Photosynthesis: Using Light to make Food CHAPTER 7 Photosynthesis: Using Light to make Food

OVERALL, PHOTOSYNTHESIS... 6 CO2 + 6 H2O  SUNLIGHT (light energy) C6H12O6 + 6O2

Important details…. Light is required. It is the source of energy. The reaction takes place in the chloroplast of the plant cell. The chloroplast is filled with green pigment called chlorophyll.

PHOTOSYNTHESIS TAKES PLACE IN... CHLOROPLASTS

7.1 Photosynthesis fuels the biosphere Plants, algae, and some photosynthetic protists and bacteria are photoautotrophs, the producers of food consumed by heterotrophic organisms. Heterotrophs are consumers that feed on plants or animals or decompose organic material. Checkpoint Question Response Light, CO2, and H2O. (Minerals are also required; you’ll learn about the needs of plants in Chapter 32). Teaching Tips The evolution of chloroplasts from photosynthetic prokaryotes living inside of eukaryotic cells is discussed in Module 4.15. If your students have not already read Chapter 4, consider discussing this theory of endosymbiosis. (7.1) Some students might think that the term “producers” applies to the production of oxygen by plants. In turn, they might think that consumers are organisms that use oxygen (which would include all aerobic organisms). Extra care may be needed to clarify the definitions of these frequently used terms. (7.1) Active Lecture Tips When introducing the diverse ways that plants impact our lives, challenge your students to work with students seated nearby to come up with a list of products made from plants that they come across on a regular basis. The collective lists from your students can be surprisingly long and might help to build up your catalog of examples. (7.1)

7.2 Photosynthesis occurs in chloroplasts in plant cells Chloroplasts are surrounded by a double membrane and contain stacks of __________ and a thick fluid called __________. Chlorophyll is a light-absorbing ________ in the chloroplasts that plays a central role in converting solar energy to chemical energy. _____________ is responsible for the green color in plants. Checkpoint Question Response CO2 enters leaves through stomata, and H2O enters the roots and is carried to leaves through veins. Teaching Tips The authors note the analogous roles of the thylakoid space and the intermembrane space of a mitochondrion. Students might be encouraged to create a list of the similarities in structure and function of mitochondria and chloroplasts through these related chapters. (7.2) The living world contains many examples of adaptations to increase surface area. Some examples are the many folds of the inner mitochondrial membrane; the highly branched surfaces of plant roots, fish gills, and human lungs; and the highly branched system of capillaries in the tissues of our bodies. Consider relating this broad principle to the extensive folding of the thylakoid membranes. (7.2)

Leaf Cross Section Mesophyll Leaf Vein How do the reactant molecules of photosynthesis reach the chloroplasts in leaves? Mesophyll Cell CO2 O2 Stoma Chloroplast Figure 7.2 Zooming in on the location and structure of chloroplasts Inner and outer membranes Granum Thylakoid Thylakoid space Stroma

7.2 Photosynthesis occurs in chloroplasts in plant cells Required for photosynthesis: Chloroplasts- Stomata- Veins- Teaching Tips 1. The authors note the analogous roles of the thylakoid space and the intermembrane space of a mitochondrion. Students might be encouraged to create a list of the similarities in structure and function of mitochondria and chloroplasts through these related chapters. 2. The living world contains many examples of adaptations to increase surface area. Some examples are the many folds of the inner mitochondrial membrane, the highly branched surfaces of plant roots, fish gills, and human lungs, and the highly branched system of capillaries in the tissues of our bodies. Consider relating this broad principle to the extensive folding of the thylakoid membranes. 9

Chloroplast – Internal Structure Pigments are located in the membranes of the thylakoids.

7.4 Photosynthesis is a redox process, as is cellular respiration Photosynthesis, like respiration, is a redox (oxidation-reduction) process. CO2 becomes reduced to sugar as electrons along with hydrogen ions from water are added to it. Water molecules are oxidized when they lose electrons along with hydrogen ions. Teaching Tips In our world, energy is frequently converted to a usable form in one place and used in another. For example, electricity is generated by power plants, transferred to our homes, and used to run computers, create light, and help us prepare foods. Consider relating this common energy transfer to the two-stage process of photosynthesis. 11

Compare Cellular Respiration with Photosynthesis Becomes oxidized Becomes reduced Figure 7.4B Cellular respiration (releases chemical energy) photosynthesis 12

In photosynthesis (overview), light energy is captured by chlorophyll molecules to boost the energy of electrons, light energy is converted to chemical energy, and chemical energy is stored in the chemical bonds of sugars. Check out this animation…..https://www.youtube.com/watch?v=PkUpmV6ADdU Teaching Tips In our world, energy is frequently converted to a usable form in one place and used in another. For example, electricity is generated by power plants, transferred to our homes, and used to run computers, create light, and help us prepare foods. Consider relating this common energy transfer to the two-stage process of photosynthesis. 13

An overview of the two stages of photosynthesis in a chloroplast CO2 Light NADP+ ADP P Calvin Cycle Light Reactions (in stroma) (in thylakoids) ATP Figure 7.5_s3 An overview of the two stages of photosynthesis in a chloroplast (step 3) NADPH Chloroplast O2 Sugar An overview of the two stages of photosynthesis in a chloroplast 14

Nicotinamide Adenine Dinucleotide Phosphate In chloroplasts, NADP is reduced in last step of the electron chain of the light dependent reactions of photosynthesis. The NADPH formed is used in the light independent reactions.

7.5 Photosynthesis occurs in two stages, which are linked by ATP and NADPH Photosynthesis occurs in two metabolic stages. The light reactions occur in the _________ ________. _______ is split, providing a source of electrons and giving off oxygen as a by-product, ______ is generated from ADP and a phosphate group, and ________energy is absorbed by the chlorophyll molecules to drive the transfer of electrons and H+ from water to the electron acceptor NADP+ reducing it to NADPH. NADPH produced by the light reactions provides the electrons for reducing carbon in the Calvin cycle. Student Misconceptions and Concerns Students may understand the overall chemical relationships between photosynthesis and cellular respiration, but many struggle to understand the use of carbon dioxide in the Calvin cycle. Photosynthesis is much more than gas exchange. Teaching Tips 1. In our world, energy is frequently converted to a usable form in one place and used in another. For example, electricity is generated by power plants, transferred to our homes, and used to run computers, create light, and help us prepare foods. Consider relating this common energy transfer to the two-stage process of photosynthesis. 2. Figure 7.5 is an important visual organizer, which notes the key structures and functions of the two stages of photosynthesis. This figure demonstrates that water and sunlight are used in the thylakoid membranes to generate oxygen, ATP, and NADPH. The second step, in the stroma, reveals the use of carbon dioxide, ATP, and NADPH to ultimately generate carbohydrates. 16

7.5 Photosynthesis occurs in two stages, which are linked by ATP and NADPH The second stage is the Calvin cycle, which occurs in the ______________ of the chloroplast. The Calvin cycle is a cyclic series of reactions that assembles ______________ molecules using CO2 and the energy-rich products of the light reactions. During the Calvin cycle, ____________ is incorporated into organic compounds in a process called carbon fixation. After carbon fixation, enzymes of the cycle make _________ by further reducing the carbon compounds. The Calvin cycle is often called the dark reactions or light-independent reactions, because none of the steps requires light directly. Student Misconceptions and Concerns Students may understand the overall chemical relationships between photosynthesis and cellular respiration, but many struggle to understand the use of carbon dioxide in the Calvin cycle. Photosynthesis is much more than gas exchange. Teaching Tips 1. In our world, energy is frequently converted to a usable form in one place and used in another. For example, electricity is generated by power plants, transferred to our homes, and used to run computers, create light, and help us prepare foods. Consider relating this common energy transfer to the two-stage process of photosynthesis. 2. Figure 7.5 is an important visual organizer, which notes the key structures and functions of the two stages of photosynthesis. This figure demonstrates that water and sunlight are used in the thylakoid membranes to generate oxygen, ATP, and NADPH. The second step, in the stroma, reveals the use of carbon dioxide, ATP, and NADPH to ultimately generate carbohydrates. 17

7.6 Visible radiation absorbed by pigments drives the light reactions Sunlight is a type of energy called electromagnetic energy or radiation. Certain wavelengths of visible light are absorbed by chlorophyll and other pigments. Carotenoids also function in photoprotection from excessive light. What color of light is least effective at driving photosynthesis? Explain. Checkpoint Question Response Green, because it is mostly transmitted and reflected—not absorbed—by photosynthetic pigments. Student Misconceptions and Concerns The authors note that electromagnetic energy travels through space in waves that are like ripples made by a pebble dropped into a puddle. This wave imagery is helpful but can confuse students when energy is also thought of as discrete packets called photons. The dual nature of light, which exhibits the properties of waves and particles, may need to be discussed further if students are to do more than just accept definitions. (7.6) The authors note that sunlight is a type of radiation. Many students think of radiation as a result of radioactive decay, a serious threat to health. The diverse types of radiation and the varying energy associated with each might need to be explained. (7.6) Even at the college level, students struggle to understand why we perceive certain colors. The authors discuss the specific absorption and reflection of certain wavelengths of light, noting which colors are absorbed and which are reflected (and thus available for our eyes to detect). Consider spending time to make sure that your students understand how photosynthetic pigments absorb and reflect certain wavelengths. (7.6–7.7) Teaching Tips Consider bringing a prism to class and demonstrating the spectrum of light. Depending on what you have available, it can be a dramatic reinforcement. (7.6) Active Lecture Tips See the Activity “Demonstration of the Light Dependent Reactions of Photosynthesis Using Students as Molecules” on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. (7.6–7.9) See the Activity “Satellite TV and Photosystems” on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. (7.6–7.9)

Light, microwaves, x-rays, and TV and radio transmissions are all kinds of electromagnetic waves. They are all the same kind of wavy disturbance that repeats itself over a distance called the wavelength. One thing that all the forms of electromagnetic radiation have in common is that they can travel through empty space. This is not true of other kinds of waves; sound waves, for example, need some kind of material, like air or water, in which to move. Visible Light is 380-750nm

Gamma rays Micro- waves Radio waves X-rays Figure 7.6a 10−5 nm 10−3 nm 1 nm 103 nm 106 nm 1 m 103 m Gamma rays Micro- waves Radio waves X-rays UV Infrared Visible light Figure 7.6a The electromagnetic spectrum 380 400 500 600 700 750 Wavelength (nm) Shorter wavelength Longer wavelength Higher energy Lower energy

Light Reflected light Chloroplast Thylakoid Absorbed Transmitted light Figure 7.6b The interaction of light with chlorophyll in a chloroplast Chloroplast Thylakoid Absorbed light Transmitted light We see the color of the wavelengths that are transmitted. For example, chlorophyll transmits green wavelengths.

Chloroplasts contain several different pigments, which absorb light of different wavelengths. Chlorophyll a absorbs blue-violet and red light and reflects green. Chlorophyll b absorbs blue and orange and reflects yellow-green. Carotenoids and other pigments broaden the spectrum of colors that can drive photosynthesis Some provide photoprotection, absorbing and dissipating excessive light energy that would otherwise damage chlorophyll Student Misconceptions and Concerns 1. The authors note that electromagnetic energy travels through space in waves that are like ripples made by a pebble dropped in a pond. This wave imagery is helpful, but can confuse students when energy is also thought of as discrete packets called photons. The dual nature of light, which exhibits the properties of waves and particles, may need to be discussed further, if students are to do more than just accept definitions. 2. The authors note that sunlight is a type of radiation. Many students think of radiation as a result of radioactive decay, a serious threat to health. The diverse types of radiation and the varying energy associated with each might need to be explained. Teaching Tips Consider bringing a prism to class and demonstrating the spectrum of light. Depending on what you have available, it can be a dramatic reinforcement. 22

Accessory Pigments- Absorb light that chlorophyll a and b cannot!

Leaves have different pigments. other pigments become visible. When (green) chlorophyll begins to breakdown due to colder temperatures and shorter days, other pigments become visible.

Reactions of Photosynthesis 2 Reaction Stages (sets) light dependent reactions sun energy chemical energy light independent reactions or Calvin Cycle synthesis of glucose

7.7 Photosystems capture solar energy Thylakoid membranes contain photosystems, each consisting of light-harvesting complexes and a reaction-center complex. A primary electron acceptor receives photoexcited electrons from reaction-center chlorophyll a. Checkpoint Question Response In the chloroplasts, a light-excited electron from the reaction-center chlorophyll molecules is passed to a primary electron acceptor before it can fall back to the ground state. Student Misconceptions and Concerns Even at the college level, students struggle to understand why we perceive certain colors. The authors discuss the specific absorption and reflection of certain wavelengths of light, noting which colors are absorbed and which are reflected (and thus available for our eyes to detect). Consider spending time to make sure that your students understand how photosynthetic pigments absorb and reflect certain wavelengths. (7.6–7.7) Teaching Tips The authors discuss a phenomenon that most students have noticed: dark surfaces heat up faster in the sun than do lighter-colored surfaces. This is an opportunity to demonstrate to your students the various depths of scientific explanations and help them appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. In high school, they may have learned more about light energy and the fact that dark surfaces absorb more of this energy than lighter surfaces. Now, in college, they are learning that at the atomic level, darker surfaces absorb the energy of more photons, exciting more electrons, which then fall back to a lower state, releasing more heat. (7.7) Active Lecture Tips See the Activity “Demonstration of the Light Dependent Reactions of Photosynthesis Using Students as Molecules” on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. (7.6–7.9) See the Activity “Satellite TV and Photosystems” on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. (7.6–7.9)

7.7 Photosystems capture solar energy Pigments in chloroplasts absorb photons (unit of solar power), which increases the potential energy of the pigment’s electrons and sends the electrons into an unstable state. These unstable electrons drop back down to their “ground state,” and as they do, release their excess energy as heat. Student Misconceptions and Concerns Even at the college level, students struggle to understand why we perceive certain colors. The authors discuss the specific absorption and reflection of certain wavelengths of light, noting which colors are absorbed and which are reflected (and thus available for our eyes to detect). Consider spending time to make sure that your students understand how photosynthetic pigments absorb and reflect certain wavelengths. Teaching Tips The authors discuss a phenomenon that most students have noticed: dark surfaces heat up faster in the sun than do lighter-colored surfaces. This is an opportunity to demonstrate to your students the various depths of scientific explanations and help them appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. In high school, they may have learned about light energy and the fact that dark surfaces absorb more of this energy than lighter surfaces. Now, in college, they are learning that at the atomic level, darker surfaces absorb the energy of more photons, exciting more electrons, which then fall back to a lower state, releasing more heat. 27

Pair of chlorophyll a molecules A light-excited pair of chlorophyll molecules in the reaction center of a photosystem passing an excited electron to a primary electron acceptor (it becomes reduced) Photosystem Reaction-center complex Light-harvesting complexes Light Primary electron acceptor Figure 7.7B A light-excited pair of chlorophyll molecules in the reaction center of a photosystem passing an excited electron to a primary electron acceptor Thylakoid membrane Transfer of energy Pair of chlorophyll a molecules Pigment molecules 28

Animation: Light and Pigments © 2018 Pearson Education, Inc.

7.7 Photosystems capture solar energy Two types of photosystems (photosystem I and photosystem II) cooperate in the light reactions. Each type of photosystem has a characteristic reaction center. Photosystem II, which functions first, is called P680 because its pigment absorbs light with a wavelength of 680 nm. Photosystem I, which functions second, is called P700 because it absorbs light with a wavelength of 700 nm. Student Misconceptions and Concerns Even at the college level, students struggle to understand why we perceive certain colors. The authors discuss the specific absorption and reflection of certain wavelengths of light, noting which colors are absorbed and which are reflected (and thus available for our eyes to detect). Consider spending time to make sure that your students understand how photosynthetic pigments absorb and reflect certain wavelengths. Teaching Tips The authors discuss a phenomenon that most students have noticed: dark surfaces heat up faster in the sun than do lighter-colored surfaces. This is an opportunity to demonstrate to your students the various depths of scientific explanations and help them appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. In high school, they may have learned about light energy and the fact that dark surfaces absorb more of this energy than lighter surfaces. Now, in college, they are learning that at the atomic level, darker surfaces absorb the energy of more photons, exciting more electrons, which then fall back to a lower state, releasing more heat. 30

7.8 Two photosystems connected by an electron transport chain convert light energy to the chemical energy of ATP and NADPH In the light reactions, light energy is transformed into the chemical energy of ATP and NADPH. To accomplish this, electrons are removed from water, passed from photosystem II to photosystem I, and accepted by NADP+, reducing it to NADPH. Between the two photosystems, the electrons move down an electron transport chain and provide energy for the synthesis of ATP. Teaching Tips The authors develop a mechanical analogy for the energy levels and movement of electrons in the light reaction. Figure 7.8B equates the height of an electron with its energy state. Thus, electrons captured at high levels carry more energy than electrons in lower positions. Although this figure can be very effective, students might need to be carefully led through the analogy to understand precisely what is represented. 31

7.8 Two photosystems connected by an electron transport chain convert light energy to the chemical energy of ATP and NADPH Electron transport chain Provides energy for synthesis of ATP by chemiosmosis NADP  H NADPH Light Light Photosystem I 6 Photosystem II Stroma 1 Primary acceptor Primary acceptor 2 Thylakoid membrane 4 5 P680 P700 Figure 7.8A Electron flow in the light reactions: light energy driving electrons from water to NADPH Thylakoid space 3 H2O 2 1 O2 H  2 32

7.9 VISUALIZING THE CONCEPT: The light reactions take place within the thylakoid membranes In photophosphorylation, the electron transport chain pumps H+ into the thylakoid space. The concentration gradient drives H+ back through ATP synthase, powering the synthesis of ATP. Teaching Tips Module 7.9 can be the basis for an excellent comparison between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts. If your students have not already read or discussed chemiosmosis in mitochondria, consider assigning Modules 6.6 and 6.9 to show the similarities of these processes. (As noted in Module 7.2, the thylakoid space is analogous to the intermembrane space of mitochondria.) (7.9) Active Lecture Tips See the Activity “Demonstration of the Light Dependent Reactions of Photosynthesis Using Students as Molecules” on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. (7.6–7.9) See the Activity “Satellite TV and Photosystems” on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. (7.6–7.9)

Thylakoid sac Chloroplast Light Electron transport chain Light H+ NADP+ + H+ Photosystem II Photosystem I – – NADPH H+ – Primary electron acceptor – Pigment molecules Reaction center pair of chlorophyll a molecules H+ – – H+ H+ H+ H+ H2O H+ 1 H+ O2 + 2 H+ To Calvin Cycle 2 H+ H+ H+ H+ H+ Figure 7.9 The light reactions take place within the thylakoid membranes H+ H+ H+ H+ H+ H+ Thylakoid membrane ATP synthase H+ H+ H+ H+ H+ ADP + P ATP

Chemiosmosis powers ATP synthesis in the light reactions Chemiosmosis is the mechanism that generates ATP in chloroplasts. In photophosphorylation, using the initial energy input from light, the electron transport chain pumps H+ into the thylakoid space, and the resulting concentration gradient drives H+ back through ATP synthase, producing ATP. Teaching Tips Module 7.9 notes the similarities between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts. If your students have not already read or discussed chemiosmosis in mitochondria, consider assigning Modules 6.6 and 6.10 to show the similarities of these processes. (As noted in Module 7.2, the thylakoid space is analogous to the intermembrane space of mitochondria.) 35

3 VERY Important Concepts How is oxygen produced in the light dependent reaction? Energy carried by the e- is captured in _____________. Chemiosmosis produces __________

Light Dependent Reaction Steps 1. 2. 3. 4. 5.

The Calvin cycle makes sugar within a chloroplast. To produce sugar, the necessary ingredients are 1. 2. 3. The Calvin cycle produces an energy-rich, three-carbon sugar called glyceraldehyde-3-phosphate G3P). A plant cell then uses G3P to make glucose. Student Misconceptions and Concerns The terms light reactions and dark reactions can lead students to conclude that each set of reactions occurs at different times of the day. However, the Calvin cycle in most plants occurs during daylight, when NADPH and ATP from the light reactions are readily available. Teaching Tips Glucose is not the direct product of the Calvin cycle, as might be expected from the general equation for photosynthesis. Instead, as noted in the text, G3P is the main product. Clarify the diverse uses of G3P in the production of many important plant molecules for students. 38

7.10 ATP and NADPH power sugar synthesis in the Calvin cycle The steps of the Calvin cycle include carbon fixation, reduction, release of G3P, and regeneration of RuBP. Using carbon from CO2, electrons from NADPH, and energy from ATP, the cycle constructs G3P, which is used to build glucose and other organic molecules. Student Misconceptions and Concerns The terms light reactions and dark reactions can lead students to conclude that each set of reactions occurs at different times of the day. However, the Calvin cycle in most plants occurs during daylight, when NADPH and ATP from the light reactions are readily available. (7.10) Teaching Tips Glucose is not the direct product of the Calvin cycle, as might be expected from the general equation for photosynthesis. Instead, as noted in the text, G3P is the main product. Clarify the diverse uses of G3P in the production of many important plant molecules. (7.10)

Input 1 Carbon fixation. RuBP 4 Regeneration of RuBP. Calvin Cycle G3P H2O CO2 Light NADP+ ADP + P Light Reactions Calvin Input Cycle ATP 3 1 Carbon fixation. CO2 − − NADPH Chloroplast Rubisco O2 Sugar 3 6 P P P RuBP 6 ATP 3 ADP 4 Regeneration of RuBP. 6 ADP + P Calvin 3 ATP Cycle − − 6 NADPH Figure 7.10 Details of the Calvin cycle, which takes place in the stroma of a chloroplast 6 NADP+ 5 P 6 P G3P G3P 2 Reduction. 3 Release of one molecule of G3P. 1 P G3P Glucose and other compounds Output

Light inDependent Reaction Steps 1. 2. 3. 4. 5.

H2O CO2 Light NADP+ ADP + P Light Reactions Calvin Cycle ATP − − NADPH Figure 7.10_1 Details of the Calvin cycle, which takes place in the stroma of a chloroplast (part 1) − − NADPH Chloroplast O2 Sugar

Different Carbon Fixing Pathways The carbon fixing pathway previously described is more accurately called the C3 pathway due to the first stable compounds formed in the Calvin-Benson Cycle being 2, 3-carbon molecules (PGA) Different environments have adapted a different pathway. C4 pathway: high daytime temperatures, high intensity of sunlight, ex.- corn, sugarcane CAM pathway: high daytime temperatures, intense sunlight, low soil moisture, ex.- cacti, pineapples

7.11 Other methods of carbon fixation have evolved in hot, dry climates Most plants use CO2 directly from the air, and carbon fixation occurs when the enzyme rubisco adds CO2 to RuBP. Such plants are called C3 plants because the first product of carbon fixation is a three-carbon compound, 3-PGA. Teaching Tips 1. If you can find examples of C3, C4, and CAM plants, consider bringing them to class. Referring to living plants helps students understand these abstract concepts. Nice photographs can serve as a substitute. 2. Relate the properties of C3 and C4 plants to the regions of the country where each is grown. Students might generally understand that crops have specific requirements, but may not specifically relate these physiological differences to their geographic sites of production or specific evolutionary histories. 44

C4 plants have evolved a means of carbon fixation that saves water during photosynthesis while optimizing the Calvin cycle. C4 plants are so named because they first fix CO2 into a four-carbon compound. When the weather is hot and dry, C4 plants keep their stomata mostly closed, thus conserving water. Teaching Tips 1. If you can find examples of C3, C4, and CAM plants, consider bringing them to class. Referring to living plants helps students understand these abstract concepts. Nice photographs can serve as a substitute. 2. Relate the properties of C3 and C4 plants to the regions of the country where each is grown. Students might generally understand that crops have specific requirements, but may not specifically relate these physiological differences to their geographic sites of production or specific evolutionary histories. 45

CO2 is fixed into a four-carbon compound, Another adaptation to hot and dry environments has evolved in the CAM plants, such as pineapples and cacti. CAM plants conserve water by opening their stomata and admitting CO2 only at night. CO2 is fixed into a four-carbon compound, which banks CO2 at night and releases it to the Calvin cycle during the day. Teaching Tips 1. If you can find examples of C3, C4, and CAM plants, consider bringing them to class. Referring to living plants helps students understand these abstract concepts. Nice photographs can serve as a substitute. 2. Relate the properties of C3 and C4 plants to the regions of the country where each is grown. Students might generally understand that crops have specific requirements, but may not specifically relate these physiological differences to their geographic sites of production or specific evolutionary histories. 47

Mesophyll cell CO2 CO2 Night 4-C compound 4-C compound Bundle- sheath cell CO2 CO2 Calvin Cycle Calvin Cycle Figure 7.11 Comparison of C4 and CAM photosynthesis 3-C sugar 3-C sugar Day C4 plant CAM plant Sugarcane Pineapple 48

7.12 Photosynthesis provides food and O2 for almost all living organisms Most of the living world depends on the food-making machinery of photosynthesis. The chloroplast integrates the two stages of photosynthesis and makes sugar from CO2. Student Misconceptions and Concerns Some students do not realize that plant cells also have mitochondria. Instead, they assume that the chloroplasts are sufficient for the plant cell’s needs. As noted in the text, nearly 50% of the carbohydrates produced by plant cells are used for cellular respiration (involving mitochondria). Teaching Tips 1. Challenge students to explain how the energy in beef is ultimately derived from the sun. 2. The authors note that G3P is also used to produce cellulose, the most abundant organic molecule in a plant and probably on the surface of the Earth! 49

Figure 7.12 A summary of photosynthesis CO2 O2 Sucrose (export) H2O Light H2O CO2 Chloroplast NADP+ ADP Light + P Reactions RuBP Calvin Cycle (in stroma) Photosystem II Figure 7.12 A summary of photosynthesis Electron transport chain Thylakoids ATP Photosystem I Stroma − − NADPH H2O G3P Cellular respiration Other organic compounds O2 Sugars Starch Cellulose

7.12 Photosynthesis provides food and O2 for almost all living organisms About half of the carbohydrates made by photosynthesis are consumed as fuel for cellular respiration in the mitochondria of plant cells. Sugars also serve as the starting material for making other organic molecules, such as proteins, lipids, and cellulose. Excess food made by plants is stockpiled as starch in roots, tubers, seeds, and fruits. Student Misconceptions and Concerns Some students do not realize that plant cells also have mitochondria. Instead, they assume that the chloroplasts are sufficient for the plant cell’s needs. As noted in the text, nearly 50% of the carbohydrates produced by plant cells are used for cellular respiration (involving mitochondria). Teaching Tips 1. Challenge students to explain how the energy in beef is ultimately derived from the sun. 2. The authors note that G3P is also used to produce cellulose, the most abundant organic molecule in a plant and probably on the surface of the Earth! 51

Calvin Cycle (in stroma) Electron transport chain H2O CO2 Chloroplast Light NADP ADP P Light Reactions RuBP Photosystem II Calvin Cycle (in stroma) 3-PGA Electron transport chain Thylakoids Photosystem I ATP Stroma Figure 7.12 A summary of photosynthesis NADPH G3P Cellular respiration Cellulose Starch O2 Sugars Other organic compounds 52

7.13 SCIENTIFIC THINKING: Rising atmospheric levels of carbon dioxide may affect plants in various ways The greenhouse effect operates on a global scale. Solar radiation includes visible light that penetrates the Earth’s atmosphere and warms the planet’s surface. Heat radiating from the warmed planet is absorbed by gases in the atmosphere, which then reflects some of the heat back to Earth. Without the warming of the greenhouse effect, the Earth would be much colder and most life as we know it could not exist. Student Misconceptions and Concerns 1. Students often do not fully understand how the burning of fossil fuels contributes to global warming. They might wonder, “How does the burning of fossil fuels differ from the burning of ethanol produced from crops?” Students might not realize that the carbon in fossil fuels was removed from the atmosphere hundreds of millions of years ago, while the carbon in crops was removed much more recently, when the crops were grown. The use of ethanol as an alternative is complicated by the typical reliance upon fossil fuels for ethanol production. 2. Students may confuse global warming with the breakdown of the ozone layer. Be prepared to explain both phenomena and the impact of human activities. Teaching Tips Some students might better relate the greenhouse effect to what happens inside their closed car on a sunny day. The glass in our automobiles functions like the glass of a greenhouse, trapping heat inside our car. This can be an advantage during the winter but is usually not welcome on a hot summer day! 53

Greenhouse Effect The gases in the atmosphere that absorb heat radiation are called greenhouse gases. These include water vapor, carbon dioxide and methane. Student Misconceptions and Concerns 1. Students often do not fully understand how the burning of fossil fuels contributes to global warming. They might wonder, “How does the burning of fossil fuels differ from the burning of ethanol produced from crops?” Students might not realize that the carbon in fossil fuels was removed from the atmosphere hundreds of millions of years ago, while the carbon in crops was removed much more recently, when the crops were grown. The use of ethanol as an alternative is complicated by the typical reliance upon fossil fuels for ethanol production. 2. Students may confuse global warming with the breakdown of the ozone layer. Be prepared to explain both phenomena and the impact of human activities. Teaching Tips Some students might better relate the greenhouse effect to what happens inside their closed car on a sunny day. The glass in our automobiles functions like the glass of a greenhouse, trapping heat inside our car. This can be an advantage during the winter but is usually not welcome on a hot summer day! 54

Global Climate Change Increasing concentrations of greenhouse gases have been linked to global climate change (also called global warming), a slow but steady rise in Earth’s surface temperature. Since 1850, the atmospheric concentration of CO2 has increased by about 40%, mostly due to the combustion of fossil fuels including coal, oil, and gasoline. Student Misconceptions and Concerns 1. Students often do not fully understand how the burning of fossil fuels contributes to global warming. They might wonder, “How does the burning of fossil fuels differ from the burning of ethanol produced from crops?” Students might not realize that the carbon in fossil fuels was removed from the atmosphere hundreds of millions of years ago, while the carbon in crops was removed much more recently, when the crops were grown. The use of ethanol as an alternative is complicated by the typical reliance upon fossil fuels for ethanol production. 2. Students may confuse global warming with the breakdown of the ozone layer. Be prepared to explain both phenomena and the impact of human activities. Teaching Tips Some students might better relate the greenhouse effect to what happens inside their closed car on a sunny day. The glass in our automobiles functions like the glass of a greenhouse, trapping heat inside our car. This can be an advantage during the winter but is usually not welcome on a hot summer day! © 2012 Pearson Education, Inc. 55

Global Climate Change Some predicted consequences of continued warming include melting of polar ice, rising sea levels, extreme weather patterns, droughts, increased extinction rates, and the spread of tropical diseases. Student Misconceptions and Concerns 1. Students often do not fully understand how the burning of fossil fuels contributes to global warming. They might wonder, “How does the burning of fossil fuels differ from the burning of ethanol produced from crops?” Students might not realize that the carbon in fossil fuels was removed from the atmosphere hundreds of millions of years ago, while the carbon in crops was removed much more recently, when the crops were grown. The use of ethanol as an alternative is complicated by the typical reliance upon fossil fuels for ethanol production. 2. Students may confuse global warming with the breakdown of the ozone layer. Be prepared to explain both phenomena and the impact of human activities. Teaching Tips Some students might better relate the greenhouse effect to what happens inside their closed car on a sunny day. The glass in our automobiles functions like the glass of a greenhouse, trapping heat inside our car. This can be an advantage during the winter but is usually not welcome on a hot summer day! 56

Deforestation Widespread deforestation has aggravated the global warming problem by reducing an effective CO2 sink. Global warming caused by increasing CO2 levels may be reduced by limiting deforestation, reducing fossil fuel consumption, and growing biofuel crops that remove CO2 from the atmosphere. Student Misconceptions and Concerns 1. Students often do not fully understand how the burning of fossil fuels contributes to global warming. They might wonder, “How does the burning of fossil fuels differ from the burning of ethanol produced from crops?” Students might not realize that the carbon in fossil fuels was removed from the atmosphere hundreds of millions of years ago, while the carbon in crops was removed much more recently, when the crops were grown. The use of ethanol as an alternative is complicated by the typical reliance upon fossil fuels for ethanol production. 2. Students may confuse global warming with the breakdown of the ozone layer. Be prepared to explain both phenomena and the impact of human activities. Teaching Tips Some students might better relate the greenhouse effect to what happens inside their closed car on a sunny day. The glass in our automobiles functions like the glass of a greenhouse, trapping heat inside our car. This can be an advantage during the winter but is usually not welcome on a hot summer day! 57

You should now be able to Define autotrophs, heterotrophs, producers, and photoautotrophs. Describe the structure of chloroplasts and their location in a leaf. Explain how plants produce oxygen. Describe the role of redox reactions in photosynthesis and cellular respiration. Compare the reactants and products of the light reactions and the Calvin cycle. 58

You should now be able to Describe the properties and functions of the different photosynthetic pigments. Explain how photosystems capture solar energy. Explain how the electron transport chain and chemiosmosis generate ATP, NADPH, and oxygen in the light reactions. Compare photophosphorylation and oxidative phosphorylation. Describe the reactants and products of the Calvin cycle. 59

You should now be able to Compare the mechanisms that C3, C4, and CAM plants use to obtain and use carbon dioxide. Review the overall process of the light reactions and the Calvin cycle, noting the products, reactants, and locations of every major step. Describe the greenhouse effect. Explain how reducing the use of fossil fuels and deforestation may moderate climate change 60

Photosynthesis converts includes both (a) (b) (c) to in which in which Figure 7.UN03 Photosynthesis converts includes both (a) (b) (c) to in which in which chemical energy light-excited electrons of chlorophyll CO2 is fixed to RuBP H2O is split and then and 3-carbon molecule is reduced (d) are passed down reduce NADP+ to Figure 7.UN03 Connecting the concepts, question 1 using (e) (f) to produce producing chemiosmosis by (g) (h)

Mitochondrion Chloroplast Intermembrane space H+ c. Membrane Matrix d. Figure 7.UN04 Mitochondrion Chloroplast Intermembrane space H+ c. Membrane Figure 7.UN04 Testing your knowledge, question 14 Matrix d. a. e. b.