, Inc. 7.1 Photosynthesis fuels the biosphere • Photosynthesis in plants takes place in chloroplasts. • Photosynthetic bacteria have infolded regions of the plasma membrane containing clusters of pigments and enzymes. © 2015 Pearson Education, Inc. 7.2 Photosynthesis occurs in chloroplasts in plant cells • Chlorophyll • is an important light-absorbing pigment in chloroplasts, • is responsible for the green color of plants, and • plays a central role in converting solar energy to chemical energy. © 2015 Pearson Education, Inc. 7.2 Photosynthesis occurs in chloroplasts in plant cells • Chloroplasts are concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf. • Stomata are tiny pores in the leaf that allow • carbon dioxide to enter and • oxygen to exit. • Veins in the leaf deliver water absorbed by roots. © 2015 Pearson Education, Inc. Figure 7.2-0 Leaf Cross Section Mesophyll Leaf Vein Mesophyll Cell CO2 O2 Stoma Chloroplast Inner and outer membranes Granum Thylakoid Thylakoid space Stroma © 2015 Pearson Education, Inc. Figure 7.2-1 Leaf Cross Section Mesophyll Leaf Vein Mesophyll Cell CO2 O2 Stoma Chloroplast © 2015 Pearson Education, Inc. 7.2 Photosynthesis occurs in chloroplasts in plant cells • Chloroplasts consist of an envelope of two membranes, which encloses an inner compartment filled with a thick fluid called stroma that contains a system of interconnected membranous sacs called thylakoids. © 2015 Pearson Education, Inc. 7.2 Photosynthesis occurs in chloroplasts in plant cells • Thylakoids • are often concentrated in stacks called grana and • have an internal compartment called the thylakoid space, which has functions analogous to the intermembrane space of a mitochondrion in the generation of ATP. © 2015 Pearson Education, Inc. 7.2 Photosynthesis occurs in chloroplasts in plant cells • Thylakoid membranes also house much of the machinery that converts light energy to chemical energy. • Chlorophyll molecules • are built into the thylakoid membrane and • capture light energy. © 2015 Pearson Education, Inc. Figure 7.2-2 Mesophyll Cell Chloroplast Inner and outer membranes Granum Thylakoid Thylakoid space Stroma © 2015 Pearson Education, Inc. Figure 7.2-3 Mesophyll Cell Chloroplast © 2015 Pearson Education, Inc. Figure 7.2-4 Chloroplast Stroma Granum © 2015 Pearson Education, Inc. 7.3 Scientists traced the process of photosynthesis using isotopes • Scientists have known since the 1800s that plants produce O2. But does this oxygen come from carbon dioxide or water? • For many years, it was assumed that oxygen was extracted from CO2 taken into the plant. • However, later research using a heavy isotope of oxygen, 18O, confirmed that oxygen produced by photosynthesis comes from H2O. © 2015 Pearson Education, Inc. Figure 7.3 © 2015 Pearson Education, Inc. 7.3 Scientists traced the process of photosynthesis using isotopes • Experiment 1: 6 CO2 12 H2O → C6H12O6 6 H2O 6 O2 • Experiment 2: 6 CO2 12 H2O → C6H12O6 6 H2O 6 O2 © 2015 Pearson Education, Inc. 7.4 Photosynthesis is a redox process, as is cellular respiration • Photosynthesis, like respiration, is a redox (oxidation-reduction) process. • CO2 becomes reduced to sugar as electrons, along with hydrogen ions (H+) from water, are added to it. • Water molecules are oxidized when they lose electrons along with hydrogen ions. © 2015 Pearson Education, Inc. Figure 7.4a Becomes reduced 6 CO2 + 6 H2O C6H12O6 + 6 O2 Becomes oxidized © 2015 Pearson Education, Inc. 7.4 Photosynthesis is a redox process, as is cellular respiration • Cellular respiration uses redox reactions to harvest the chemical energy stored in a glucose molecule. • This is accomplished by oxidizing the sugar and reducing O2 to H2O. • The electrons lose potential as they travel down the electron transport chain to O2. • In contrast, the food-producing redox reactions of photosynthesis require energy. © 2015 Pearson Education, Inc. 7.4 Photosynthesis is a redox process, as is cellular respiration • In photosynthesis, • light energy is captured by chlorophyll molecules to boost the energy of electrons, • light energy is converted to chemical energy, and • chemical energy is stored in the chemical bonds of sugars. © 2015 Pearson Education, Inc. Figure 7.4b Becomes oxidized C6H12O6 + 6 O2 6 CO2 + 6 H2O Becomes reduced © 2015 Pearson Education, Inc. 7.5 The two stages of photosynthesis are linked by ATP and NADPH • Photosynthesis occurs in two stages. 1. The light reactions occur in the thylakoid membranes. In these reactions • water is split, providing a source of electrons and giving off oxygen as a by-product, • ATP is generated from ADP and a phosphate group, and • light energy is absorbed by the chlorophyll molecules to drive the transfer of electrons and H+ from water to the electron acceptor NADP+, reducing it to NADPH. • NADPH, produced by the light reactions, provides the “reducing power” to the Calvin cycle. © 2015 Pearson Education, Inc. 7.5 The two stages of photosynthesis are linked by ATP and NADPH 2. The second stage is the Calvin cycle, which occurs in the stroma of the chloroplast. • The Calvin cycle is a cyclic series of reactions that assembles sugar molecules using CO2 and the energyrich products of the light reactions. • During the Calvin cycle, CO2 is incorporated into organic compounds in a process called carbon fixation. • After carbon fixation, the carbon compounds are reduced to sugars. • The Calvin cycle is often called the dark reactions, or light-independent reactions, because none of the steps requires light directly. © 2015 Pearson Education, Inc. Figure 7.5-1 H2O Light NADP + ADP + P Light Reactions (in thylakoids) Chloroplast © 2015 Pearson Education, Inc. Figure 7.5-2 H2O Light NADP + ADP + P Light Reactions (in thylakoids) ATP NADPH Chloroplast O2 © 2015 Pearson Education, Inc. Figure 7.5-3 H2O CO2 Light NADP + ADP + P Calvin Cycle (in stroma) Light Reactions (in thylakoids) ATP NADPH Chloroplast O2 © 2015 Pearson Education, Inc. Sugar THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY © 2015 Pearson Education, Inc. 7.6 Visible radiation absorbed by pigments drives the light reactions • Sunlight contains energy called electromagnetic energy or radiation. • Visible light is only a small part of the electromagnetic spectrum, the full range of electromagnetic wavelengths. • Electromagnetic energy travels in waves. • The wavelength is the distance between the crests of two adjacent waves. © 2015 Pearson Education, Inc. 7.6 Visible radiation absorbed by pigments drives the light reactions • Light behaves as discrete packets of energy called photons. • A photon is a fixed quantity of light energy. • The shorter the wavelength, the greater the energy. © 2015 Pearson Education, Inc. Figure 7.6a Shorter wavelength Higher energy 10−5 nm 10−3 nm Gamma rays X-rays Longer wavelength Lower energy 103 nm 1 nm UV 106 nm Infrared Microwaves 103 m 1m Radio waves Visible light 380 400 © 2015 Pearson Education, Inc. 500 600 Wavelength (nm) 700 750 7.6 Visible radiation absorbed by pigments drives the light reactions • Plant pigments • are built into the thylakoid membrane, • absorb some wavelengths of light, and • reflect or transmit other wavelengths. • We see the color of the wavelengths that are transmitted. For example, chlorophyll transmits green wavelengths. © 2015 Pearson Education, Inc. Animation: Light and Pigments © 2015 Pearson Education, Inc. Figure 7.6b-0 Light Chloroplast Thylakoid Absorbed light © 2015 Pearson Education, Inc. Transmitted light Reflected light Figure 7.6b-1 Light Chloroplast Thylakoid Absorbed light © 2015 Pearson Education, Inc. Transmitted light Reflected light Figure 7.6b-2 © 2015 Pearson Education, Inc. 7.6 Visible radiation absorbed by pigments drives the light reactions • Chloroplasts contain several different pigments, which absorb light of different wavelengths. • Chlorophyll a absorbs mainly blue-violet and red light and reflects mainly green light. • Chlorophyll b absorbs mainly blue and orange and reflects (appears) olive-green. • Carotenoids • broaden the spectrum of colors that can drive photosynthesis and • provide photoprotection, absorbing and dissipating excessive light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules. © 2015 Pearson Education, Inc. 7.7 Photosystems capture solar energy • Pigments in chloroplasts absorb photons (capturing solar power), which • increases the potential energy of the pigments’ electrons and • sends the electrons into an unstable state. • Generally, when isolated pigment molecules absorb light, their excited electrons drop back down to the ground state and release their excess energy as heat. © 2015 Pearson Education, Inc. Figure 7.7a-0 Excited state Heat Photon of light Photon (fluorescence) Ground state Chlorophyll molecule © 2015 Pearson Education, Inc. Figure 7.7a-2 © 2015 Pearson Education, Inc. Figure 7.7a-1 Excited state Heat Photon of light Photon (fluorescence) Ground state Chlorophyll molecule © 2015 Pearson Education, Inc. 7.7 Photosystems capture solar energy • Within a thylakoid membrane, chlorophyll and other pigment molecules • absorb photons and • transfer the energy to other pigment molecules. • In the thylakoid membrane, chlorophyll molecules are organized along with other pigments and proteins into photosystems. © 2015 Pearson Education, Inc. 7.7 Photosystems capture solar energy • A photosystem consists of a number of lightharvesting complexes surrounding a reactioncenter complex. • A light-harvesting complex contains various pigment molecules bound to proteins. • Collectively, the light-harvesting complexes function as a light-gathering antenna. © 2015 Pearson Education, Inc. Figure 7.7b Photosystem Light Light-harvesting Reaction-center complex complexes Thylakoid membrane STROMA Primary electron acceptor THYLAKOID SPACE Pair of Transfer of energy chlorophyll a molecules © 2015 Pearson Education, Inc. Pigment molecules 7.7 Photosystems capture solar energy • The light energy is passed from molecule to molecule within the photosystem. • Finally it reaches the reaction center, where a primary electron acceptor accepts these electrons and consequently becomes reduced. • The solar-powered transfer of an electron from the reaction-center chlorophyll a pair to the primary electron acceptor is the first step in the transformation of light energy to chemical energy in the light reactions. © 2015 Pearson Education, Inc. 7.7 Photosystems capture solar energy • Two types of photosystems (photosystem I and photosystem II) cooperate in the light reactions. • Each photosystem has a characteristic reactioncenter complex, with a special pair of chlorophyll a molecules associated with a particular primary electron acceptor. © 2015 Pearson Education, Inc. 7.8 Two photosystems connected by an electron transport chain generate ATP and NADPH • In the light reactions, light energy is transformed into the chemical energy of ATP and NADPH. • To accomplish this, electrons are • removed from water, • passed from photosystem II to photosystem I, and • accepted by NADP+, reducing it to NADPH. • Between the two photosystems, the electrons • move down an electron transport chain and • provide energy for the synthesis of ATP. © 2015 Pearson Education, Inc. 7.8 Two photosystems connected by an electron transport chain generate ATP and NADPH • A simple mechanical analogy helps unpack this rather complicated system. • This construction analogy shows how the coupling of two photosystems and an electron transport chain can transform the energy of light to the chemical energy of ATP and NADPH. © 2015 Pearson Education, Inc. Figure 7.8 ATP NADPH Electron transport chain ramp Photosystem II © 2015 Pearson Education, Inc. Photosystem I 7.9 VISUALIZING THE CONCEPT: The light reactions take place within the thylakoid membranes • A thylakoid membrane includes numerous copies of • the two photosystems and • the electron transport chain. © 2015 Pearson Education, Inc. 7.9 VISUALIZING THE CONCEPT: The light reactions take place within the thylakoid membranes • Light energy absorbed by the two photosystems drives the flow of electrons from water to NADPH. • The electron transport chain helps to produce the concentration gradient of H+ across the thylakoid membrane, which drives H+ through ATP synthase, producing ATP. • Because the initial energy input is light (“photo”), this chemiosmotic production of ATP is called photophosphorylation. © 2015 Pearson Education, Inc. Figure 7.9-0 Thylakoid sac Chloroplast Light H+ Photosystem II Electron Light Photosystem transport chain H2O 1 2 H+ H+ H+ H+ O2 + 2 H+ NADPH I H+ H+ NADP+ + H+ H+ H+ H+ To Calvin Cycle H+ H+ H+ THYLAKOID SPACE H+ H+ H+ H+ Thylakoid membrane H+ H+ H+ H+ ATP synthase STROMA H+ H+ H+ ADP + P ATP H+ © 2015 Pearson Education, Inc. H+ Figure 7.9-1 Light Photosystem II Primary electron acceptor Pigment molecules Reaction center pair of chlorophyll a molecules H2O 1 2 O 2 + 2 H+ © 2015 Pearson Education, Inc. Figure 7.9-2 Light H+ Photosystem II H+ H+ H2O 1 2 O 2 + 2 H+ © 2015 Pearson Education, Inc. Electron transport chain H+ H+ H+ Figure 7.9-3 Light H+ Photosystem II Light Electron Photosystem transport chain I H+ Primary electron acceptor Pigment molecules H+ H2O 1 2 O 2 + 2 H+ © 2015 Pearson Education, Inc. Reaction center pair of chlorophyll a molecules H+ H+ H+ H+ H+ Figure 7.9-4 Light H+ Photosystem II 1 2 O 2 + 2 H+ H+ NADP+ H+ NADPH I H+ H+ H2O Light Electron Photosystem transport chain H+ H+ H+ H+ H+ H+ H+ H+ H+ © 2015 Pearson Education, Inc. To Calvin Cycle Figure 7.9-5 Light H+ Photosystem II Light Electron Photosystem transport chain H2O 1 2 H+ H+ H+ H+ O 2 + 2 H+ H+ NADPH I H+ H+ NADP+ H+ H+ H+ To Calvin Cycle H+ H+ H+ THYLAKOID SPACE H+ H+ H+ H+ Thylakoid membrane H+ H+ H+ H+ ATP synthase STROMA H+ H+ H+ ADP + P ATP H+ © 2015 Pearson Education, Inc. H+ THE CALVIN CYCLE: REDUCING CO2 TO SUGAR © 2015 Pearson Education, Inc. 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle • The Calvin cycle makes sugar within a chloroplast. • To produce sugar, the necessary ingredients are • atmospheric CO2 and • ATP and NADPH generated by the light reactions. © 2015 Pearson Education, Inc. 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle • The Calvin cycle uses these three ingredients to produce an energy-rich, three-carbon sugar called glyceraldehyde 3-phosphate (G3P). • A plant cell uses G3P to make • glucose, • the disaccharide sucrose, and • other organic molecules as needed. © 2015 Pearson Education, Inc. 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle • The steps of the Calvin cycle include • • • • carbon fixation, reduction, release of one molecule of G3P, and regeneration of the starting molecule, ribulose bisphosphate (RuBP). • Photosynthesis is an emergent property of the structural organization of a chloroplast, which integrates the two stages of photosynthesis. © 2015 Pearson Education, Inc. Figure 7.10-0 CO2 H2O Light NADP+ ADP + P Light Reactions Input Calvin Cycle 3 CO2 ATP Step 1 Carbon fixation NADPH Chloroplast O2 Rubisco Sugar 3 P 6 P P 3-PGA RuBP Step 4 Regeneration of RuBP 3 ADP 6 ATP P 6 ADP + P CALVIN CYCLE 3 ATP 6 NADPH 6 NADP+ 5 6 P G3P G3P Step 2 Reduction Step 3 Release of one molecule of G3P 1 P G3P Output © 2015 Pearson Education, Inc. P Glucose and other compounds Figure 7.10-1 CO2 H2O Light NADP+ ADP + P Light Reactions Calvin Cycle ATP NADPH Chloroplast O2 © 2015 Pearson Education, Inc. Sugar Figure 7.10-2 Input 3 CO2 Step 1 Carbon fixation Rubisco 3 P 6 P P 3-PGA RuBP 6 ATP 3 ADP Step 4 Regeneration 3 ATP of RuBP 6 ADP + P CALVIN CYCLE 6 NADPH 6 NADP+ 5 6 P G3P G3P Step 2 Reduction Step 3 Release of one molecule of G3P P 1 G3P Output © 2015 Pearson Education, Inc. P Glucose and other compounds Figure 7.10-3-1 Input 3 CO2 Step 1 Carbon fixation Rubisco 3 P 6 P 3-PGA RuBP CALVIN CYCLE © 2015 Pearson Education, Inc. P Figure 7.10-3-2 Input 3 CO2 Step 1 Carbon fixation Rubisco 3 P 6 P P 3-PGA RuBP 6 ATP 6 ADP + P CALVIN CYCLE 6 NADPH 6 NADP+ 6 P G3P Step 2 Reduction © 2015 Pearson Education, Inc. Figure 7.10-3-3 Input 3 CO2 Step 1 Carbon fixation Rubisco 3 P 6 P P 3-PGA RuBP 6 ATP 6 ADP + P CALVIN CYCLE 6 NADPH 6 NADP+ 5 6 P G3P G3P Step 2 Reduction Step 3 Release of one molecule of G3P P 1 G3P Output © 2015 Pearson Education, Inc. P Glucose and other compounds Figure 7.10-3-4 Input 3 CO2 Step 1 Carbon fixation Rubisco 3 P 6 P P 3-PGA RuBP 6 ATP 3 ADP Step 4 Regeneration 3 ATP of RuBP 6 ADP + P CALVIN CYCLE 6 NADPH 6 NADP+ 5 6 P G3P G3P Step 2 Reduction Step 3 Release of one molecule of G3P P 1 G3P Output © 2015 Pearson Education, Inc. P Glucose and other compounds 7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates • Most plants use CO2 directly from the air, and carbon fixation occurs when the enzyme rubisco adds CO2 to RuBP. • Such plants are called C3 plants because the first product of carbon fixation is a three-carbon compound, 3-PGA. © 2015 Pearson Education, Inc. 7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates • In hot and dry weather, C3 plants close their stomata to reduce water loss, but this prevents CO2 from entering the leaf and O2 from leaving. • As O2 builds up in a leaf, rubisco adds O2 instead of CO2 to RuBP, and a two-carbon product of this reaction is then broken down in the cell. • This process is called photorespiration because it occurs in the light, consumes O2, and releases CO2. • But unlike cellular respiration, it uses ATP instead of producing it. © 2015 Pearson Education, Inc. 7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates • An alternate mode of carbon fixation has evolved that • minimizes photorespiration and • optimizes the Calvin cycle. • C4 plants are so named because they first fix CO2 into a four-carbon compound. • When the weather is hot and dry, C4 plants keep their stomata mostly closed, thus conserving water. © 2015 Pearson Education, Inc. 7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates • Another adaptation to hot and dry environments has evolved in the CAM plants, such as pineapples, cacti, aloe, and jade. • CAM plants conserve water by opening their stomata and admitting CO2 only at night. • CO2 is fixed into a four-carbon compound, which banks CO2 at night and releases it to the Calvin cycle during the day. © 2015 Pearson Education, Inc. Figure 7.11-0 CO2 Mesophyll cell Bundlesheath cell © 2015 Pearson Education, Inc. CO2 4-C compound 4-C compound CO2 CO2 Calvin Cycle Calvin Cycle Sugar Sugar C4 plant Sugarcane Night Day CAM plant Pineapple Figure 7.11-1 CO2 Mesophyll cell Bundlesheath cell 4-C compound 4-C compound CO2 CO2 Calvin Cycle Calvin Cycle Sugar Sugar C4 plant © 2015 Pearson Education, Inc. CO2 Night Day CAM plant Figure 7.11-2 Sugarcane © 2015 Pearson Education, Inc. Figure 7.11-3 Pineapple © 2015 Pearson Education, Inc. THE GLOBAL SIGNIFICANCE OF PHOTOSYNTHESIS © 2015 Pearson Education, Inc. 7.12 Photosynthesis makes sugar from CO2 and H2O, providing food and O2 for almost all living organisms • Two photosystems in the thylakoid membranes capture solar energy, energizing electrons in chlorophyll molecules. • Simultaneously, • water is split, • O2 is released, and • electrons are funneled to the photosystems. © 2015 Pearson Education, Inc. 7.12 Photosynthesis makes sugar from CO2 and H2O, providing food and O2 for almost all living organisms • The photoexcited electrons are transferred through an electron transport chain, where energy is harvested to make ATP by the process of chemiosmosis, and finally to NADP+, reducing it to the high-energy compound NADPH. © 2015 Pearson Education, Inc. Figure 7.12 H2O Chloroplast CO2 Light NADP+ ADP Light Reactions + P RuBP Photosystem II Electron transport chain Thylakoids Photosystem I ATP NADPH O2 © 2015 Pearson Education, Inc. Calvin Cycle 3-PGA (in stroma) Stroma G3P Sugars Cellular respiration Cellulose Starch Other organic compounds 7.12 Photosynthesis makes sugar from CO2 and H2O, providing food and O2 for almost all living organisms • About 50% of the carbohydrates made by photosynthesis is consumed as fuel for cellular respiration in the mitochondria of plant cells. • Sugars also serve as the starting material for making other organic molecules, such as proteins, lipids, and cellulose. • Many glucose molecules are linked together to make cellulose, the main component of cell walls. © 2015 Pearson Education, Inc. 7.12 Photosynthesis makes sugar from CO2 and H2O, providing food and O2 for almost all living organisms • Most plants make much more food each day than they need. They store the excess in • • • • roots, tubers, seeds, and fruits. • Plants (and other photosynthesizers) are the ultimate source of food for virtually all other organisms. © 2015 Pearson Education, Inc. 7.13 SCIENTIFIC THINKING: Rising atmospheric levels of carbon dioxide and global climate change will affect plants in various ways • The greenhouse effect operates on a global scale. • Solar radiation passes through the atmosphere and warms Earth’s surface. • Heat radiating from the warmed planet is absorbed by greenhouse gases, such as CO2, water vapor, and methane, which then reflect some of the heat back to Earth. • Without this natural heating effect, the average air temperature would be a frigid –18C (–0.4F), and most life as we know it could not exist. © 2015 Pearson Education, Inc. 7.13 SCIENTIFIC THINKING: Rising atmospheric levels of carbon dioxide and global climate change will affect plants in various ways • Increasing concentrations of greenhouse gases have been linked to global climate change, of which one major aspect is global warming. © 2015 Pearson Education, Inc. 7.13 SCIENTIFIC THINKING: Rising atmospheric levels of carbon dioxide and global climate change will affect plants in various ways • The predicted consequences of global climate change include • • • • • • melting of polar ice, rising sea levels, extreme weather patterns, droughts, increased extinction rates, and the spread of tropical diseases. • Many of these effects are already being documented. © 2015 Pearson Education, Inc. 7.13 SCIENTIFIC THINKING: Rising atmospheric levels of carbon dioxide and global climate change will affect plants in various ways • How may global climate change affect plants? • Increasing CO2 levels can increase plant productivity. • Research has documented such an increase, although results often indicate that the growth rates of weeds, such as poison ivy, increase more than those of crop plants and trees. © 2015 Pearson Education, Inc. 7.13 SCIENTIFIC THINKING: Rising atmospheric levels of carbon dioxide and global climate change will affect plants in various ways • How do scientists study the effects of increasing CO2 on plants? • Many experiments are done in small growth chambers in which variables can be carefully controlled. • Other scientists are turning to long-term field studies that include large-scale manipulations of CO2 levels. © 2015 Pearson Education, Inc. 7.13 SCIENTIFIC THINKING: Rising atmospheric levels of carbon dioxide and global climate change will affect plants in various ways • In the Free-Air CO2 Enrichment (FACE) experiment set up in Duke University’s experimental forest, scientists monitored the effects of elevated CO2 levels on an intact forest ecosystem over a period of 15 years, using six study sites, each 30 m in diameter and ringed by 16 towers. © 2015 Pearson Education, Inc. Figure 7.13a © 2015 Pearson Education, Inc. 7.13 SCIENTIFIC THINKING: Rising atmospheric levels of carbon dioxide and global climate change will affect plants in various ways • Another study compared the growth of poison ivy in experimental and control plots. The results showed that poison ivy in the elevated CO2 plots • showed an average annual growth increase of 149% compared to control plots and • produced a more potent form of poison ivy’s allergenic compound. © 2015 Pearson Education, Inc. Mean plant dry biomass (g) Figure 7.13b 10 9 8 7 6 5 4 3 2 1 0 Key Control plots Elevated CO2 plots 1999 2000 2001 2002 2003 2004 Year Source: Adaptation of Figure 1A from “Biomass and Toxicity Responses of Poison Ivy (Toxicodendron Radicans) to Elevated Atmospheric CO2” by Jacqueline E. Mohan, et al., from PNAS, June 2006, Volume 103(24). Copyright © 2006 by National Academy of Sciences. Reprinted with permission. © 2015 Pearson Education, Inc. 7.14 Scientific research and international treaties have helped slow the depletion of Earth’s ozone layer • Solar radiation converts O2 high in the atmosphere to ozone (O3), which shields organisms from damaging UV radiation. • A gigantic hole in the ozone layer has appeared every spring over Antarctica since the late 1970s and continues today. © 2015 Pearson Education, Inc. Figure 7.14a Southern tip of South America Antarctica September 2012 © 2015 Pearson Education, Inc. 7.14 Scientific research and international treaties have helped slow the depletion of Earth’s ozone layer • Industrial chemicals called CFCs have caused dangerous thinning of the ozone layer. • This destruction of the ozone layer was documented by scientists from the British Antarctic Survey, NASA, and National Oceanic and Atmospheric Administration. © 2015 Pearson Education, Inc. Figure 7.14b © 2015 Pearson Education, Inc. 7.14 Scientific research and international treaties have helped slow the depletion of Earth’s ozone layer • In response to these scientific findings, the first treaty to address Earth’s environment was signed in 1987. • As research continued to establish the extent and danger of ozone depletion, these agreements were strengthened in 1990, and now nearly 200 nations participate in the protocol. © 2015 Pearson Education, Inc. 7.14 Scientific research and international treaties have helped slow the depletion of Earth’s ozone layer • In 1995, Molina and Rowland shared a Nobel Prize for their work in determining how CFCs were damaging the atmosphere. • Global emissions of CFCs are near zero now, but because these compounds are so stable, recovery of the ozone layer is not expected until around 2060. • Meanwhile, unblocked UV radiation is predicted to increase skin cancer and cataracts and damage crops and phytoplankton in the oceans. © 2015 Pearson Education, Inc. You should now be able to 1. Define autotrophs, heterotrophs, producers, and photoautotrophs. 2. Describe the structure of chloroplasts and their location in a leaf. 3. Explain how plants produce oxygen. 4. Describe the role of redox reactions in photosynthesis and cellular respiration. 5. Compare the reactants and products of the light reactions and the Calvin cycle. © 2015 Pearson Education, Inc. You should now be able to 6. Describe the properties and functions of the different photosynthetic pigments. 7. Explain how photosystems capture solar energy. 8. Explain how the electron transport chain and chemiosmosis generate ATP, NADPH, and oxygen in the light reactions. 9. Compare photophosphorylation and oxidative phosphorylation. 10. Describe the reactants and products of the Calvin cycle. © 2015 Pearson Education, Inc. You should now be able to 11. Compare the mechanisms that C3, C4, and CAM plants use to obtain and use carbon dioxide. 12. Review the overall process of the light reactions and the Calvin cycle, noting the products, reactants, and locations of every major step. 13. Describe the greenhouse effect. 14. Explain how the ozone layer forms, how human activities have damaged it, and the consequences of the destruction of the ozone layer. © 2015 Pearson Education, Inc. Figure 7.UN01 Light energy 6 CO2 6 Carbon dioxide © 2015 Pearson Education, Inc. H 2O Water C6H12O6 Photosynthesis Glucose 6 O2 Oxygen gas Figure 7.UN02 H2O Chloroplast CO2 Light NADP+ Stroma ADP Thylakoids + P Light Reactions Calvin Cycle ATP NADPH O2 © 2015 Pearson Education, Inc. Sugar Figure 7.UN03 Photosynthesis converts includes both (a) (b) (c) to in which chemical energy light-excited electrons of chlorophyll H2O is split in which CO2 is fixed to RuBP and and then are passed down (d) Reduce NADP+ to 3-PGA is reduced using (e) (f) to produce producing chemiosmosis © 2015 Pearson Education, Inc. by (g) (h) Figure 7.UN04 Mitochondrion Chloroplast Intermembrane space H+ c. Membrane Matrix d. a. b. © 2015 Pearson Education, Inc. e.