chapter-6-photosynthesis.pdf

P HOTOSYNTHESIS CHAPTER 6 Alfalfa, corn, and soybeans are growing on this farmland in Pennsylvania. Through photosynthesis, these plants obtain energy from the sun and store it in organic compounds. Humans and other living organisms depend on organic compound...

P HOTOSYNTHESIS CHAPTER 6 Alfalfa, corn, and soybeans are growing on this farmland in Pennsylvania. Through photosynthesis, these plants obtain energy from the sun and store it in organic compounds. Humans and other living organisms depend on organic compounds—and therefore on photosynthesis—to obtain the energy necessary for living. SECTION 1 The Light Reactions Unit 2—Photosynthesis Topics 1–6 SECTION 2 The Calvin Cycle 112 CHAPTER 6 Copyright © by Holt, Rinehart and Winston. All rights reserved.  SECTION 1 T H E L I G H T R E AC T I O N S OBJECTIVES Explain why almost all organisms All organisms use energy to carry out the functions of life. depend on photosynthesis. Describe the role of chlorophylls Where does this energy come from? Directly or indirectly, and other pigments in almost all of the energy in living systems comes from the sun. photosynthesis. Energy from the sun enters living systems when plants, algae, Summarize the main events of the some unicellular protists, and some prokaryotes absorb sunlight light reactions. Explain how ATP is made during and use it to make organic compounds. the light reactions. VOCABULARY OBTAINING ENERGY autotroph photosynthesis Organisms can be classified according to how they get energy. heterotroph Organisms that use energy from sunlight or from chemical bonds in light reactions inorganic substances to make organic compounds are called chloroplast autotrophs (AWT-oh-TROHFS). Most autotrophs use the process of thylakoid photosynthesis to convert light energy from the sun into chemical granum energy in the form of organic compounds, primarily carbohydrates. stroma The tree in Figure 6-1 is an autotroph because it converts light pigment energy from the sun into organic compounds. The tree uses these chlorophyll compounds for energy. The caterpillar also depends on the tree carotenoid for energy, as it cannot manufacture organic compounds itself. photosystem Animals and other organisms that must get energy from food primary electron acceptor instead of directly from sunlight or inorganic substances are electron transport chain called heterotrophs (HEHT-uhr-oh-TROHFS). The bird is also a het- chemiosmosis erotroph. The food that fuels the bird originates with an autotroph (the tree), but it passes indirectly to the bird through the cater- pillar. In similar ways, almost all organisms ultimately depend on autotrophs to obtain the energy necessary to carry out the processes of life. Photosynthesis involves a complex series of chemical reactions in which the product of one reaction is consumed in the next reac- tion. A series of chemical reactions linked in this way is referred to FIGURE 6-1 as a biochemical pathway. The tree depends on the sun for energy. Like all heterotrophs, the caterpillar and bird depend on an autotroph (the tree and its leaves) for energy. Light energy Plants convert light Caterpillars get energy Birds get energy by energy to chemical energy. by eating plants. eating caterpillars. PHOTOSYNTHESIS 113 Copyright © by Holt, Rinehart and Winston. All rights reserved.  PHOTOSYNTHESIS by autotrophs OVERVIEW OF Light energy PHOTOSYNTHESIS Carbon Organic Figure 6-2 shows how autotrophs use photosynthesis to produce dioxide compounds and water and oxygen organic compounds from carbon dioxide (CO2) and water. The oxy- gen (O2) and some of the organic compounds produced are then used by cells in a process called cellular respiration. During cellular CELLULAR RESPIRATION respiration, CO2 and water are produced. Thus, the products of by autotrophs and photosynthesis are reactants in cellular respiration. Conversely, the heterotrophs products of cellular respiration are reactants in photosynthesis. FIGURE 6-2 Photosynthesis can be divided into two stages: Many autotrophs produce organic 1. Light Reactions Light energy (absorbed from the sun) is con- compounds and oxygen through verted to chemical energy, which is temporarily stored in ATP photosynthesis. Both autotrophs and and the energy carrier molecule NADPH. heterotrophs produce carbon dioxide 2. Calvin Cycle Organic compounds are formed using CO2 and through cellular respiration. the chemical energy stored in ATP and NADPH. Photosynthesis can be summarized by the following equation: 6CO2 ! 6H2O light energy C6H12O6 ! 6O2 This equation, however, does not explain how photosynthesis occurs. It is helpful to examine the two stages separately in order to better understand the overall process of photosynthesis. CAPTURING LIGHT ENERGY The first stage of photosynthesis includes the light reactions, so named because they require light to happen. The light reactions begin with the absorption of light in chloroplasts, organelles found in the cells of plants and algae. Most chloroplasts are similar in structure. As shown in Figure 6-3, each chloroplast is surrounded FIGURE 6-3 by a pair of membranes. Inside the inner membrane is another sys- Photosynthesis in eukaryotes occurs tem of membranes called thylakoids (THIE-luh-koydz) that are inside the chloroplast. The light reactions of photosynthesis take arranged as flattened sacs. The thylakoids are connected and layered place in the thylakoids, which are to form stacks called grana (GRAY-nuh) (singular, granum). stacked to form grana. Surrounding the grana is a solution called the stroma (STROH-muh). CHLOROPLAST PLANT CELL LEAF Thylakoid Stroma Outer membrane Granum Inner membrane 114 CHAPTER 6 Copyright © by Holt, Rinehart and Winston. All rights reserved. Light and Pigments Sun To understand how chloroplasts absorb light in photosynthesis, it is important to understand some of the properties of light. Light from the sun appears white, but it is actually made of a variety of White light colors. As shown in Figure 6-4, white light can be separated into its component colors by passing the light through a prism. The result- Prism ing array of colors, ranging from red at one end to violet at the other, is called the visible spectrum. When white light strikes an object, its component colors can be reflected, transmitted, or absorbed by the object. Many objects contain pigments, compounds that absorb light. Most pigments absorb certain colors more strongly than others. By absorbing cer- tain colors, a pigment subtracts those colors from the visible spec- Visible spectrum trum. Therefore, the light that is reflected or transmitted by the pigment no longer appears white. For example, the lenses in green- tinted sunglasses contain a pigment that reflects and transmits 400 nm 700 nm green light and absorbs the other colors. As a result, the lenses Increasing wavelength look green. FIGURE 6-4 Chloroplast Pigments White light contains a variety of colors called the visible spectrum. Each color Located in the membrane of the thylakoids are several pigments, has a different wavelength, measured in the most important of which are called chlorophylls (KLAWR-uh-FILZ). nanometers. There are several different types of chlorophylls. The two most common types are known as chlorophyll a and chlorophyll b. As Figure 6-5 shows, chlorophyll a absorbs less blue light but more red light than chlorophyll b absorbs. Neither chlorophyll a nor chlorophyll b absorbs much green light. Instead, they allow green light to be reflected or transmitted. For this reason, leaves and other plant structures that contain large amounts of chlorophyll look green. FIGURE 6-5 Only chlorophyll a is directly involved in the light reactions of The three curves on this graph show photosynthesis. Chlorophyll b assists chlorophyll a in capturing how three pigments involved in light energy, and therefore chlorophyll b is called an accessory pig- photosynthesis differ in the colors of ment. Other compounds found in the thylakoid membrane, includ- light they absorb. Where a curve has a peak, much of the light at that ing the yellow, orange, and brown carotenoids (kuh-RAHT’n-OYDZ), wavelength is absorbed. Where a curve also function as accessory pigments. Looking again at Figure 6-5, has a trough, much of the light at that notice that the pattern of light absorption of one of the carotenoids wavelength is reflected or transmitted. differs from the pattern of either type of chlorophyll. By absorbing colors that chloro- Absorption Spectra of Photosynthetic Pigments phyll a cannot absorb, the accessory pig- ments enable plants to capture more of the Relative absorption energy in light. Chlorophyll b In the leaves of a plant, the chlorophylls are Chlorophyll a generally much more abundant and therefore Carotenoid mask the colors of the other pigments. But in the nonphotosynthetic parts of a plant, such as fruits and flowers, the colors of the other pig- ments may be quite visible. During the fall, many plants lose their chlorophylls, and their 400 500 600 700 Wavelength (nm) leaves take on the rich hues of the carotenoids. PHOTOSYNTHESIS 115 Copyright © by Holt, Rinehart and Winston. All rights reserved.  Word Roots and Origins CONVERTING LIGHT ENERGY TO CHEMICAL ENERGY thylakoid from the Greek thylakos, Once the pigments in the chloroplast have captured light energy, meaning “pocket” the light energy must then be converted to chemical energy. The chemical energy is temporarily stored in ATP and NADPH. During these reactions, O2 is given off. The chlorophylls and carotenoids are grouped in clusters of a few hundred pigment molecules in the thylakoid membrane. Each cluster of pigment molecules and the proteins that the pigment molecules are embedded in are referred to collectively as a photosystem. Two types of photosystems are known: photosystem I and photosystem II. They contain similar kinds of pigments, but they have different roles in the light reactions. The light reactions begin when accessory pigment molecules in both photosystems absorb light. By absorbing light, those molecules acquire some of the energy carried by the light. In each photosystem, the acquired energy is passed quickly to the other pigment molecules until it reaches a specific pair of chloro- phyll a molecules. Chlorophyll a can also absorb light. The events that occur next can be divided into five steps, as shown FIGURE 6-6 in Figure 6-6. The light reactions, which take place in the thylakoid membrane, use energy In step 1 , light energy forces electrons to enter a higher energy from sunlight to produce ATP, NADPH, level in the two chlorophyll a molecules of photosystem II. These and oxygen. energized electrons are said to be “excited.” CHLOROPLAST THYLAKOID STROMA Light Primary electron Primary electron NADP+ Light acceptor acceptor 4 + NADPH 3 H+ 1 2 5 e– Thylakoid e– e– membrane e– H+ Photosystem II Electron Photosystem I Electron transport chain transport chain INSIDE OF THYLAKOID 116 CHAPTER 6 Copyright © by Holt, Rinehart and Winston. All rights reserved.  The excited electrons have enough energy to leave the chlorophyll a molecules. Because they have lost electrons, the chlorophyll a mol- ecules have undergone an oxidation reaction. Remember that each www.scilinks.org Topic: Photosynthesis oxidation reaction must be accompanied by a reduction reaction. Keyword: HM61140 So, some substance must accept the electrons that the chlorophyll a molecules have lost, as shown in step 2. The acceptor of the electrons lost from chlorophyll a is a molecule in the thylakoid membrane called the primary electron acceptor. In step 3 , the primary electron acceptor donates the electrons to the first of a series of molecules located in the thylakoid mem- brane. These molecules are called an electron transport chain because they transfer electrons from one molecule to the next. As the electrons pass from molecule to molecule in the chain, they lose most of the energy that they acquired when they were excited. The energy they lose is used to move protons (H!) into the thylakoid. In step 4 , light is absorbed by photosystem I. This happens at the same time that light is absorbed by photosystem II. Electrons move from a pair of chlorophyll a molecules in photosystem I to another primary electron acceptor. The electrons lost by these chlorophyll a molecules are replaced by the electrons that have passed through the electron transport chain from photosystem II. The primary electron acceptor of photosystem I donates elec- trons to a different electron transport chain. This chain brings the electrons to the side of the thylakoid membrane that faces the stroma. There the electrons combine with a proton and NADP!, an organic molecule that accepts electrons during oxidation/reduc- tion reactions. As you can see in step 5 , this reaction causes NADP! to be reduced to NADPH. Replacing Electrons in Light Reactions In step 4 , electrons from chlorophyll molecules in photosystem II FIGURE 6-7 replace electrons that leave chlorophyll molecules in photosystem I. The splitting of water inside the If this replacement did not occur, both electron transport chains thylakoid releases electrons, which would stop, and photosynthesis would not happen. The replace- replace the electrons that leave ment electrons for photosystem II are provided by photosystem II. water molecules. As Figure 6-7 shows, an enzyme Light Primary inside the thylakoid splits water molecules into electron acceptor protons, electrons, and oxygen. The following equation summarizes the reaction: STROMA 2H2O 4H! ! 4e" ! O2 Water-splitting enzyme For every two molecules of water that are split, Thylakoid four electrons become available to replace those membrane e– lost by the chlorophyll molecules in photosystem II. The protons that are produced are left inside Photosystem II the thylakoid, and the oxygen diffuses out of the 4H+ chloroplast and can then leave the plant. Thus, oxy- gen is not needed for photosynthesis to occur but is INSIDE OF essential for cellular respiration in most organisms, THYLAKOID 2H2O O2 including plants themselves. PHOTOSYNTHESIS 117 Copyright © by Holt, Rinehart and Winston. All rights reserved.  FIGURE 6-8 During chemiosmosis, the movement Making ATP in Light Reactions of protons into the stroma of the ATP is the main energy currency of cells. An important part of the chloroplast releases energy, which is light reactions is the synthesis of ATP through a process called used to produce ATP. chemiosmosis (KEM-i-ahs-MOH-sis). Chemiosmosis relies on a concen- Thylakoid tration gradient of protons across the thylakoid membrane. Recall membrane that some protons are produced from the splitting of water mole- cules inside the thylakoid. Other protons are pumped from the stroma to the interior of the thylakoid. The energy required to pump these protons is supplied by the Thylakoid excited electrons as they pass along the electron transport chain of photosystem II. Both of these mechanisms act to build up a concentration gradi- Thylakoid space ent of protons. That is, the concentration of protons H+ is higher inside the thylakoid than in the stroma. (low concentration) The concentration gradient of protons represents ATP potential energy. That energy is harnessed by an ADP + enzyme called ATP synthase, which is located in the ATP phosphate thylakoid membrane, as Figure 6-8 shows. ATP syn- synthase thase makes ATP by adding a phosphate group to STROMA adenosine diphosphate, or ADP. The energy driving this reaction is provided by the movement of pro- tons from inside the thylakoid to the stroma Thylakoid through the ATP synthase complex. Thus, ATP syn- membrane thase converts the potential energy of the proton concentration gradient into chemical energy stored in ATP. As you learned earlier, some of the protons INSIDE OF in the stroma are used to make NADPH from NADP! THYLAKOID at the end of the electron transport chain of photo- system I. Together, NADPH and ATP provide energy for the second set of reactions in photosynthesis, H+ (high concentration) which are described in the next section. SECTION 1 REVIEW 1. Explain why both autotrophs and heterotrophs CRITICAL THINKING depend on photosynthesis to obtain the energy 6. Making Comparisons Thinking about the main they need for life processes. role of pigments in photosynthesis, explain how 2. Describe the role of chlorophylls in the biochem- the pigments in colored objects such as clothes ical pathways of photosynthesis. differ from plant pigments. 3. List the three substances that are produced 7. Analyzing Information The molecule that when water molecules are broken down during precedes the electron transport chains of both the light reactions. photosystem I and photosystem II is an electron 4. Explain why the splitting of water is important acceptor. What is the original molecule that is to the continuation of the light reactions. the electron donor for both of these systems? 5. Name the product of the process known as 8. Predicting Results Explain how the light reac- chemiosmosis. tions would be affected if there were no concen- tration gradient of protons across the thylakoid membrane. 118 CHAPTER 6 Copyright © by Holt, Rinehart and Winston. All rights reserved.  Science in Action How Can Scientists Replicate Photosynthesis in the Lab? Life on Earth is powered by the sun’s energy. The chloroplasts found in plants and some other organisms can efficiently capture and convert solar energy to sugars through a process known as photosynthesis. Now, scientists have developed an artificial photosystem that may one day allow for solar-powered food or fuel production in the lab. Dr. Darius Kuciauskas HYPOTHESIS: Synthetic Compounds Can METHODS: Design and Build a Imitate a Photosynthetic System Pigment-Protein Light- In nature, a photosystem contains pigments and Harvesting Complex proteins embedded in a linear sequence in the mem- Kuciauskas and his team set out to create the brane of a chloroplast. This arrangment allows for photosystem. The team started with purple pig- effective capture and orderly transfer of electrons ment molecules called porphyrins. These molecules from one photosystem to another and eventually absorb light energy and are related to naturally to a final electron acceptor for powering synthesis occurring chlorophyll. The scientists joined four of of sugar. the porphyrins to each other. To the center-most Dr. Darius Kuciauskas (koo-si-AHS-kuhs) of Virginia porphyrin, they attached a ball-shaped cluster of Commonwealth University and his colleagues 60 carbon atoms called C60, or fullerene. thought they could produce an artificial photosys- tem that carried out the initial steps of photosyn- RESULTS: The Artificial Photosystem Captured thesis. Kuciauskas and his team predicted that when and Transferred Electrons they shined light on four pigment molecules with The four porphyrin pigments captured light energy, energy-conducting chemical bonds, the pigment and the bonds passed electrons to the C60 acceptor molecules would capture and channel light energy molecule. The acceptor held the charge for only one through the “wirelike” bonds. The electrons traveling microsecond—about one millionth of a second. through the bonds would then pass to, and electrically CONCLUSION: Artificial Photosystems charge, an acceptor molecule that was placed at the Are Possible center of the pigment complex. The Kuciauskas team concluded that their complex does act as an artificial photosystem, at least for a fleeting instant. Kuciauskas and his colleagues have continued their work, aiming to modify the complex further so it can hold its energy longer. Ultimately, they hope their photosystem can power chemical reactions for making important organic molecules that serve as food or fuel. REVIEW 1. What are the essential parts of a photosystem? 2. Describe the method by which the Kuciauskas team built an artificial photosystem. www.scilinks.org 3. Critical Thinking Review the Topic: Plant methods that Kuciauskas and Photosynthetic his team used to test their Pigments hypothesis. Determine one Keyword: HM61164 Kuciauskas and his colleagues use laser systems such as this variable the scientists might one to study light absorption in molecules that might one day be used in an artificial photosystem. change to modify their results. 119 Copyright © by Holt, Rinehart and Winston. All rights reserved.  SECTION 2 OBJECTIVES Summarize the main events of the T H E C A LV I N C YC L E Calvin cycle. In the second set of reactions in photosynthesis, plants use the Describe what happens to the energy that was stored in ATP and NADPH during the light compounds that are made in the Calvin cycle. reactions to produce organic compounds in the form of sugars. Distinguish between C , C , and These organic compounds are then consumed by autotrophs 3 4 CAM plants. Summarize how the light reactions and heterotrophs alike for energy. The most common way that and the Calvin cycle work together plants produce organic compounds is called the Calvin cycle. to create the continuous cycle of photosynthesis. Explain how environmental factors influence photosynthesis. CARBON FIXATION VOCABULARY The Calvin cycle is a series of enzyme-assisted chemical reactions that produces a three-carbon sugar. In the Calvin cycle, carbon Calvin cycle atoms from CO2 in the atmosphere are bonded, or “fixed,” into carbon fixation organic compounds. This incorporation of CO2 into organic com- stomata pounds is called carbon fixation. A total of three CO2 molecules C4 pathway must enter the Calvin cycle to produce each three-carbon sugar CAM pathway that will be used to make the organic compounds. The Calvin cycle occurs within the stroma of the chloroplast. Figure 6-9 shows the events that occur when three CO2 molecules enter the Calvin cycle. In step 1 , CO2 diffuses into the stroma from the surrounding cytosol. An enzyme combines each CO2 molecule with a five-carbon FIGURE 6-9 molecule called ribulose bisphosphate (RuBP). The six-carbon mole- The Calvin cycle, in which carbon is cules that result are very unstable, and they each immediately split fixed into organic compounds, takes into two three-carbon molecules. These three-carbon molecules are place in the stroma of the chloroplast. called 3-phosphoglycerate (3-PGA). 1 Each of the three CO2 C 3 CO2 molecules combines with a molecule of RuBP. Each resulting six-carbon molecule 3 molecules immediately splits into two of RuBP molecules of 3-PGA. 3 P- C C C C C -P 3 ADP Stroma 6 molecules 4 The rest of the of 3-PGA CHLOROPLAST 3 ATP G3P is converted back into RuBP. 6 C C C -P 6 ATP Organic 1 molecule compounds of G3P 2 Each molecule of 6 ADP 6 molecules 3-PGA is converted 1 C C C -P of G3P into a molecule of G3P. 6 NADPH 3 One molecule of 6 C C C -P G3P is used to make organic compounds. 6 NADP+ 6 Phosphate 120 CHAPTER 6 Copyright © by Holt, Rinehart and Winston. All rights reserved.  In step 2 , each molecule of 3-PGA is converted into another three-carbon molecule, glyceraldehyde 3-phosphate (G3P), in a two-part process. First, each 3-PGA molecule receives a phosphate group from a molecule of ATP. The resulting compound then receives a proton ( H!) from NADPH and releases a phosphate group, producing G3P. The ADP, NADP!, and phosphate that are also produced can be used again in the light reactions to make more ATP and NADPH. In step 3 , one of the G3P molecules leaves the Calvin cycle and is used to make organic compounds (carbohydrates) in which energy is stored for later use. In step 4 , the remaining G3P molecules are converted back into RuBP through the addition of phosphate groups from ATP molecules. The resulting RuBP molecules then enter the Calvin cycle again. The Calvin cycle (named for Melvin Calvin, the American bio- chemist who worked out the chemical reactions in the cycle) is the most common pathway for carbon fixation. Plant species that fix carbon exclusively through the Calvin cycle are known as C3 plants because of the three-carbon compound that is initially formed in this process. ALTERNATIVE PATHWAYS Many plant species that evolved in hot, dry climates fix carbon Word Roots and Origins through alternative pathways. Under hot and dry conditions, stomata plants can rapidly lose water to the air through small pores called stomata (STOH-muh-tuh). Stomata (singular, stoma), shown in Figure from the Greek stoma, meaning 6-10, are usually located on the undersurface of the leaves. Plants “mouth” can reduce water loss by partially closing their stomata when the air is hot and dry. Stomata are the major passageways through which CO2 enters and O2 leaves a plant. When a plant’s stomata are partly closed, the level of CO2 in the plant falls as CO2 is consumed in the Calvin cycle. At the same time, the level of O2 in the plant rises as the light reactions generate O2. Both a low CO2 level and a high O2 level inhibit carbon fixation by the Calvin cycle. Alternative pathways for carbon fixation help plants deal with this problem. FIGURE 6-10 These photos show stomata in the leaf of a tobacco plant, Nicotiana tabacum. (a) When a stoma is open, water, carbon dioxide, and other gases can pass through it to enter or leave a plant (814"). (b) When a stoma is closed, passage through it is greatly restricted (a) OPEN STOMA (b) CLOSED STOMA (878"). PHOTOSYNTHESIS 121 Copyright © by Holt, Rinehart and Winston. All rights reserved.  The C4 Pathway One alternative pathway enables certain plants to fix CO2 into four- carbon compounds. This pathway is thus called the C4 pathway, and plants that use it are known as C4 plants. During the hottest part of the day, C4 plants have their stomata partially closed. However, certain cells in C4 plants have an enzyme that can fix CO2 into four-carbon compounds even when the CO2 level is low and the O2 level is high. These compounds are then transported to other cells, where CO2 is released and enters the Calvin cycle. C4 plants include corn, sugar cane, and crab grass. Such plants lose only about half as much water as C3 plants when producing the same amount of carbohydrates. Many plants that use the C4 path- way evolved in tropical climates. The CAM Pathway Cactuses, pineapples, and certain other plants have a different adaptation to hot, dry climates. Such plants fix carbon through a pathway called the CAM pathway. CAM is an abbreviation for crassulacean acid metabolism, because this water-conserving path- way was first discovered in plants of the family Crassulaceae, such as the jade plant. Plants that use the CAM pathway open their stomata at night and close them during the day—just the opposite of what other plants do. At night, CAM plants take in CO2 and fix it into a variety of organic compounds. During the day, CO2 is released from these compounds and enters the Calvin cycle. Because CAM plants have their stomata open at night, when the temperature is lower, they grow fairly slowly. However, they lose less water than either C3 or C4 plants. Eco Connection Photosynthesis and the Global Greenhouse A SUMMARY OF With the beginning of the Industrial PHOTOSYNTHESIS Revolution around 1850, the atmospheric concentration of CO2 Photosynthesis happens in two stages, both of which occur inside started to increase. This increase the chloroplasts of plant cells and algae. has resulted largely from the burn- 1. The light reactions—Energy is absorbed from sunlight and con- ing of fossil fuels, which releases verted into chemical energy, which is temporarily stored in CO2 as a byproduct. You might expect plants to benefit from the ATP and NADPH. buildup of CO2 in the atmosphere. 2. The Calvin cycle—Carbon dioxide and the chemical energy In fact, the rise in CO2 levels may stored in ATP and NADPH are used to form organic compounds. harm photosynthetic organisms Photosynthesis itself is an ongoing cycle: the products of the light more than it helps them. CO2 and reactions are used in the Calvin cycle, and some of the products of other gases in the atmosphere the Calvin cycle are used in the light reactions. The other products of retain some of the Earth’s heat, the Calvin cycle are used to produce a variety of organic compounds, causing the Earth to become such as amino acids, lipids, and carbohydrates. Many plants produce warmer. This warming could reduce the amount of worldwide precipita- more carbohydrates than they need. These extra carbohydrates can tion, creating deserts that would be be stored as starch in the chloroplasts and in structures such as inhospitable to most plants. roots and fruits. These stored carbohydrates provide the chemical energy that both autotrophs and heterotrophs depend on. 122 CHAPTER 6 Copyright © by Holt, Rinehart and Winston. All rights reserved.  O2 H2O CO2 Light NADPH LIGHT ATP CALVIN REACTIONS CYCLE NADP+ PLANT CELL ADP +P FIGURE 6-11 Photosynthesis is an ongoing cycle. Organic CHLOROPLAST compounds Quick Lab Analyzing Photosynthesis Figure 6-11 above illustrates how the light reactions in the Materials disposable gloves, lab thylakoid and the Calvin cycle in the stroma actually work apron, safety goggles, 250 mL together as one continuous cycle—photosynthesis. Amazingly, Erlenmeyer flasks (3), bromothymol this complicated process can occur in every one of the thousands blue, 5 cm sprigs of elodea (2), of chloroplasts present in a plant if the conditions are right. water, drinking straw, plastic wrap, Recall that water is split during the light reactions, yielding 100 mL graduated cylinder electrons, protons, and oxygen as a byproduct. Thus, the simplest Procedure overall equation for photosynthesis, including both the light reactions and the Calvin cycle, can be written as follows: 1. Put on your disposable gloves, CO2 ! H2O light energy (CH2O) ! O2 lab apron, and safety goggles. 2. Label the flasks “1,” “2,” and In the equation above, (CH2O) represents the general formula “3.” Add 200 mL of water and for a carbohydrate. It is often replaced in this equation by the car- 20 drops of bromothymol blue to bohydrate glucose, C6H12O6, giving the following equation: each flask. 3. Put the drinking straw in flask 1, 6CO2 ! 6H2O light energy C6H12O6 ! 6O2 and blow into the blue solution until the solution turns yellow. However, glucose is not actually a direct product of photosyn- Repeat this step with flask 2. thesis. Glucose is often included in the equation mainly to empha- 4. Put one elodea sprig in flask 1. size the relationship between photosynthesis and cellular Do nothing to flask 2. Put the respiration, in which glucose plays a key role. Just as the light reac- other elodea sprig in flask 3. tions and the Calvin cycle together create an ongoing cycle, pho- 5. Cover all flasks with plastic tosynthesis and cellular respiration together also create an wrap. Place the flasks in a ongoing cycle. well-lighted location, and leave The light reactions are sometimes referred to as light-dependent them overnight. Record your reactions, because the energy from light is required for the reac- observations. tions to occur. The Calvin cycle is sometimes referred to as the Analysis Describe your results. light-independent reactions or the dark reactions, because the Explain what caused one of the solutions to change color. Why did Calvin cycle does not require light directly. But the Calvin cycle the other solutions not change usually proceeds during daytime, when the light reactions are pro- color? Which flask is the control ducing the materials that the Calvin cycle uses to fix carbon into in this experiment? organic compounds. PHOTOSYNTHESIS 123 Copyright © by Holt, Rinehart and Winston. All rights reserved.  FACTORS THAT AFFECT PHOTOSYNTHESIS Photosynthesis is affected by the environment. Three factors with the most influence are light intensity, CO2 level, and temperature. Light Intensity As shown in Figure 6-12a, the rate of photosynthesis increases as light intensity increases. Higher light intensity excites more elec- trons in both photosystems. Thus, the light reactions occur more rapidly. However, at some point all of the available electrons are excited, and the maximum rate of photosynthesis is reached. The rate stays level regardless of further increases in light intensity. FIGURE 6-12 Carbon Dioxide Levels Environmental factors affect the rate of photosynthesis in plants. (a) As light Increasing levels of CO2 also stimulate photosynthesis until the rate intensity increases, the rate of of photosynthesis levels off. Thus, a graph of the rate of photosyn- photosynthesis increases and then levels thesis versus CO2 concentration would resemble Figure 6-12a. off at a maximum. (b) As temperature increases, the rate of photosynthesis Temperature increases to a maximum and then decreases with further rises in Increasing temperature accelerates the chemical reactions involved temperature. in photosynthesis. As a result, the rate of photosynthesis increases as temperature increases, over a cer- Environmental Influences on Photosynthesis tain range. This effect is illustrated by the left half of the curve in Figure 6-12b. Rate of photosynthesis Rate of photosynthesis The rate peaks at a certain tempera- ture, at which many of the enzymes that catalyze the reactions become ineffective. Also, the stomata begin to close, limiting water loss and CO2 entry into the leaves. These conditions cause the rate of photosynthesis to decrease when the temperature is fur- (a) Light intensity (b) Temperature ther increased, as shown by the right half of the curve in Figure 6-12b. SECTION 2 REVIEW 1. Name the part of the chloroplast where the CRITICAL THINKING Calvin cycle takes place. 6. Predicting Results What would happen to 2. Describe what can happen to the three-carbon photosynthesis if all of the three-carbon sugars molecules made in the Calvin cycle. produced in the Calvin cycle were used to make 3. Distinguish between C3, C4, and CAM plants. organic compounds? 4. Explain why the light reactions and the Calvin 7. Inferring Relationships Explain how a global cycle are dependent on each other. temperature increase could affect plants. 5. Explain why increased light intensity might not 8. Evaluating Differences Explain how the world result in an increased rate of photosynthesis. would be different if C4 plants and CAM plants had not evolved. 124 CHAPTER 6 Copyright © by Holt, Rinehart and Winston. All rights reserved.  CHAPTER HIGHLIGHTS SECTION 1 The Light Reactions Photosynthesis converts light energy into chemical Excited electrons that leave chlorophyll a travel along energy through series of reactions known as biochemical two electron transport chains, resulting in the production pathways. Almost all life depends on photosynthesis. of NADPH. The electrons are replaced when water is split into electrons, protons, and oxygen in the thylakoid. Autotrophs use photosynthesis to make organic Oxygen is released as a byproduct of photosynthesis. compounds from carbon dioxide and water. Heterotrophs cannot make their own organic compounds from As electrons travel along the electron transport chains, inorganic compounds and therefore depend on protons move into the thylakoid and build up a autotrophs. concentration gradient. The movement of protons down this gradient of protons and through ATP synthase results White light from the sun is composed of an array of in the synthesis of ATP through chemiosmosis. colors called the visible spectrum. Pigments absorb certain colors of light and reflect or transmit the other colors. The light reactions of photosynthesis begin with the absorption of light by chlorophyll a and accessory pigments in the thylakoids. Vocabulary autotroph (p. 113) thylakoid (p. 114) carotenoid (p. 115) electron transport photosynthesis (p. 113) granum (p. 114) photosystem (p. 116) chain (p. 117) heterotroph (p. 113) stroma (p. 114) primary electron chemiosmosis (p. 118) light reactions (p. 114) pigment (p. 115) acceptor (p. 117) chloroplast (p. 114) chlorophyll (p. 115) SECTION 2 The Calvin Cycle The ATP and NADPH produced in the light reactions drive Photosynthesis occurs in two stages. In the light the second stage of photosynthesis, the Calvin cycle. In reactions, energy is absorbed from sunlight and the Calvin cycle, CO2 is incorporated into organic converted into chemical energy; in the Calvin cycle, compounds, a process called carbon fixation. carbon dioxide and chemical energy are used to form organic compounds. The Calvin cycle produces a compound called G3P. Most G3P molecules are converted into RuBP to keep the The rate of photosynthesis increases and then reaches a Calvin cycle operating. However, some G3P molecules are plateau as light intensity or CO2 concentration increases. used to make other organic compounds, including amino Below a certain temperature, the rate of photosynthesis acids, lipids, and carbohydrates. increases as temperature increases. Above that temperature, the rate of photosynthesis decreases as Plants that fix carbon using only the Calvin cycle are temperature increases. known as C3 plants. Some plants that evolved in hot, dry climates fix carbon through alternative pathways—the C4 and CAM pathways. These plants carry out carbon fixation and the Calvin cycle either in different cells or at different times. Vocabulary Calvin cycle (p. 120) stomata (p. 121) CAM pathway (p. 122) carbon fixation (p. 120) C4 pathway (p. 122) PHOTOSYNTHESIS 125 Copyright © by Holt, Rinehart and Winston. All rights reserved.  CHAPTER REVIEW USING VOCABULARY 17. CONCEPT MAPPING Create a concept map that shows the steps of photosyn- 1. For each pair of terms, explain the relationship thesis. Include the following terms in your between the terms. concept map: light reactions, Calvin cycle, a. electron transport chain and primary photosystem I, photosystem II, carbon fixation, electron acceptor accessory pigment, chlorophyll, electron trans- b. photosystem and pigment port chain, ATP synthase, chemiosmosis, and c. thylakoid and stroma photosynthesis. d. carbon fixation and C4 pathway 2. Use the following terms in the same sentence: photosynthesis, light reactions, pigment, and CRITICAL THINKING photosystem. 18. Predicting Results When the CO2 concentration in 3. Choose the term that does not belong in the fol- the cells of a C3 plant is low compared with the lowing group, and explain why it does not belong: O2 concentration, an enzyme combines RuBP electron transport chain, chemiosmosis, Calvin with O2 rather than with CO2. What effect would cycle, and photosystem II. this enzymatic change have on photosynthesis? Under what environmental conditions would it be 4. Word Roots and Origins The prefix chloro- is most likely to occur? derived from the Greek word that means “green.” Using this information, explain why the chloro- 19. Evaluating Information All of the major compo- phylls are well named. nents of the light reactions, including the pigment molecules clustered in photosystems I and II, are located in the thylakoid membrane. What is the UNDERSTANDING KEY CONCEPTS advantage of having these components confined to the same membrane rather than dissolved in 5. Distinguish between autotrophs and heterotrophs. the stroma or the cytosol? 6. Identify the primary source of energy for humans. 20. Inferring Relationships When the sun’s rays are 7. Relate the types of pigments involved in photo- blocked by a thick forest, clouds, or smoke from synthesis and their roles. a large fire, what effect do you think there will be on photosynthesis? How might it affect the levels 8. Compare the different roles of photosystem I and of atmospheric carbon dioxide and oxygen? What photosystem II in photosynthesis. experiments could scientists conduct in the labo- 9. Describe what happens to the extra energy in ratory to test your predictions? excited electrons as they pass along an electron 21. Interpreting Graphics The graph below shows transport chain in a chloroplast. how the percentage of stomata that are open 10. Explain how oxygen is generated in photo- varies over time for two different plants. One synthesis. curve represents the stomata of a geranium, and 11. Summarize the events of chemiosmosis. the other curve represents the stomata of a pineapple. Which curve corresponds to the 12. State the three major steps of the Calvin cycle. pineapple stomata? Explain your reasoning. 13. Explain what happens to the 3-carbon com- pounds that do not leave the Calvin cycle to be Daily Cycle of Stomatal Opening made into organic compounds. 14. Propose what might happen to photosynthesis if ATP were not produced in the light reactions. A 100 15. Relate the rate of photosynthesis to carbon Percentage of open stomata dioxide levels. B 16. Unit 2–Photosynthesis 50 Many plants have stomata that take in CO2 at night and release it during the day. Write a report about these types of plants, and summarize why 0 this adaptation is an advantage for plants living in Noon Midnight Noon Midnight a hot, dry climate. 126 CHAPTER 6 Copyright © by Holt, Rinehart and Winston. All rights reserved.  Standardized Test Preparation DIRECTIONS: Choose the letter of the answer choice DIRECTIONS: Complete the following analogy. that best answers the question. 5. light reactions : ATP :: Calvin cycle : 1. Which of the following is a reactant in the Calvin A. H! cycle? B. O2 A. O2 C. G3P B. CO2 D. H2O C. H2O INTERPRETING GRAPHICS: The diagram below D. C6H12O6 shows a step in the process of chemiosmosis. Use the 2. Which of the following statements is correct? diagram to answer the question that follows. F. Accessory pigments are not involved in photosynthesis. Y G. Accessory pigments add color to plants but do not absorb light energy. ATP ADP + phosphate H. Accessory pigments absorb colors of light that chlorophyll a cannot absorb. J. Accessory pigments receive electrons STROMA from the electron transport chain of photo- system I. 3. Oxygen is produced at what point during Thylakoid photosynthesis? membrane A. when CO2 is fixed B. when water is split C. when ATP is converted into ADP INSIDE OF D. when 3-PGA is converted into G3P THYLAKOID Y INTERPRETING GRAPHICS: The diagram below shows a portion of a chloroplast. Use the diagram to 6. What is the substance identified as Y in the answer the question that follows. image? F. H! G. NAD! H. NADPH J. ADP synthase SHORT RESPONSE Chloroplasts are organelles with areas that conduct different specialized activities. Where in the chloroplast do the light reactions and the Calvin cycle occur? EXTENDED RESPONSE X The reactions of photosynthesis make up a biochemi- cal pathway. Part A What are the reactants and products for both 4. Which of the following correctly identifies the the light reactions and the Calvin cycle? structure marked X and the activities that take Part B Explain how the biochemical pathway of pho- place there? F. stroma—Calvin cycle tosynthesis recycles many of its own reac- G. stroma—light reactions tants, and identify the recycled reactants. H. thylakoid—Calvin cycle J. thylakoid—light reactions For a question involving a bio- chemical pathway, write out all of the reactants and products of each step before answering the question. PHOTOSYNTHESIS 127 Copyright © by Holt, Rinehart and Winston. All rights reserved.  EXPLORATION LAB Measuring the Rate of Photosynthesis OBJECTIVES Procedure Measure the amount of oxygen produced by an PART A Setting Up the Experiment elodea sprig. CAUTION Always wear Determine the rate of photosynthesis for elodea. safety goggles and a lab apron to protect your eyes and clothing. Glassware PROCESS SKILLS is fragile. Notify the teacher of broken glass or cuts. making observations Do not clean up broken glass or spills with broken measuring glass unless the teacher tells you to do so. Wear collecting data disposable polyethylene gloves when handling any graphing plant. Do not eat any part of a plant or plant seed MATERIALS used in the lab. Wash hands thoroughly after han- 500 mL of 5% baking-soda-and-water solution dling any part of a plant. 600 mL beaker 1. Add 450 mL of baking-soda-and-water solution 20 cm long elodea sprigs (2–3) to a beaker. glass funnel 2. Put two or three sprigs of elodea in the beaker. The test tube baking soda will provide the elodea with the carbon metric ruler dioxide it needs for photosynthesis. 3. Place the wide end of the funnel over the elodea. Background The end of the funnel with the small opening should 1. Summarize the main steps of photosynthesis. be pointing up. The elodea and the funnel should be 2. State the source of the oxygen produced during completely under the solution. photosynthesis. 4. Fill a test tube with the remaining baking-soda-and- 3. Identify factors that can affect the rate of water solution. Place your thumb over the end of photosynthesis. the test tube. Turn the test tube upside down, taking care that no air enters. Hold the opening of the test tube under the solution, and place the test tube over the small end of the funnel. Try not to let any solution leak out of the test tube as you do this. 5. Place the beaker setup in a well-lighted area near a lamp or in direct sunlight. 128 CHAPTER 6 Copyright © by Holt, Rinehart and Winston. All rights reserved. PART B Collecting Data 6. Create a data table like the one below. 7. Record that there was 0 mm gas in the test tube on day 0. (If you were unable to place the test tube without getting air in the tube, measure the height of the column of air in the test tube in millimeters. Record this value for day 0.) In this lab, change in gas volume is indicated by a linear measurement expressed in millimeters. 8. For days 1 through 5, measure the amount of gas in the test tube. Record the measurements in your data table under the heading “Total amount of gas present (mm).” 9. Calculate the amount of gas produced each day by subtracting the amount of gas present on the previous day from the amount of gas present today. Record these amounts under the heading “Amount of gas produced per day (mm).” Analysis and Conclusions 5. What may happen to the oxygen level if an animal, 1. Using the data from your table, prepare a graph. such as a snail, were put in the beaker with the elodea 2. Using information from your graph, describe what sprig while the elodea sprig was making oxygen? happened to the amount of gas in the test tube. 3. How much gas was produced in the test tube Further Inquiry after day 5? Design an experiment for predicting the effects of tempera- 4. Write the equation for photosynthesis. Explain each ture on the amount of oxygen produced or rate of photo- part of the equation. For example, which ingredients synthesis by elodea. are necessary for photosynthesis to take place? Which substances are produced by photosynthesis? Which gas is produced that we need in order to live? AMOUNT OF GAS PRESENT IN THE TEST TUBE Days of exposure to light Total amount of gas present (mm) Amount of gas produced per day (mm) 0 1 2 3 4 5 PHOTOSYNTHESIS 129 Copyright © by Holt, Rinehart and Winston. All rights reserved.