Photosynthesis: Using Light to Make Food PDF

7 Why Photosynthesis Matters Photosynthesis: Using Light to Make Food If you want ▶ to reduce the rate of global climate change, plant a tree....

7 Why Photosynthesis Matters Photosynthesis: Using Light to Make Food If you want ▶ to reduce the rate of global climate change, plant a tree. ◀ Nearly all life on Earth—including you—can trace its source of energy back to the sun. Protecting yourself ▶ from short wavelengths of light can be lifesaving. 140 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 140 C/M/Y/K M07_SIMO2368_05_GE_CH07.indd 140 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:08 AM  CHAPTER CONTENTS CHAPTER THREAD The Basics of Photosynthesis 142 Biofuels The Light Reactions: Converting Solar Energy to Chemical BIOLOGY AND SOCIETY A Greasy Crime Wave 141 Energy 144 THE PROCESS OF SCIENCE What Colors of Light Drive The Calvin Cycle: Making Sugar from Carbon Dioxide 149 Photosynthesis? 145 EVOLUTION CONNECTION Creating a Better Biofuel Factory 149 Biofuels BIOLOGY AND SOCIETY A Greasy Crime Wave In September 2013, police in Ocala, Florida, arrested two men and charged them with organized fraud and grand theft. Their crime? The men were caught red-handed with more than 700 gallons of stolen used cook- ing oil pilfered from a variety of local eateries. Why would anyone steal that nasty stuff? The reason is sim- ple: Remnants of restaurant deep fryers, sometimes called “liquid gold,” fetch about $2 per pound when sold to recyclers. That makes the burglars’ haul worth more than $5,000. Why is grease so valuable? As fossil fuel supplies dwindle and prices rise, the need for reliable, renewable sources of energy increases. In response, scientists are researching better ways to harness biofuels, energy obtained from living material. Some researchers focus on burning plant matter directly (wood pellet boilers, for example), and others focus on using plant material to produce biofuels that can be burned. There are several types of biofuels. Bioethanol is a type of alcohol (the same kind found in alcoholic drinks) that is made from wheat, corn, sugar beets, and other food crops. Starch made naturally by plants is converted to glucose and then fermented to ethanol by micro- organisms such as single-celled algae. Bioethanol can be used directly as a fuel source in specially designed vehicles, but it is more commonly used as a gasoline additive that can increase fuel efficiency while decreasing vehicle emissions. You may have noticed a sticker on a gas pump that declares the percentage of ethanol in that gasoline; most cars today run on a blend of 85% gasoline and 15% ethanol. Many car manufacturers are producing “flex- ible-fuel” vehicles that can run on any combination of gasoline and bioethanol. Although bioethanol does reduce carbon emissions and is a renewable resource, its production raises the prices of food crops (which become more expensive as acreage is diverted to biofuel production). Cellulosic ethanol is a form of bioethanol made from cellulose found in nonedible plant material such as wood, grass, or scraps from crops. Biodiesel, the most common biofuel in Europe, is made from plant oils such as recycled frying oil. Like bioethanol, it can be used on its own or as an emissions-reducing additive to standard diesel. In a strange twist, rising Using biofuels. Biofuels are values for diesel have sparked a greasy crime wave as thieves tap into this new and largely added to most gasoline in the unguarded source of raw material. Today, only about 2.7% of the world’s fuel used for driving United States. is provided by biofuels, but the International Energy Agency has set a goal of 25% by 2050. When we derive energy from biofuels, we are actually tapping into the energy of the sun, which drives photosynthesis in plants. Photosynthesis is the process by which plants use light to make sugars from carbon dioxide—sugars that are food for the plant and the starting point for most of our own food. In this chap- ter, we’ll first examine some basic concepts of photosynthesis; then we’ll look at the specific mechanisms involved in this process. 141 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 141 C/M/Y/K M07_SIMO2368_05_GE_CH07.indd Title: EBP 5e 141 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:08 AM  The Basics of Photosynthesis CHAPTER 7 PHOTOSYNTHESIS: USING LIGHT TO MAKE FOOD The process of photosynthesis is the ultimate source chloroplasts (about 500,000 per square millimeter of of energy for nearly every ecosystem on Earth. leaf surface—that’s equivalent to about 300 million chlo- Photosynthesis is a process whereby plants, algae roplasts in a leaf the size of a standard postage stamp). (which are protists), and certain bacteria trans- Their green color is from chlorophyll, a pigment (light- Nearly all life on Earth— form light energy into chemical energy, using absorbing molecule) in the chloroplasts that plays a cen- including you—can trace its carbon dioxide and water as starting materials tral role in converting solar energy to chemical energy. source of energy back and releasing oxygen gas as a by-product. The Chloroplasts are concentrated in the interior cells of to the sun. chemical energy produced via photosynthe- leaves (Figure 7.2), with a typical cell containing sis is stored in the bonds of sugar molecules. 30–40 chloroplasts. Carbon dioxide (CO2) enters a Organisms that generate their own organic matter leaf, and oxygen (O2) exits, by way of tiny pores called from inorganic ingredients are called autotrophs (see stomata (singular, stoma, meaning “mouth”). The carbon Chapter 6). Plants and other organisms that do this by dioxide that enters the leaf is the source of carbon for photosynthesis—photoautotrophs—are the producers much of the body of the plant, including the sugars and for most ecosystems (Figure 7.1). Not only do photo- starches that we eat. So the bulk of the body of a plant autotrophs feed us, they also clothe us (as the source of derives from the air, not the soil. As proof of this idea, cotton fibers), house us (wood), and provide energy for consider hydroponics, a means of growing plants using warmth, light, and transportation (biofuels). only air and water; no soil whatsoever is involved. In addition to carbon dioxide, photosynthesis requires water, which is absorbed by the plant’s roots and transported to Chloroplasts: Sites the leaves, where veins carry it to the photosynthetic cells. Membranes within the chloroplast form the frame- of Photosynthesis work where many of the reactions of photosynthe- Photosynthesis in plants and algae occurs within light- sis occur. Like a mitochondrion, a chloroplast has a absorbing organelles called chloroplasts (see Chapter 4, double-membrane envelope. The chloroplast’s inner especially Figure 4.17). All green parts of a plant have membrane encloses a compartment filled with stroma, chloroplasts and thus can carry out photosynthesis. a thick fluid. (It’s easy to confuse two terms associated ▼Figure 7.1 A diversity of In most plants, however, the leaves have the most with photosynthesis: stomata are pores through which photoautotrophs. PHOTOSYNTHETIC AUTOTROPHS Plants Photosynthetic Protists Photosynthetic Bacteria (mostly on land) (aquatic) (aquatic) LM 375× Forest plants Kelp, a large, multicellular alga Micrograph of cyanobacteria 142 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 142 C/M/Y/K M07_SIMO2368_05_GE_CH07.indd 142 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:08 AM  gases are exchanged, and stroma is the fluid THE BASICS within the chloroplast.) Suspended in the stroma OF PHOTOSYNTHESIS are interconnected membranous sacs called thylakoids. The thylakoids are concentrated in stacks called grana (singular, granum). Inner and outer membranes The chlorophyll molecules that capture light energy are built into the thylakoid mem- branes. The structure of a chloroplast— LM 2,000× with its stacks of disks—aids its function by providing a large surface area for the reactions of Photosynthetic cells Vein photosynthesis. (transports water and nutrients) Stroma Thylakoid Granum Thylakoid Interior cell space Chloroplast CHECKPOINT ▶ Figure 7.2 Journey Photosynthesis takes place within organelles called into a leaf. This series of Colorized TEM 1,000× Stomata _________ using gases that blowups takes you into a O2 leaf’s interior, then into a CO2 are exchanged via pores plant cell, and finally into called _________. a chloroplast, the site of Leaf cross section Answer: chloroplasts; stomata photosynthesis. MAJOR THEMES IN BIOLOGY Interconnections ution Structure/Function Information Flow Energy Transformations within Systems Energy ral selection is fying theme and ery level in the The structure of an object, such as a molecule or a body part, provides insight into its function, and vice versa. Within biological systems, information stored in DNA is transmitted and expressed. All biological systems depend on obtaining, converting, and releasing energy and matter. Transformations All biological systems, from molecules to ecosystems, depend on interactions between components. An Overview of Photosynthesis The following chemical equation, simplified to highlight sugar. Hydrogen is moved along with the electrons being the relationship between photosynthesis and cellular transferred from water to carbon dioxide. This transfer ▼ Figure 7.3 A road map for photosynthesis. We’ll respiration, provides a summary of the reactants and of hydrogen requires the chloroplast to split water mol- use a smaller version of this products of photosynthesis: ecules into hydrogen and oxygen. The hydrogen is trans- road map for orientation as we ferred along with electrons to carbon dioxide to form take a closer look at the light Light energy sugar. The oxygen escapes through stomata in leaves into reactions and the Calvin cycle. the atmosphere as O2, a waste prod- 6 CO2 + 6 H 2O C 6H12O 6 + 6 O2 Photo- uct of photosynthesis. H 2O CO2 Chloroplast Carbon Water synthesis Glucose Oxygen gas The overall equation dioxide Light Notice that the reactants of photosynthesis—carbon for photosynthesis is a dioxide (CO2) and water (H2O)—are the same as the simple summary of a com- waste products of cellular respiration (see Figure 6.2). plex process. Like many NADP+ Also notice that photosynthesis produces what respira- energy-producing processes ADP tion uses—glucose (C6H12O6) and oxygen (O2). In other within cells, photosynthesis is + P words, photosynthesis recycles the “exhaust” of cellular a multistep chemical pathway, Light Calvin cycle respiration and rearranges its atoms to produce food with each step in the path pro- reactions and oxygen. Photosynthesis is a chemical transforma- ducing products that are used as ATP tion that requires a lot of energy, and sunlight absorbed reactants in the next step. This is by chlorophyll provides that energy. a clear example of one of biology’s – – NADPH Recall that cellular respiration is a process of electron major themes: the use of metabolic transfer (see Chapter 6). A “fall” of electrons from food pathways to obtain, process, and molecules to oxygen to form water releases the energy store energy. To help get a better over- that mitochondria can use to make ATP (see Figure 6.9). view, let’s take a look at the two stages of O2 Sugar The opposite occurs in photosynthesis: Electrons are photosynthesis: the light reactions and boosted “uphill” and added to carbon dioxide to produce the Calvin cycle (Figure 7.3). 143 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 143 C/M/Y/K M07_SIMO2368_05_GE_CH07.indd Title: EBP 5e 143 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:09 AM  CHAPTER 7 In the light reactions, chlorophyll in the thylakoid Calvin cycle indirectly depends on light to produce PHOTOSYNTHESIS: USING membranes absorbs solar energy (the “photo” part of sugar because it requires the supply of ATP and LIGHT TO MAKE FOOD photosynthesis), which is then converted to the chemi- NADPH produced by the light reactions. cal energy of ATP (the molecule that drives most cel- The initial incorporation of carbon from CO2 lular work) and NADPH (an electron carrier). During into organic compounds is called carbon fixation. the light reactions, water is split, providing a source of This process has important implications for global electrons and giving off O2 gas as a by-product. climate, because the removal of carbon from the CHECKPOINT The Calvin cycle uses the products of the air and its incorporation into plant material light reactions to power the production of If you want to can help reduce the concentration of carbon 1. What molecules are the inputs of photosynthesis? sugar from carbon dioxide (the “synthesis” reduce the rate dioxide in the atmosphere. Deforestation, What molecules are the part of photosynthesis). The enzymes that of global climate which removes a lot of photosynthetic outputs? drive the Calvin cycle are dissolved in the change, plant plant life, thereby reduces the ability of the 2. Name the two stages of stroma. ATP generated by the light reactions a tree. biosphere to absorb carbon. Planting new photosynthesis in their provides the energy for sugar synthesis. And forests can have the opposite effect of fixing proper order. and O2 2. light reactions, Calvin cycle the NADPH produced by the light reactions carbon from the atmosphere, potentially reduc- Answers: 1. CO2 and H2O; glucose provides the high-energy electrons that drive the ing the effect of the gases that contribute to global synthesis of glucose from carbon dioxide. Thus, the climate change. The Light Reactions: Converting Solar Energy to Chemical Energy Chloroplasts are solar-powered CO2 Increasing wavelength Light H2O sugar factories. Let’s look at 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1m 103 m how they convert sunlight to NADP+ ADP chemical energy. Light + P Calvin Gamma X-rays UV Infrared Micro- Radio reactions cycle rays waves waves ATP Highest Lowest – – energy energy The Nature of NADPH Sunlight O2 Sugar Visible light Sunlight is a type of energy called radiation or electromagnetic energy. Electromagnetic 380 400 500 600 700 750 energy travels through space as rhythmic waves, like Wavelength (nm) the ripples made by a pebble dropped into a pond. The distance between the crests of two adjacent waves is called a wavelength. The full range of radiation, from the very short wavelengths of gamma rays to the Wavelength = very long wavelengths of radio signals, is called the 580 nm electromagnetic spectrum (Figure 7.4). Visible light is the fraction of the spectrum that our eyes see as differ- ▲ Figure 7.4 The electromagnetic spectrum. The middle of ent colors. the figure expands the thin slice of the spectrum that is visible When sunlight shines on a pigmented material, to us as different colors of light, from about 380 nanometers certain wavelengths (colors) of the visible light are ab- (nm) to about 750 nm in wavelength. The bottom of the figure shows electromagnetic waves of one particular wavelength of sorbed and disappear from the light that is reflected by visible light. the material. For example, we see a pair of jeans as blue because pigments in the fabric absorb the other colors, botanists (biologists who study plants) discovered that leaving only light in the blue part of the spectrum to only certain wavelengths of light are used by plants, as be reflected from the fabric to our eyes. In the 1800s, we’ll see next. 144 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 144 C/M/Y/K M07_SIMO2368_05_GE_CH07.indd 144 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:09 AM  THE LIGHT REACTIONS: Biofuels THE PROCESS OF SCIENCE CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY What Colors of Light Drive Variations of this classic experiment are still per- formed today. For example, biofuel researchers test dif- Photosynthesis? ferent species of algae to determine which wavelengths of light result in optimal fuel production. Biofuel facilities of In 1883, German biologist Theodor Engelmann made the future may use a variety of species that take advantage the observation that certain bacteria living in water of the full spectrum of light that shines down on them. tend to cluster in areas with higher oxygen concentra- tions. He already knew that light passed through Light ◀ Figure 7.5 Investigating how light a prism would separate into the different wavelengths wavelength affects photosynthesis. Prism When algal cells are placed on a microscope (colors). Engelmann soon began to question whether slide, oxygen-seeking bacteria migrate toward he could use this information to determine which wave- algae exposed to certain colors of light. These lengths of light work best for photosynthesis. results suggest that blue-violet and orange-red Microscope slide Engelmann’s hypothesis was that oxygen-seeking bac- wavelengths best drive photosynthesis, while green wavelengths do so only a little bit. teria would congregate near regions of algae performing the most photosynthesis (and hence producing the most oxygen). Engelmann began his experiment by laying a string of freshwater algal cells within a drop of water on a microscope slide. He then added oxygen-sensitive bacte- ria to the drop. Next, using a prism, he created a spectrum Number of bacteria of light and shined it on the slide. His results, summa- Bacteria Bacteria rized in Figure 7.5, showed that most bacteria congre- gated around algae exposed to red-orange and blue-violet light, with very few bacteria moving to the area of green light. Other experiments have since verified that chlo- Algal cells roplasts absorb light mainly in the blue-violet and red- orange part of the spectrum and that those wavelengths of 400 500 600 700 light are the ones mainly responsible for photosynthesis. Wavelength of light (nm) Chloroplast Pigments Light The selective absorption of light by leaves explains why they appear green to us; Reflected light of that color is poorly light absorbed by chloroplasts Chloroplast and is thus reflected or trans- mitted toward the observer (Figure 7.6). Energy cannot be destroyed, so the absorbed energy must be converted to other forms. Chloroplasts contain several different pigments that absorb light of different wavelengths. Absorbed light ▶ Figure 7.6 Why are leaves green? Transmitted Chlorophyll and other pigments in light (detected chloroplasts reflect or transmit green light by your eye) while absorbing other colors. 145 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 145 C/M/Y/K M07_SIMO2368_05_GE_CH07.indd Title: EBP 5e 145 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:09 AM  CHAPTER 7 Chlorophyll a, the pigment that participates directly in When a pigment molecule absorbs a photon, one of PHOTOSYNTHESIS: USING the light reactions, absorbs mainly blue-violet and red the pigment’s electrons gains energy. This electron is LIGHT TO MAKE FOOD light. A very similar molecule, chlorophyll b, absorbs now said to be “excited”; that is, the electron has been mainly blue and orange light. Chlorophyll b does not raised from its starting state (called the ground state) participate directly in the light reactions, but it conveys to an excited state. The excited state is highly unstable, absorbed energy to chlo- so an excited electron usually loses its excess energy rophyll a, which then puts and falls back to its ground state almost immediately the energy to work in the (Figure 7.8a). Most pigments release heat energy as light reactions. their light-excited electrons fall back to their ground Chloroplasts also con- state. (That’s why a surface with a lot of pigment, such tain a family of yellow- as a black driveway, gets so hot on a sunny day.) But orange pigments called some pigments emit light as well as heat after absorbing carotenoids, which absorb mainly blue-green light. Some carotenoids have a Excited state protective function: They The electron falls to its dissipate excess light en- Absorption of a e– ground state, releasing photon excites ergy that would otherwise an electron. heat and light. damage chlorophyll. Some carotenoids are human Heat nutrients: beta-carotene ▲ Figure 7.7 Photosynthetic (a bright orange/red pigment found in pumpkins, sweet pigments. Falling autumn Light potatoes, and carrots) is converted to vitamin A in the temperatures cause a decrease in the levels of body, and lycopene (a bright red pigment found in to- Light green chlorophyll within the matoes, watermelon, and red peppers) is an antioxidant (fluorescence) foliage of leaf-bearing trees. that is being studied for potential anti-cancer properties. This decrease allows the colors Photon Additionally, the spectacular colors of fall foliage in some Ground of the carotenoids to be seen. Chlorophyll state parts of the world are due partly to the yellow-orange molecule light reflected from carotenoids (Figure 7.7). The decreas- ing temperatures in autumn cause a decrease in the levels CHECKPOINT (a) Absorption of a photon of chlorophyll, allowing the colors of the longer-lasting What is the specific name carotenoids to be seen in all their fall glory. of the pigment that absorbs energy during the light All of these chloroplast pigments are built into the reactions? thylakoid membranes (see Figure 7.2). There the pig- Answer: chlorophyll a ments are organized into light-harvesting complexes called photosystems, our next topic. How Photosystems Harvest Light Energy Thinking about light as waves explains most of light’s properties. However, light also behaves as discrete pack- ets of energy called photons. A photon is a fixed quan- tity of light energy. The shorter the wavelength of light, the greater the energy of a photon. A photon of violet light, for example, packs nearly twice as Protecting yourself from much energy as a photon of red light. This is short wavelengths of light why short-wavelength light—such as ultravio- can be lifesaving. (b) Fluorescence of a glow stick. Breaking a vial within a glow let light and X-rays—can be damaging; pho- stick starts a chemical reaction that excites electrons within a tons at these wavelengths carry enough energy fluorescent dye. As the electrons fall from their excited state to the ground state, the excess energy is emitted as light. to damage proteins and DNA, potentially leading to cancerous mutations. ▲ Figure 7.8 Excited electrons in pigments. 146 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 146 C/M/Y/K M07_SIMO2368_05_GE_CH07.indd 146 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:09 AM  THE LIGHT REACTIONS: Chloroplast CONVERTING SOLAR Cluster of pigment ENERGY TO CHEMICAL molecules associated Photon ENERGY with membrane proteins Primary electron acceptor e– Electron Reaction transfer Reaction- center center chlorophyll a Pigment ▲ Figure 7.9 A photosystem: molecules light-gathering molecules that focus light energy onto Thylakoid membrane Transfer of energy a reaction center. Photosystem photons. The fluorescent light emitted by a glow stick molecules functions as a light-gathering antenna. When a is caused by a chemical reaction that excites electrons photon strikes one of the pigment molecules, the energy CHECKPOINT of a fluorescent dye (Figure 7.8b). The excited electrons jumps from molecule to molecule until it arrives at the quickly fall back down to their ground state, releasing reaction center of the photosystem. The reaction center What is the role of a reaction center during energy in the form of fluorescent light. consists of chlorophyll a molecules that sit next to another photosynthesis? In the thylakoid membrane, chlorophyll molecules molecule called a primary electron acceptor. This primary chemical reactions. are organized with other molecules into photosystems. electron acceptor traps the light-excited electron ( e– ) use this trapped energy to drive Each photosystem has a cluster of a few hundred pig- from the chlorophyll a in the reaction center. Another molecules to molecules that can a light-excited photon from pigment ment molecules, including chlorophylls a and b and team of molecules built into the thylakoid membrane then Answer: A reaction center transfers some carotenoids (Figure 7.9). This cluster of pigment uses that trapped energy to make ATP and NADPH. How the Light Reactions Generate ATP and NADPH Primary NADP+ Two photosystems cooperate in the light reactions electron 2e– (Figure 7.10). Photons excite electrons in the chlo- acceptor rophyll of the first photosystem. These photons are Energy 3 to make ATP 2e– then trapped by the primary electron acceptor. Primary electron 2 This photosystem then replaces the lost electrons acceptor – – NADPH by extracting new ones from water. This is the step that releases O2 during photosyn- 2e– Light thesis. Energized electrons from Ele ctro the first photosystem pass down an nt ran spo electron transport chain to the sec- Light rt c hai ond photosystem. The chloroplast n uses the energy released by this electron “fall” to make ATP. The Reaction- center second photosystem transfers its chlorophyll 1 light-excited electrons to NADP+, reducing it to NADPH. Reaction- center Second photosystem H 2O chlorophyll ▶ Figure 7.10 The light reactions 2e– of photosynthesis. The orange First photosystem arrows trace a light-driven flow of electrons from H2O to NADPH. These 2 H+ + 1 O2 2 electrons also produce ATP. 147 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 147 C/M/Y/K M07_SIMO2368_05_GE_CH07.indd Title: EBP 5e 147 Short / Normal S4CARLISLEPublishing Services 25/09/15 10:09 AM  CHAPTER 7 Figure 7.11 shows the location of the light reactions ▼ Figure 7.12 The light reactions illustrated using PHOTOSYNTHESIS: USING in the thylakoid membrane. The two photosystems and a hard-hat analogy. LIGHT TO MAKE FOOD the electron transport chain that connects them transfer electrons from H2O to NADP+, producing NADPH. Notice that the mechanism of ATP production during e– the light reactions is very similar to the mechanism we saw in cellular respiration (see Figure 6.10). In both ATP cases, an electron transport chain pumps hydrogen ions (H+) across a membrane—the inner mitochondrial e– e– – – CHECKPOINT membrane in the case of respiration and the thylakoid NADPH 1. Why is water required as a membrane in photosynthesis. And in both cases, ATP e– reactant in photosynthesis? synthases use the energy stored by the H+ gradient to (Hint: Review Figures 7.10 e– make ATP. The main difference is that food provides the e– and 7.11.) high-energy electrons in cellular respiration, whereas 2. In addition to conveying Photon electrons between light-excited electrons flow down the transport chain Electron photosystems, the during photosynthesis. The traffic of electrons shown transport chain electron transport chains in Figures 7.10 and 7.11 is analogous to the cartoon in of chloroplasts provide Figure 7.12. the energy for the We have seen how the light reactions absorb solar e– synthesis of _________. energy and convert it to the chemical energy of ATP and Photon NADPH. Notice again, however, that the light reactions NAD+ to NADPH. 2. ATP provides electrons for converting Answers: 1. The splitting of water did not produce any sugar. That’s the job of the Calvin cycle, as we’ll see next. First photosystem Second photosystem ▼ Figure 7.11 How the thylakoid membrane converts light energy to the chemical energy of NADPH and ATP. Calvin cycle Light Light – – H+ NADPH ATP + ADP + P NADP H+ Stroma Photosystem Electron transport chain Photosystem Thylakoid ATP membrane synthase Inside thylakoid Electron flow 2e– H+ H+ H 2O H+ H+ H+ 1 O2 2 Thylakoid membrane 148 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 148 C/M/Y/K M07_SIMO2368_05_GE_CH07.indd 148 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:09 AM  The Calvin Cycle: Making Sugar THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE from Carbon Dioxide CO2 (from air) If chloroplasts are CO2 Light H2O solar-powered sugar 1 An enzyme adds each CO2 to a five-carbon sugar called RuBP. The factories, then the + NADP resulting molecule breaks into two Calvin cycle is ADP P three-carbon molecules. + P Calvin Light the actual sugar- reactions cycle ATP 4 Using energy from ATP, RuBP sugar Three-carbon molecule manufacturing – – enzymes rearrange the machinery. This NADPH remaining G3P sugars to regenerate RuBP. ATP process is called a O2 P Sugar cycle because its start- P ing material is regenerated. ADP + P ADP + P Calvin With each turn of the cycle, there are chemical cycle – – inputs and outputs. The inputs are CO2 from NADPH ATP the air as well as ATP and NADPH produced by the light reactions. Using carbon from CO2, en- NADP+ ergy from ATP, and high-energy electrons from G3P sugar G3P sugar NADPH, the Calvin cycle constructs an energy- P P rich sugar molecule called glyceraldehyde 3-phosphate (G3P). The plant cell can then use 3 For every three molecules of CO2 2 Using energy from ATP and NADPH G3P as the raw material to make the glucose that enter the cycle, the net output is produced by the light reactions, one G3P sugar. The other G3P sugars enzymes convert each three-carbon and other organic compounds (such as cellulose continue in the cycle. molecule to the three-carbon sugar G3P. and starch) that it needs. Figure 7.13 presents the basics of the Calvin cycle, emphasizing in- puts and outputs. Each symbol represents a ▲ Figure 7.13 The Calvin cycle. With carbon atom, and each P symbol represents a the energy of ATP and the electrons phosphate group. G3P sugar from NADPH, the carbons from CO2 are used to make a three-carbon sugar Glucose (and P other organic called G3P. compounds) Biofuels EVOLUTION CONNECTION Creating a Better Biofuel Biomechanical engineers are working to solve this Factory dilemma by turning to an obvious example: evolution by natural selection. In nature, organisms with genes that Throughout this chapter, we’ve studied how plants make them better suited to their local environment will, convert solar energy to chemical energy via photosyn- on average, more often survive and pass those genes on thesis. Such transformations are vital to our welfare and to the next generation. Repeated over many generations, to Earth’s ecosystems. As discussed in the Biology and genes that enhance survival within that environment CHECKPOINT Society section, scientists are attempting to tap into the will become more common, and the species evolves. What is the function of “green energy” of photosynthesis to produce biofuels. When trying to solve an engineering problem, sci- NADPH in the Calvin cycle? But the production of biofuels is highly inefficient. In entists can impose their own desired outcomes using form G3P (a sugar). electrons that are added to CO2 to fact, it is usually far more costly to produce biofuels than a process called directed evolution (see the Process Answer: It provides the high-energy to extract the equivalent amount of fossil fuels. of Science section in Chapter 5 for another example). 149 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 149 C/M/Y/K M07_SIMO2368_05_GE_CH07.indd Title: EBP 5e 149 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:09 AM  CHAPTER 7 During this process, scientists in the laboratory ▼ Figure 7.14 Microscopic biofuel factories. This researcher is PHOTOSYNTHESIS: USING (instead of the natural environment) determine monitoring a reaction chamber in which microscopic algae are using LIGHT TO MAKE FOOD light to produce biofuels. which organisms are the fittest. Directed evolu- tion of biofuel production often involves mi- croscopic algae (Figure 7.14) rather than plants because algae are easier to manipulate and main- tain within the laboratory. Furthermore, some al- gae produce nearly half their own body weight in hydrocarbons that are only a few chemical steps away from useful biofuels. In a typical directed evolution experiment, the researcher starts with a large collection of individual alga—sometimes naturally occurring species and sometimes transgenic algae that have been engineered to carry useful genes, such as fungal genes for enzymes that break down cellulose. The algae are exposed to mutation- promoting chemicals. This produces a highly varied collection of algae that can be screened for the desired outcome: the ability to produce the most useful biofuel in the largest quantity. The tiny fraction of total algae that can best per- form this task is grown and subjected to another round of mutation and selection. After many repetitions, the algae may slowly improve their ability to efficiently produce biofuels. Many re- search laboratories—some within major petro- leum companies—are using such methods and may someday produce an alga that can provide the ultimate source of green energy, an achieve- ment that would highlight how lessons from natural evolution can be applied to improve our lives. Chapter Review Energy Transformation: An Overview of Photosynthesis SUMMARY OF KEY CONCEPTS Light energy The Basics of Photosynthesis Photosynthesis is a process whereby light energy is transformed into chemical energy stored as bonds in sugars made from carbon dioxide and water. 6 CO2 + 6 H2O C6H12O6 + 6 O2 Photosynthesis Carbon Water Glucose Oxygen gas Chloroplasts: Sites of Photosynthesis dioxide Chloroplasts contain a thick fluid called stroma surrounding a network of membranes called thylakoids. 150 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 150 C/M/Y/K M07_SIMO2368_05_GE_CH07.indd 150 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:09 AM