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5 Photosynthesis Learning Outline 5.1 Life Depends on Photosynthesis 5.2 Photosynthetic Pigments Capture Sunlight 5.3 Chloroplasts Are the Sites of Photosynthesis 5.4 Photosynthesis Occurs in Two Stages 5.5 The Light Reactions Begin Photosynthesis A. Photosystem II Produces ATP B. Photosystem I Produces NADPH 5.6 The Carbon Reactions Produce Carbohydrates 5.7 C3, C4, and CAM Plants Use Different Carbon Fixation Pathways 5.8 Investigating Life: Solar-Powered Sea Slugs Food from Plants. A farmer works his way through a rice terrace in China. Rice grains are a food staple for much of the world’s population. Learn How to Learn A Quick Once-Over Unless your instructor requires you to read your textbook in detail before class, try a quick preview. At the very least, read the Learning Outline to identify the main ideas. It is also a good idea to look at the figures and the key terms in the narrative. Previewing a chapter should help you understand the lecture, because you will already know the main ideas. In addition, note-taking will be easier if you recognize new vocabulary words from your quick once-over. Return to your book for an in-depth reading after class to help nail down the details. hoe96928_ch05.indd 2 5/19/11 12:50 PM 5.1 Life Depends on Photosynthesis What’s the Point? Most plants are easy to grow (compared with animals, anyway) because their needs are simple. Give a plant water, essential elements in soil, carbon dioxide, and light, and it will produce food and oxygen. These products build the plant’s body and sustain its life. Meanwhile, animals and other consumers eat plants. A leafy foundation therefore supports Earth’s ecosystems. How can plants do so much with such simple raw materials? The answer lies in chloroplasts, microscopic solar panels inside each green cell. This chapter explains how chloroplasts use the sun’s energy to conjure sugar out of thin air. It is spring. A seed germinates, its tender roots and pale yellow stem extending rapidly in a race against time. For now, the seedling’s sole energy source is food stored in the seed itself. If the shoot does not reach light before its reserves run out, the seedling will die. But if it makes it, green leaves will unfurl and catch the light. The seedling begins to feed itself, and an independent new life begins. The plant is an autotroph (“self feeder”), meaning it uses inorganic substances such as water and carbon dioxide (CO2 ) to produce organic compounds. The opposite of an autotroph is a heterotroph, which is an organism that obtains carbon by consuming preexisting organic molecules. You are a heterotroph, and so are all other animals, all fungi, and many microorganisms. Organisms that can produce their own food underlie every ecosystem on Earth. It is not surprising, therefore, that if asked to designate the most important metabolic pathway, most biologists would not hesitate to cite photosynthesis: the process by which plants, algae, and some microorganisms harness solar energy and convert it into chemical energy. Photosynthesis is a series of chemical reactions that use light energy to assemble CO2 into glucose (C6H12O6), the carbohydrate that feeds plants (figure 5.1). The plant uses water in the process and releases oxygen gas (O2) as a byproduct. These chemical reactions are summarized as follows: light energy 6CO2 + 6H2O ⎯→ C6H12O6 + 6O2 Photosynthesis Carbon dioxide and water consumed CO2 + H2O + light energy Glucose and oxygen produced C6H12O6 + O2 Leaf cell Chloroplasts TEM 15 μm (false color) Figure 5.1 Sugar from the Sun. In photosynthesis, a plant produces glucose and O2 from simple starting materials: carbon dioxide, water, and sunlight. This process provides not only food for the plant but also the energy, raw materials, and oxygen for most heterotrophs (see figure 4.2). Animals, fungi, and other consumers eat the leaves, stems, roots, flowers, nectar, fruits, and seeds of the world’s producers. Even the waste product of photosynthesis, O2, is essential to most life on Earth. Because humans live on land, we are most familiar with the contribution that plants make to Earth’s terrestrial ecosystems. In fact, however, more than half of the world’s photosynthesis occurs in the oceans, courtesy of countless algae and bacteria. On land or in the water, Earth without photosynthesis would not be a living world for long. If the sky were blackened by a nuclear holocaust, cataclysmic volcanic eruption, or massive meteor impact, the light intensity reaching Earth’s surface would decline to about a tenth of its normal level. Photosynthetic organisms would die as they depleted their energy reserves faster than they could manufacture more food. Animals that normally ate these producers would go hungry, as would the animals that ate them. A year or even two might pass before enough life-giving light could penetrate the hazy atmosphere, but by then, it would be too late. The lethal chain reaction would already be well into motion, destroying food webs at their bases. No wonder biologists consider photosynthesis to be Earth’s most important metabolic process. 5.1 Mastering Concepts 1. How is an autotroph different from a heterotroph? 2. What is photosynthesis? 3. Why is photosynthesis essential to life? 3 hoe96928_ch05.indd 3 5/19/11 12:50 PM 4 UNIT 1 Science, Chemistry, and Cells Short wavelength (high energy) 5.2 Photosynthetic Pigments Capture Sunlight Gamma rays Visible light 400 Each minute, the sun converts more than 100 metric tons of matter to energy, releasing much of it outward as waves Blue of electromagnetic radiation. After an 8-minute journey, Cyan 500 Portion of Ultraviolet about two billionths of this energy reaches Earth’s upper Green spectrum radiation 550 atmosphere. Of this, only about 1% is used for photosynthat Yellow thesis, yet this tiny fraction of the sun’s power ultimately 600 reaches Orange Infrared Earth's produces nearly 2 quadrillion kilograms of carbohydrates 650 radiation Wavelength surface a year! Light may seem insubstantial, but it is a powerful 700 force on Earth. Microwaves Visible light is a small sliver of a much larger 750 electromagnetic spectrum, the range of possible frequencies of radiation (figure 5.2). All electromagnetic radiation, including light, consists of Radio waves photons, discrete packets of kinetic energy. A photon’s wavelength is the distance it moves during a complete vibration. The shorter a photon’s Long wavelength (low energy) wavelength, the more energy it contains. The visible light that provides Figure 5.2 The Electromagnetic Spectrum. Sunlight reaching the energy that powers photosynthesis is in the middle range of the elecEarth consists of ultraviolet radiation, visible light, and infrared tromagnetic spectrum. We perceive visible light of different wavelengths radiation, all of which is just a small part of a continuous spectrum as distinct colors. of electromagnetic radiation. The shorter the wavelength, the more Plant cells contain several pigment molecules that capture light energy (see energy associated with the radiation. this chapter’s Burning Question). The most abundant is chlorophyll a, a green photosynthetic pigment in plants, algae, and cyanobacteria. Photosynthetic organisms usually also have several types of accessory pigments, which are Chlorophyll a Sunlight Chlorophyll b energy-capturing pigment molecules other than chlorophyll a. Chlorophyll b Reflected 80 Carotenoids and carotenoids are accessory pigments in plants. light The photosynthetic pigments have distinct colors because they absorb 60 only some wavelengths of visible light, while transmitting or reflecting a. others (figure 5.3). Chlorophylls a and b absorb red and blue wavelengths; they appear green because they reflect green light. Carotenoids, on the 40 other hand, reflect longer wavelengths of light, so they appear red, orange, or yellow. (Carrots, tomatoes, lobster shells, and the flesh of salmon all owe their distinctive colors to carotenoid pigments, which the animals 20 must obtain from their diets.) Only absorbed light is useful in photosynthesis. Accessory pigments ab0 sorb wavelengths that chlorophyll a cannot, so they extend the range of light 400 500 600 700 wavelengths that a cell can harness. This is a little like the members of the Wavelength of light (nanometers) b. same team on a quiz show, each contributing answers from a different area of expertise. Figure 5.3 Everything but Green. (a) Chlorophyll molecules Violet 450 Relative absorption (percent) Wavelength in nanometers X-rays reflect green and yellow wavelengths of light and absorb the other wavelengths. (b) Each pigment absorbs some wavelengths of light and reflects others. 5.2 Mastering Concepts 1. What is the relationship between visible light and the electromagnetic spectrum? 2. How does it benefit a plant to have multiple types of pigments? Figure It Out If you could expose plants to just one wavelength of light at a time, would a wavelength of 300 nm, 450 nm, or 600 nm produce the highest photosynthetic rate? Answer: 450 nm Life Depends on Photosynthesis hoe96928_ch05.indd 4 Photosynthetic Pigments Capture Sunlight Chloroplasts Are the Sites of Photosynthesis Photosynthesis Occurs in Two Stages 5/19/11 12:50 PM 5 Chapter 5 Photosynthesis Leaf 5.3 Chloroplasts Are the Sites of Photosynthesis In plants, leaves are the main organs of photosynthesis. Their broad, flat surfaces expose abundant surface area to sunlight. But light is just one requirement for photosynthesis. Water is essential, too; roots absorb this vital ingredient, which moves up stems and into the leaves. Plants also must exchange CO2 and O2 with the atmosphere through stomata (singular: stoma), tiny openings in the epidermis of a leaf or stem. (The word stoma comes from the Greek word for “mouth”). leaf epidermis, p. 000 a. Most photosynthesis occurs in cells filling the leaf’s interior (figure 5.4). Mesophyll is the collective term for these internal cells (meso- means “middle,” and -phyll means “leaf”). Leaf mesophyll cells contain abundant chloroplasts, the organelles of photosynthesis in plants and algae. Most photosynthetic cells contain 40 to 200 chloroplasts, which add up to about 500,000 per square millimeter of leaf—an impressive array of solar energy collectors. Each chloroplast contains tremendous surface area for the reactions of photosynthesis. Two membranes enclose the stroma, a gelatinous fluid containing ribosomes, DNA, and enzymes. (Be careful not to confuse the stroma with a stoma, or leaf pore). Suspended in the stroma of each chloroplast are between 10 and 100 grana (singular: granum), each composed of a stack of 10 to 20 disk-shaped thylakoids. Each thylakoid, in turn, consists of a membrane studded with photosynthetic pigments and enclosing a volume called the thylakoid space. Mesophyll cells Stoma CO2 O2 + H2O Mesophyll cell Nucleus Central vacuole Mitochondrion Chloroplasts TEM 15 μm (false color) b. Granum Chloroplast Thylakoid Thylakoid space DNA Pigment molecules embedded in thylakoid membrane Outer membrane Inner membrane Granum d. Stroma Ribosomes c. The Light Reactions Begin Photosynthesis hoe96928_ch05.indd 5 The Carbon Reactions Produce Carbohydrates Figure 5.4 Leaf and Chloroplast Anatomy. Leaf mesophyll tissue consists of cells that contain many chloroplasts. C3, C4, and CAM Plants Investigating Life: Solar-Powered Sea Slugs 5/19/11 12:50 PM 6 UNIT 1 Science, Chemistry, and Cells Chlorophyll H2 C H3C CH N N Chloroplast Thylakoid CH2CH3 N Mg N H3 C CH3 CH2 CH2 CO CH O 2 3 O C O CH2 CH C CH3 CH2 CH2 CH2 HC CH3 CH2 Hydrophobic CH2 tail CH2 HC CH3 CH2 CH2 CH2 HC CH3 CH3 Thylakoid membrane Proteins Chlorophyll Thylakoid space Photosystem Figure 5.5 Thylakoid Membrane. This diagram of a photosystem shows a complex grouping of proteins and pigments (including chlorophyll) embedded in the chloroplast’s thylakoid membrane. Light CO2 H2O Chloroplast ATP Light reactions NADPH NADP+ Carbon reactions ADP O2 Glucose Figure 5.6 Overview of Photosynthesis. In the light reactions, pigment molecules capture sunlight energy and transfer it to molecules of ATP and NADPH. The carbon reactions use this energy to build glucose out of carbon dioxide. Life Depends on Photosynthesis hoe96928_ch05.indd 6 The pigments and proteins that participate in photosynthesis are grouped into photosystems in the thylakoid membrane (figure 5.5). One photosystem consists of chlorophyll a aggregated with other pigment molecules and the proteins that anchor the entire complex in the membrane. Within each photosystem are some 300 chlorophyll molecules and 50 accessory pigments. Although all of the pigment molecules absorb light energy, only one chlorophyll a molecule per photosystem actually uses the energy in photosynthetic reactions. The photosystem’s reaction center is this chlorophyll a molecule and its associated proteins. All other pigment molecules in the photosystem are called antenna pigments because they capture photon energy and funnel it to the reaction center. If the different pigments are like a quiz show team, then the reaction center is analogous to the one member who announces the team’s answer to the show’s moderator. Why does only one chlorophyll molecule out of a few hundred actually participate in photosynthetic reactions? A single chlorophyll a molecule can absorb only a small amount of light energy. Several pigment molecules near each other capture much more energy because they can pass the energy on to the reaction center, freeing them to absorb other photons as they strike. Thus, the photosystem’s organization greatly enhances the efficiency of photosynthesis. 5.3 Mastering Concepts 1. Describe the relationship among the chloroplast, stroma, grana, and thylakoids. 2. How does the reaction center chlorophyll interact with the antenna pigments in a photosystem? 5.4 Photosynthesis Occurs in Two Stages Inside a chloroplast, photosynthesis occurs in two stages: the light reactions and the carbon reactions. Figure 5.6 summarizes the entire process, and sections 5.5 and 5.6 describe each part in greater detail. The light reactions convert solar energy to chemical energy. (You can think of the light reactions as the “photo-” part of photosynthesis.) In the chloroplast’s thylakoid membranes, pigment molecules in two linked photosystems capture kinetic energy from photons and store it as potential energy in the chemical bonds of two molecules: ATP and NADPH. Recall from chapter 4 that ATP is a nucleotide that stores potential energy in the covalent bonds between its phosphate groups. ATP forms when a phosphate group is added to ADP (see figure 4.00). The other energy-rich product of the light reactions, NADPH, is a molecule that carries pairs of energized electrons. In photosynthesis, these electrons come from chlorophyll molecules. Once the light reactions are underway, chlorophyll, in turn, replaces its “lost” electrons by splitting water molecules, yielding O2 as a waste product. These two resources (energy and “loaded” electron carriers) set the stage for the second part of photosynthesis. The carbon reactions use ATP and the high-energy electrons in NADPH to reduce CO2 to glucose molecules. (These reactions are the “-synthesis” part of photosynthesis.) The ATP and NADPH Photosynthetic Pigments Capture Sunlight Chloroplasts Are the Sites of Photosynthesis Photosynthesis Occurs in Two Stages 5/19/11 12:50 PM 7 Chapter 5 Photosynthesis come from the light reactions, and the CO2 comes from the atmosphere. Once inside the leaf, CO2 diffuses into a mesophyll cell and across the chloroplast membrane into the stroma, where the carbon reactions occur. Overall, photosynthesis is an oxidation–reduction (redox) process. “Oxidation” means that electrons are removed from an atom or molecule; “reduction” means electrons are added. As you will see, photosynthesis strips electrons from the oxygen atoms in H2O (i.e., the oxygen atoms are oxidized). These electrons reduce the carbon in CO2. Because oxygen atoms attract electrons more strongly than do carbon atoms (see chapter 2), moving electrons from oxygen to carbon requires energy. The energy source for this reaction is, of course, light. redox reactions, p. 000 5.4 Mastering Concepts Light 1. What happens in each of the two main stages of photosynthesis? 2. Where in the chloroplast does each stage occur? CO2 H2O Chloroplast ATP Light reactions The Light Reactions Begin Photosynthesis hoe96928_ch05.indd 7 O2 Photosystem II Light energy Electron transport chain H+ Reaction center chlorophyll Photosystem I NADP+ NADPH 6 Reaction center chlorophyll 3 2e– 1/2 Electron transport chain 5 Pigment molecules 2 Glucose Light energy H+ Stroma 1 H2O Carbon reactions ADP 5.5 The Light Reactions Begin Photosynthesis A plant placed in a dark closet literally starves. Without light, the plant cannot generate ATP or NADPH. And without these critical energy and electron carriers, the plant cannot feed itself. Once its stored reserves are gone, the plant dies. The plant’s life thus depends on the light reactions of photosynthesis, which occur in the membranes of chloroplasts. We have already seen that the pigments and proteins of the chloroplast’s thylakoid membranes are organized into photosystems (see figure 5.5). More specifically, the thylakoid membranes contain two types of photosystems, dubbed I and II. An electron transport chain connects the two photosystems. Recall from chapter 4 that an electron transport chain is a group of proteins that shuttle electrons like a bucket brigade, releasing energy with each step. As you will see, the electron transport chain that links photosystems I and II stores potential energy used in ATP synthesis. A second electron transport chain extending from photosystem I ends with the production of NADPH. Figure 5.7 depicts the arrangement of the photosystems and electron transport chains in the thylakoid membrane. Refer to this illustration as you work through the rest of this section. NADPH NADP+ H+ O2 + 2H+ H+ H+ Thylakoid space 4 Stroma ATP synthase H+ ADP + P ATP Figure 5.7 Two Photosystems Participate in the Light Reactions. [1] Chlorophyll molecules in photosystem II transfer light energy to electrons. [2] The electrons are stripped from water molecules, releasing oxygen. [3] The energized electrons pass to photosystem I via an electron transport chain. Each transfer releases energy that is used to pump hydrogen ions into the thylakoid space. [4] The resulting hydrogen gradient is used to generate ATP. [5] In photosystem I, the electrons absorb more light energy and [6] are passed to NADP+, creating the energy-rich NADPH. The Carbon Reactions Produce Carbohydrates C3, C4, and CAM Plants Investigating Life: Solar-Powered Sea Slugs 5/19/11 12:50 PM 8 UNIT 1 Science, Chemistry, and Cells A. Photosystem II Produces ATP Burning Questions Why do leaves change colors in the fall? Most leaves are green throughout a plant’s growing season, although there are exceptions; some ornamental plants, for example, have yellow or purple leaves. The near-ubiquitous green color comes from chlorophyll a, the most abundant pigment in photosynthetic plant parts. But the leaf also has other photosynthetic pigments. Carotenoids contribute brilliant yellow, orange, and red hues. Purple pigments, such as anthocyanins, are not photosynthetically active, but they do protect leaves from damage by ultraviolet radiation. These accessory pigments are less abundant than chlorophyll, so they usually remain invisible to the naked eye during the growing season. As winter approaches, however, deciduous plants prepare to shed their leaves. The chlorophyll degrades, and the now “unmasked” accessory pigments reveal their colors for a short time as a spectacular autumn display. These pigments soon disappear as well, and the dead leaves turn brown. Spring brings a flush of fresh, green leaves. The energy to produce the foliage comes from glucose the plant produced during the last growing season and stored as starch. The new leaves make food throughout the spring and summer, so the tree can grow— both above ground and below—and produce fruits and seeds. As the days grow shorter and cooler in autumn, the cycle will continue, and the colorful pigments will again participate in one of nature’s great disappearing acts. Submit your burning question to: [email protected] Photosynthesis begins in the cluster of pigment molecules of photosystem II. These pigments absorb light and transfer the energy to a chlorophyll a reaction center, where it boosts two electrons to a higher energy level. The “excited” electrons, now packed with potential energy, are ejected from this chlorophyll a molecule and grabbed by the first protein in the electron transport chain that links the two photosystems (figure 5.7, step 1). electron orbitals, p. 000 How does the chlorophyll a molecule replace these two electrons? They come from water (H2O), which donates two electrons when it splits into oxygen gas and two protons (H+). Chlorophyll a picks up the electrons, and O2 is the waste product that the plant releases to the environment (step 2). Meanwhile, the chloroplast uses the potential energy in the electrons to create a proton gradient (step 3). As the electrons pass along the electron transport chain, the energy they lose drives the active transport of protons from the stroma into the thylakoid space. The resulting proton gradient between the stroma and the inside of the thylakoid represents a form of potential energy. active transport, p. 000 An enzyme complex called ATP synthase transforms the gradient’s potential energy into chemical energy in the form of ATP (step 4). A channel in ATP synthase allows protons trapped inside the thylakoid space to return to the chloroplast’s stroma. As the gradient dissipates, energy is released. The ATP synthase enzyme uses this energy to add phosphate to ADP, generating ATP. (As described in chapter 6, the same process also produces ATP in cellular respiration). This mechanism is similar to using a dam to produce electricity. As water accumulates, tremendous pressure (a form of potential energy) builds on the face of the dam. That pressure is released by diverting water through a large pipe at the base of the dam, turning massive blades that spin an electric generator. B. Photosystem I Produces NADPH Photosystem I functions much as photosystem II does. Photon energy strikes energy-absorbing molecules of chlorophyll a, which pass the energy to the reaction center. The reactive chlorophyll molecules eject electrons to an electron carrier molecule in a second electron transport chain (figure 5.7, step 5). The boosted electrons in photosystem I are then replaced with electrons passing down the first electron transport chain from photosystem II. Unlike in photosystem II, however, the second electron transport chain does not generate ATP, nor does it pass its electrons to yet another photosystem. Instead, the electrons reduce a molecule of NADP+ to NADPH (step 6). This NADPH is the electron carrier that will reduce carbon dioxide in the carbon reactions, while the ATP generated in photosystem II will provide the energy. 5.5 Mastering Concepts 1. Describe the events in photosystem II, beginning with light and ending with the production of ATP. 2. How do electrons pass from photosystem II to photosystem I? 3. How are the electrons from photosystem II replaced? 4. What happens in photosystem I? Life Depends on Photosynthesis hoe96928_ch05.indd 8 Photosynthetic Pigments Capture Sunlight Chloroplasts Are the Sites of Photosynthesis Photosynthesis Occurs in Two Stages 5/19/11 12:50 PM 9 Chapter 5 Photosynthesis 5.6 The Carbon Reactions Produce Carbohydrates Light H2O CO2 Chloroplast The carbon reactions, also called the Calvin cycle, occur in the chloroplast’s ATP stroma. The Calvin cycle is the metabolic pathway that uses NADPH and ATP from the light reactions to assemble CO2 molecules into three-carbon carbohyNADPH Light Carbon drate molecules (figure 5.8). These products are eventually assembled into glureactions reactions NADP+ cose and other sugars. ADP The first step of the Calvin cycle is carbon fixation—the initial incorporation of carbon from CO2 into an organic compound. Specifically, CO2 combines with ribulose bisphosphate (RuBP), a five-carbon sugar with two O2 Glucose phosphate groups. An enzyme called rubisco catalyzes this first reaction. The six-carbon product of the initial reaction immediately breaks down 3 CO2 into two three-carbon molecules (PGA). Further steps in the cycle convert PGA to a carbohydrate called phosphoglyceraldehyde (PGAL). Some of the PGAL is rearranged to form additional Rubisco RuBP, continuing the cycle. But the cell can also use PGAL enzyme to build larger carbohydrates such as glucose and sucrose, the most familiar products of photosynthesis. Several fates await the carbohydrates produced CARBON FIXATION in the carbon reactions. A plant’s cells use about P 3 P 3 P P half of the glucose as fuel for their own cellular 1 Carbon dioxide is added Unstable intermediates RuBP respiration, the metabolic pathway described in to RuBP, creating an unstable molecule. chapter 6. Roots, flowers, fruits, seeds, and other nonphotosynthetic plant parts could not grow without sugar shipments from green leaves and 6 P REGENERATION stems. Plants also combine glucose with other PGAL SYNTHESIS PGA From light OF RuBP substances to manufacture additional comreactions 4 RuBP is regenerated 2 The unstable pounds, including amino acids and a host of ecintermediate splits by rearranging the 6 ATP onomically important products such as rubber, remaining molecules. to form PGAL. 3 ADP medicines, and spices. 6 NADPH Moreover, glucose molecules are the building 6 NADP+ blocks of the cellulose wall that surrounds every 3 ATP 6 ADP + 6 P plant cell. Wood is mostly made of cellulose. The timP P 5 6 ber in the world’s forests therefore stores enormous PGAL PGAL amounts of carbon. So do vast deposits of coal and other fossil fuels, which are the remains of plants and other organisms that lived long ago. Burning wood or fossil fuels releases this stored carbon into the atmosphere as CO2. As the PGAL 3 PGAL molecules amount of CO2 in the atmosphere has increased, Earth’s average temfrom are combined P other perature has risen. global climate change, p. 000 to form glucose, 1 PGAL turns which is used If a plant produces more glucose than it immediately needs for respiraof the to form starch, tion or building cell walls, it may store the excess as starch. CarbohydrateCalvin sucrose, and other cycle rich tubers and grains, such as potatoes, rice, corn, and wheat, are all organic molecules. energy-storing plant organs. Some plants, including sugarcane and sugar beets, Glucose store energy as sucrose instead. Table sugar comes from these crops. In addition, people use starch (from corn kernels) and sugar (from sugarcane) to proFigure 5.8 The Calvin Cycle. ATP and NADPH from the light duce biofuels such as ethanol. biofuels, p. 000 ( ) reactions power the Calvin cycle, simplified here. The cycle generates a three-carbon molecule, PGAL, which is used to build glucose and other carbohydrates. 5.6 Mastering Concepts 1. What happens in the carbon reactions? 2. What are the roles of CO2, ATP, and NADPH in the Calvin cycle? The Light Reactions Begin Photosynthesis hoe96928_ch05.indd 9 The Carbon Reactions Produce Carbohydrates C3, C4, and CAM Plants Investigating Life: Solar-Powered Sea Slugs 5/19/11 12:50 PM 10 UNIT 1 Science, Chemistry, and Cells Why We Care Weed Killers One low-tech way to kill an unwanted plant is to deprive it of light. Gardeners who want to convert a lawn into a garden, for example, might kill the grass by covering it with layers of newspaper or cardboard for several weeks. The light reactions of photosynthesis cannot occur in the dark; the plants die. Many herbicides also stop the light reactions. For example, a weed killer called diuron blocks electron flow in photosystem II. Paraquat, noted for its use in destroying marijuana plants, diverts electrons from photosystem I. Other herbicides take a different approach. Accessory pigments called carotenoids protect plants from damage caused by free radicals. Triazole herbicides kill plants by blocking carotenoid synthesis. No longer protected from free-radical damage, the cell’s organelles are destroyed. Still other weed killers exploit pathways not directly related to photosynthesis. For instance, glyphosate (Roundup) inhibits an enzyme that plants require for amino acid synthesis. Another herbicide, 2,4-D, mimics a plant hormone called auxin (see chapter 22). C3 plant Stoma BundleVein sheath (vascular tissue) Mesophyll cell cell C4 plant Vein Bundle(vascular sheath cell Mesophyll tissue) cell Stoma Figure 5.9 C3 and C4 Leaf Anatomy. In C3 plants, the light reactions and the Calvin cycle occur in mesophyll cells. In C4 plants, the light reactions occur in mesophyll, but the inner ring of bundle-sheath cells houses the Calvin cycle. Life Depends on Photosynthesis hoe96928_ch05.indd 10 5.7 C3, C4, and CAM Plants Use Different Carbon Fixation Pathways The Calvin cycle is also known as the C3 pathway because a three-carbon molecule, PGA, is the first stable compound in the pathway. Although all plants use the Calvin cycle, C3 plants use only this pathway to fix carbon from CO2. About 95% of plant species are C3, including cereals, peanuts, tobacco, spinach, sugar beets, soybeans, most trees, and some lawn grasses. C3 photosynthesis is obviously a successful adaptation, but it does have a weakness: inefficiency. Photosynthesis has a theoretical efficiency rate of 30%, but on cloudy days, individual plants average only from 0.1% to 3% photosynthetic efficiency. How do plants waste so much solar energy? One contributing factor is a metabolic pathway called photorespiration, a series of reactions that begin when the rubisco enzyme uses O2 instead of CO2 as a substrate. The net result of photorespiration is that the plant loses CO2 that it has already fixed, wasting both ATP and NADPH. Photorespiration is most likely in hot, dry climates. Plants in these habitats therefore face a trade-off. If the stomata remain open too long, a plant may lose water, wilt, and die. If the plant instead closes its stomata, CO2 supplies in the leaves run low while O2 builds up. Under those conditions, photorespiration becomes much more likely, and photosynthetic efficiency plummets. Plants may lose as much as 30% of their fixed carbon to this pathway, which has no known benefit. In hot climates, plants that minimize photorespiration may therefore have a significant competitive advantage. One way to improve efficiency is to ensure that rubisco always encounters high CO2 concentrations. The C4 and CAM pathways are two adaptations that do just that. C4 plants physically separate the light reactions and the carbon reactions into different cells (figure 5.9). The light reactions occur in mesophyll cells, as does a carbon-fixation reaction called the C4 pathway. In the C4 pathway, CO2 combines with a three-carbon molecule to form a four-carbon compound (hence the name C4). This molecule then moves into adjacent bundle-sheath cells that surround the leaf veins. The CO2 is liberated inside these cells, where the Calvin cycle fixes the carbon a second time by the C3 pathway. Unlike mesophyll cells, bundle-sheath cells are not exposed directly to atmospheric O2. The rubisco in bundle-sheath cells is therefore much more likely to bind CO2 instead of O2, reducing photorespiration. Meanwhile, at the cost of two ATP molecules, the threecarbon “ferry” returns to the mesophyll to pick up another CO2. About 1% of plants use the C4 pathway. All are flowering plants growing in hot, open environments, including crabgrass and crop plants such as sugarcane and corn. C4 plants are less abundant, however, in cooler, moister habitats. In those environments, the ATP cost of ferrying each CO2 from a mesophyll cell to a bundle-sheath cell apparently exceeds the benefits of reduced photorespiration. Another energy- and water-saving strategy, called crassulacean acid metabolism (CAM), occurs in about 3% to 4% of plant species, including pineapple and cacti. Plants that use the CAM pathway open their stomata to fix CO2 only at night, when the temperature drops and the humidity rises. CO2 diffuses in. Mesophyll cells incorporate the CO2 into a four-carbon compound, which they store in large vacuoles. The stomata close during the heat of the day, but the stored molecule moves from the vacuole to a chloroplast and releases its CO2. The chloroplast then fixes the CO2 in the Calvin cycle. The CAM pathway reduces photorespiration by generating high CO2 concentrations inside chloroplasts. Photosynthetic Pigments Capture Sunlight Chloroplasts Are the Sites of Photosynthesis Photosynthesis Occurs in Two Stages 5/19/11 12:50 PM 11 Chapter 5 Photosynthesis All CAM plants are adapted to dry habitats. In cool environments, however, CAM plants cannot compete with C3 plants. Their stomata are only open at night, so CAM plants have much less carbon available to their cells for growth and reproduction. Figure 5.10 compares and contrasts C3, C4, and CAM plants. 5.7 Mastering Concepts 1. 2. 3. 4. Why is the Calvin cycle also called the C3 pathway? How does photorespiration counter photosynthesis? Describe how a C4 plant minimizes photorespiration. How is the CAM pathway like C4 metabolism, and how is it different? C4 plant C3 plant CAM plant Example CO2 CO2 or O2 Night Mesophyll cell Mesophyll cell 4-carbon molecule 4-carbon molecule Mesophyll cell Calvin cycle Pathway Bundlesheath cell CO2 CO2 CO2 Calvin cycle Calvin cycle Glucose Glucose Glucose Day Limitation How plant avoids photorespiration Habitat % of plant species Figure 5.10 11 ATP cost Reduced carbon availability N/A Light reactions and carbon reactions occur in separate cells. Cool, moist Hot, dry Hot, dry 95% 1% 3–4% CO2 is absorbed at night; light reactions and carbon reactions occur during the day. C3, C4, and CAM Pathways Compared. The C4 and CAM pathways are adaptations that minimize photorespiration. The Light Reactions Begin Photosynthesis hoe96928_ch05.indd Photorespiration The Carbon Reactions Produce Carbohydrates C3, C4, and CAM Plants Investigating Life: Solar-Powered Sea Slugs 5/19/11 12:51 PM 12 UNIT 1 Science, Chemistry, and Cells Investigating Life 5.8 Solar-Powered Sea Slugs Most animals have an indirect relationship with photosynthesis: autotrophs make food, which animals eat. But Elysia chlorotica is an unusual animal (figure 5.11). This sea slug, which lives in salt marshes along the eastern coast of North America, is solar-powered: it has chloroplasts in the lining of its gut. These invertebrate animals do not inherit their solar panels from their parents; instead, they acquire the chloroplasts by eating algae. As a young sea slug grazes, it punctures the filaments of the algae and sucks out the contents. The animal digests most of the nutrients, but cells lining the slug’s gut absorb the chloroplasts. The organelles stay there for the rest of the animal’s life. Like a plant, the solar-powered sea slug can live on sunlight and air. Head The Question: A chloroplast requires a few thousand genes to carry out photosynthesis, yet chloroplast DNA encodes less than 10% of the required proteins. DNA in a plant cell’s nucleus makes up the difference. But slugs are animals, so their nuclei presumably lack these genes. How can the chloroplasts operate inside their mollusk partners? Digestive tract Figure 5.11 A Slug with Solar Panels. The sea slug Elysia chlorotica owes its green color to chloroplasts harvested from algae. Alga Water DNA Slug (control) ladder The Approach: Mary E. Rumpho, of the University of Maine, collaborated with James R. Manhart, of Texas A&M University, to find out the answer. They considered two possibilities. Either the chloroplasts can work inside the host slug’s digestive tract without the help of supplemental genes, or the slug’s own cells provide the necessary proteins. The researchers searched the chloroplast’s DNA for genes essential to photosynthesis and found that a gene that encodes part of photosystem II was missing. Without this gene, photosynthesis is impossible. The researchers therefore rejected the hypothesis that the chloroplasts are autonomous. That left the second possibility, which suggested that the slug’s cells contain the DNA necessary to support the chloroplasts. The team looked for the critical missing gene in the animal’s DNA, and they found it (figure 5.12). Moreover, when they sequenced the gene from the slug’s genome, it was identical to the same gene in algae. The Conclusion: At some point, a gene required for photosynthesis moved from algae to the genome of a sea slug. The researchers speculate that cells in a slug’s gut may have taken up DNA fragments that spilled from partially eaten algae. This study provides convincing evidence that gene transfer can occur not only in bacteria but among distantly related eukaryotes, too. Apparently, organisms have traded DNA throughout life’s long history. Many biologists are therefore discarding the notion of a tidy evolutionary “tree” in favor of a messier, but perhaps more fascinating, evolutionary thicket. Gene encoding part of photosystem II Rumpho, Mary E., and seven colleagues, including James R. Manhart. 2008. Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica. Proceedings of the National Academy of Sciences, vol. 105, pages 17867–17871. Figure 5.12 Photosynthesis Gene. Both algae and the solar-powered sea slug contain a particular gene required for photosynthesis. This electrophoresis gel sorts DNA fragments by size as they migrate from the top to the bottom of the gel. The “ladder” contains DNA pieces of known size, allowing the researchers to estimate the size of the DNA being studied. Life Depends on Photosynthesis hoe96928_ch05.indd 12 Photosynthetic Pigments Capture Sunlight 5.8 Mastering Concepts 1. Explain the most important finding of this study. 2. What evidence led the researchers to their conclusion? Chloroplasts Are the Sites of Photosynthesis Photosynthesis Occurs in Two Stages 5/19/11 12:51 PM 13 Chapter 5 Photosynthesis Cell Leaf Chapter Summary 5.1 Life Depends on Photosynthesis • Photosynthesis converts kinetic energy in light to potential energy in the covalent bonds of glucose, according to the following chemical equation: Chloroplast light energy 6CO2 + 6H2O ⎯→ C6H12O6 + 6O2 • Autotrophs produce their own organic molecules from atmospheric CO2. Plants, algae, and some bacteria are autotrophs. Heterotrophs rely on organic molecules produced by other organisms. • Food and oxygen produced in photosynthesis are critical to life in terrestrial and aquatic habitats. 5.2 Photosynthetic Pigments Capture Sunlight • Visible light is a small part of the electromagnetic spectrum. • Photons move in waves. The longer the wavelength, the less kinetic energy per photon. Visible light occurs in a spectrum of colors representing different wavelengths. • Chlorophyll a is the primary photosynthetic pigment in plants. Accessory pigments absorb wavelengths of light that chlorophyll a cannot absorb, extending the range of wavelengths useful for photosynthesis. Stroma Light reactions (in thylakoid membranes) Light energy Light energy Chlorophyll H2O NADP+ H+ H+ 2H+ H+ 5.3 Chloroplasts Are the Sites of Photosynthesis • Plants exchange gases with the environment through pores called stomata. • Leaf mesophyll cells contain abundant chloroplasts. • A chloroplast includes a gelatinous matrix called the stroma. This fluid surrounds the grana, which are composed of stacked thylakoid membranes. Photosynthetic pigments are embedded in the thylakoid membranes, which enclose the thylakoid space. • A photosystem consists of antenna pigments and a reaction center. NADPH H+ 2e– 1/2 O + 2 Granum 3 CO2 Carbon reactions (in stroma) H+ ADP + P Rubisco enzyme ATP ATP ADP + P NADPH 5.4 Photosynthesis Occurs in Two Stages NADP+ • The light reactions of photosynthesis produce ATP and NADPH; these molecules provide energy and electrons for the glucose-producing carbon reactions. • Photosynthesis is a redox reaction in which water is oxidized and CO2 is reduced to glucose. ATP 1 PGAL 5.5 The Light Reactions Begin Photosynthesis A. Photosystem II Produces ATP • Photosystem II captures light energy and sends electrons from reactive chlorophyll a to an electron transport chain that joins photosystem II to photosystem I. • Electrons from chlorophyll are replaced with electrons from water. O2 is the waste product. • The energy released in the electron transport chain drives the active transport of protons into the thylakoid space. The protons diffuse out through channels in ATP synthase. This movement powers the production of ATP. B. Photosystem I Produces NADPH • Photosystem I receives electrons from the electron transport chain and uses them to reduce NADP+, producing NADPH. Light provides the energy. ADP + P P Glucose, starch, sucrose • In the Calvin cycle, rubisco catalyzes the reaction of CO2 with ribulose bisphosphate (RuBP) to yield two molecules of PGA. These are converted to PGAL, the immediate carbohydrate product of photosynthesis. PGAL later becomes glucose. • Plants use glucose to generate ATP, grow, nourish nonphotosynthetic plant parts, and produce cellulose and many other biochemicals. Most store excess glucose as starch or sucrose. 5.6 The Carbon Reactions Produce Carbohydrates 5.7 C3, C4, and CAM Plants Use Different Carbon Fixation Pathways • The carbon reactions use energy from ATP and electrons from NADPH in carbon fixation reactions that incorporate CO2 into organic compounds. • The Calvin cycle is also called the C3 pathway. Most plant species are C3 plants, which use only this pathway to fix carbon. The Light Reactions Begin Photosynthesis hoe96928_ch05.indd 13 The Carbon Reactions Produce Carbohydrates C3, C4, and CAM Plants Investigating Life: Solar-Powered Sea Slugs 5/19/11 12:51 PM 14 UNIT 1 Science, Chemistry, and Cells • Photorespiration wastes carbon and energy when rubisco reacts with O2 instead of CO2. • The C4 pathway reduces photorespiration by separating the light and carbon reactions into different cells. In mesophyll cells, CO2 is fixed as a fourcarbon molecule, which moves to a bundle-sheath cell and liberates CO2 to be fixed again in the Calvin cycle. • In the CAM pathway, desert plants such as cacti open their stomata and take in CO2 at night, storing the fixed carbon in vacuoles. During the day, they split off CO2 and fix it in chloroplasts in the same cells. 9. What happens to the enzyme rubisco during photorespiration? a. The enzyme speeds up the formation of glucose. b. The enzyme’s active site binds to O2 instead of CO2. c. It becomes denatured. d. The enzyme catalyzes the breakdown of glucose. 10. A plant that only opens its stomata at night is a a. C2 plant. c. C4 plant. b. C3 plant. d. CAM plant. 5.8 Investigating Life: Solar-Powered Sea Slugs • The sea slug Elysia chlorotica contains chloroplasts acquired from its food, a filamentous alga. The slug’s DNA includes a gene required for photosynthesis. Multiple-Choice Questions 1. Where does the energy come from to drive photosynthesis? a. A chloroplast c. The sun b. ATP d. Glucose 2. Algae in a swimming pool are ____; Escherichia coli bacteria in the human intestine are ___. a. autotrophs ... autotrophs b. heterotrophs ... heterotrophs c. autotrophs ... heterotrophs d. heterotrophs ... autotrophs 3. Photosynthesis is essential to animal life because it provides ___. a. CO2 required in respiration b. O2 c. organic molecules d. Both b and c are correct. 4. A plant appears green because a. its chloroplasts use green wavelengths of light. b. chlorophyll a absorbs red and blue light. c. chlorophyll a absorbs ultraviolet light. d. Both a and c are correct. 5. Only high-energy light can penetrate the ocean and reach photosynthetic organisms in coral reefs. What color light would you predict these organisms use? a. Red c. Blue b. Yellow d. Orange 6. Which part of the chloroplast is associated with the production of carbohydrates? a. The thylakoid c. The thylakoid space b. The grana d. The stroma 7. The ATP that is produced in the light reactions is used by the cell to a. reproduce and grow. b. build carbohydrate molecules. c. move electrons through the electron transport chain. d. split water into H+ and O2. 8. Can carbon fixation occur at night? a. Yes, because CO2 can always enter a leaf. b. No, because a plant cell is not active at night. c. Yes, if there is a source of ATP and NADPH. d. No, because photorespiration occurs at night. hoe96928_ch05.indd 14 Write It Out 1. Photosynthesis takes place in plants, algae, and some microbes. How does it affect a meat-eating animal? 2. What color would plants be if they absorbed all wavelengths of visible light? Why? 3. Define these terms and arrange them from smallest to largest: thylakoid membrane; photosystem; chloroplast; electron transport chain; reaction center. 4. Determine whether each of the following molecules is involved in the light reactions, the carbon reactions, or both and explain how: O2, CO2, carbohydrate, chlorophyll a, photons, NADPH, ATP, H2O. 5. One of the first investigators to explore photosynthesis was Flemish physician and alchemist Jan van Helmont. In the early 1600s, he grew willow trees in weighed amounts of soil, applied known amounts of water, and noted that in 5 years the trees gained more than 45 kg, but the soil had lost only a little weight. Because he had applied large amounts of water, van Helmont concluded (incorrectly) that plants grew solely by absorbing water. What is the actual source of the added biomass? Explain your answer. 6. One of the classic experiments in photosynthesis occurred in 1771, when Joseph Priestley found that if he placed a mouse in an enclosed container with a lit candle, the mouse would die. But if he also added a plant to the container, the mouse could live. Priestley concluded that plants “purify” air, allowing animals to breathe. What is the biological basis for this observation? 7. In 1941, biologists exposed photosynthesizing cells to water containing a heavy oxygen isotope, designated 18O. The “labeled” isotope appears in the O2 gas released in photosynthesis, showing that the oxygen came from the water. Where would the 18O have ended up if the researchers had used 18 O-labeled CO2 instead of H2O? 8. Over the past decades, the CO2 concentration in the atmosphere has increased. a. Predict the effect of increasing carbon dioxide concentrations on photorespiration. b. Scientists suggest that increasing CO2 concentrations are leading to higher average global temperatures. If temperatures are increasing, does this change your answer to part (a)? 9. How is the CAM pathway adaptive in a desert habitat? 10. Explain why each of the following misconceptions about photosynthesis is false: a. Only plants are autotrophs. b. Plants do not need cellular respiration because they carry out photosynthesis. c. Chlorophyll is the only photosynthetic pigment. 5/19/11 12:51 PM Chapter 5 Photosynthesis Pull It Together PHOTOSYNTHESIS Light O2 + C6H12O6 CO2 + H2O occurs in two stages Light reactions H2O Light Carbon reactions CO2 Light ATP NADPH NADP+ NADPH NADP+ ADP O2 is energy source for Glucose ATP is energy source for P require photosynthetic pigments such as P Chlorophyll H3C ADP O2 ATP NADPH N Light N N Mg CH2CH3 N CH3 Glucose CH2 CH2 CO CH O 2 3 O C O CH2 CH C CH3 CH2 CH2 CH2 HC CH3 CH2 CH2 CH2 HC CH3 CH2 CH2 CH2 HC CH3 CH3 15 produce CH H3C hoe96928_ch05.indd Glucose is an electron source for P produce H2C 1. Where does the electron transport chain fit into this concept map? 2. What specific process in the light reactions gives rise to the waste product, O2? 3. How would you incorporate the Calvin cycle, rubisco, C3 plants, C4 plants, and CAM plants into this concept map? 4. Where do humans and other heterotrophs fit into this concept map? 5. Build another small concept map showing the relationships among the terms chloroplast, stroma, grana, thylakoid, photosystem, and chlorophyll. 6. What happens to the glucose produced in photosynthesis? CO2 H2O produce ATP 15 H C HO absorbs specific wavelengths of CH2OH C H OH C H O H C H OH C OH Enhance your study of this chapter with practice quizzes, animations and videos, answer keys, and downloadable study tools. www.mhhe.com/hoefnagels. 5/19/11 12:51 PM