Photosynthesis: Using Light to Make Food Chapter 7 Photosynthesis: Using Light to Make Food

Biology and Society: Green Energy Wood has historically been the main fuel used to: Cook Warm homes Provide light at night Industrialized societies replaced wood with fossil fuels. To limit the damaging effects of fossil fuels, researchers are investigating the use of biomass (living material) as efficient and renewable energy sources. © 2010 Pearson Education, Inc.

Figure 7.0 Figure 7.0 Sunlight and leaves

Fast-growing trees, such as willows: Can be cut every three years Do not need to be replanted Are a renewable energy source Produce fewer sulfur compounds Reduce erosion Provide habitat for wildlife

THE BASICS OF PHOTOSYNTHESIS Is used by plants, some protists, and some bacteria Transforms light energy into chemical energy Uses carbon dioxide and water as starting materials The chemical energy produced via photosynthesis is stored in the bonds of sugar molecules. Organisms that use photosynthesis are: Photosynthetic autotrophs The producers for most ecosystems Student Misconceptions and Concerns 1. 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. 2. Students may not connect the growth in plant mass to the fixation of carbon during the Calvin cycle. It can be difficult for many students to appreciate that molecules in air can contribute significantly to the mass of plants. Teaching Tips 1. When introducing the diverse ways that plants impact our lives, consider challenging your students to come up with a list of products made from plants that they encounter regularly. Perhaps you might only list those encountered in a single day of college life. The list can be surprising and help to build up your “catalog of examples.” 2. Within the living world, there are many examples of adaptations to increase surface area. Some examples are the many folds of the inner mitochondrial membrane, the highly branched surfaces of fish gills and human lungs, and the highly branched system of capillaries in the tissues of our bodies. Consider relating this broad principle seen elsewhere to the extensive folding of the thylakoid membranes. 3. 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. 4. You might wish to discuss the evolution of chloroplasts from photosynthetic prokaryotes if you will not address this subject elsewhere in your course. 5. Students who have not read all of chapter 7 may not realize that glucose is not the direct product of photosynthesis. Although glucose is shown as a product of photosynthesis, a three-carbon sugar is directly produced (G3P). A plant can use G3P to make many types of organic molecules, including glucose. (The authors address the production of G3P under the section “The Calvin Cycle” later in this chapter.) 6. Figure 7.3 is an important visual organizer that notes the key structures and functions of the two stages of photosynthesis. This figure reminds students where 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 generate carbohydrates.

Photosynthetic Protists Photosynthetic Bacteria PHOTOSYNTHETIC AUTOTROPHS Plants (mostly on land) Photosynthetic Protists (aquatic) Photosynthetic Bacteria (aquatic) LM Forest plants Kelp, a large alga Micrograph of cyanobacteria Figure 7.1 Figure 7.1 Photosynthetic autotrophs

Chloroplasts: Sites of Photosynthesis Chloroplasts are: The site of photosynthesis Found mostly in the interior cells of leaves Inside chloroplasts are membranous sacs called thylakoids, which are suspended in a thick fluid, called stroma. Thylakoids are concentrated in stacks called grana. The green color of chloroplasts is from chlorophyll, a light-absorbing pigment. Stomata are tiny pores in leaves where carbon dioxide enters and oxygen exits. Student Misconceptions and Concerns 1. 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. 2. Students may not connect the growth in plant mass to the fixation of carbon during the Calvin cycle. It can be difficult for many students to appreciate that molecules in air can contribute significantly to the mass of plants. Teaching Tips 1. When introducing the diverse ways that plants impact our lives, consider challenging your students to come up with a list of products made from plants that they encounter regularly. Perhaps you might only list those encountered in a single day of college life. The list can be surprising and help to build up your “catalog of examples.” 2. Within the living world, there are many examples of adaptations to increase surface area. Some examples are the many folds of the inner mitochondrial membrane, the highly branched surfaces of fish gills and human lungs, and the highly branched system of capillaries in the tissues of our bodies. Consider relating this broad principle seen elsewhere to the extensive folding of the thylakoid membranes. 3. 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. 4. You might wish to discuss the evolution of chloroplasts from photosynthetic prokaryotes if you will not address this subject elsewhere in your course. 5. Students who have not read all of chapter 7 may not realize that glucose is not the direct product of photosynthesis. Although glucose is shown as a product of photosynthesis, a three-carbon sugar is directly produced (G3P). A plant can use G3P to make many types of organic molecules, including glucose. (The authors address the production of G3P under the section “The Calvin Cycle” later in this chapter.) 6. Figure 7.3 is an important visual organizer that notes the key structures and functions of the two stages of photosynthesis. This figure reminds students where 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 generate carbohydrates.

Vein Stomata Leaf cross section Figure 7.2-1 CO2 Stomata O2 Leaf cross section Figure 7.2-1 Figure 7.2 Journey into a leaf (Step 1)

Inner membrane Outer membrane Chloroplast Vein Granum Stroma Thylakoid CO2 Stomata O2 LM Leaf cross section Interior cell TEM Figure 7.2-2 Figure 7.2 Journey into a leaf (Step 2)

CO2 O2 Vein Stomata Leaf cross section Figure 7.2a Figure 7.2a Leaf cross section

Chloroplast Inner membrane Outer membrane Granum Stroma Thylakoid TEM Figure 7.2b Figure 7.2b Chloroplast

The Overall Equation for Photosynthesis In the overall equation for photosynthesis, notice that: The reactants of photosynthesis are the waste products of cellular respiration. Student Misconceptions and Concerns 1. 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. 2. Students may not connect the growth in plant mass to the fixation of carbon during the Calvin cycle. It can be difficult for many students to appreciate that molecules in air can contribute significantly to the mass of plants. Teaching Tips 1. When introducing the diverse ways that plants impact our lives, consider challenging your students to come up with a list of products made from plants that they encounter regularly. Perhaps you might only list those encountered in a single day of college life. The list can be surprising and help to build up your “catalog of examples.” 2. Within the living world, there are many examples of adaptations to increase surface area. Some examples are the many folds of the inner mitochondrial membrane, the highly branched surfaces of fish gills and human lungs, and the highly branched system of capillaries in the tissues of our bodies. Consider relating this broad principle seen elsewhere to the extensive folding of the thylakoid membranes. 3. 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. 4. You might wish to discuss the evolution of chloroplasts from photosynthetic prokaryotes if you will not address this subject elsewhere in your course. 5. Students who have not read all of chapter 7 may not realize that glucose is not the direct product of photosynthesis. Although glucose is shown as a product of photosynthesis, a three-carbon sugar is directly produced (G3P). A plant can use G3P to make many types of organic molecules, including glucose. (The authors address the production of G3P under the section “The Calvin Cycle” later in this chapter.) 6. Figure 7.3 is an important visual organizer that notes the key structures and functions of the two stages of photosynthesis. This figure reminds students where 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 generate carbohydrates.

Light energy 6 CO2 6 H2O C6H12O6 6 O2 Photo- synthesis Carbon dioxide Water Glucose Oxygen gas Figure 7.UN1 Figure 7.UN1 Photosynthesis equation

During photosynthesis, water is split into: In photosynthesis: Sunlight provides the energy Electrons are boosted “uphill” and added to carbon dioxide Sugar is produced During photosynthesis, water is split into: Hydrogen Oxygen Hydrogen is transferred along with electrons and added to carbon dioxide to produce sugar. Oxygen escapes through stomata into the atmosphere. Student Misconceptions and Concerns 1. 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. 2. Students may not connect the growth in plant mass to the fixation of carbon during the Calvin cycle. It can be difficult for many students to appreciate that molecules in air can contribute significantly to the mass of plants. Teaching Tips 1. When introducing the diverse ways that plants impact our lives, consider challenging your students to come up with a list of products made from plants that they encounter regularly. Perhaps you might only list those encountered in a single day of college life. The list can be surprising and help to build up your “catalog of examples.” 2. Within the living world, there are many examples of adaptations to increase surface area. Some examples are the many folds of the inner mitochondrial membrane, the highly branched surfaces of fish gills and human lungs, and the highly branched system of capillaries in the tissues of our bodies. Consider relating this broad principle seen elsewhere to the extensive folding of the thylakoid membranes. 3. 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. 4. You might wish to discuss the evolution of chloroplasts from photosynthetic prokaryotes if you will not address this subject elsewhere in your course. 5. Students who have not read all of chapter 7 may not realize that glucose is not the direct product of photosynthesis. Although glucose is shown as a product of photosynthesis, a three-carbon sugar is directly produced (G3P). A plant can use G3P to make many types of organic molecules, including glucose. (The authors address the production of G3P under the section “The Calvin Cycle” later in this chapter.) 6. Figure 7.3 is an important visual organizer that notes the key structures and functions of the two stages of photosynthesis. This figure reminds students where 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 generate carbohydrates.

A Photosynthesis Road Map Photosynthesis occurs in two stages: The light reactions convert solar energy to chemical energy The Calvin cycle uses the products of the light reactions to make sugar from carbon dioxide Student Misconceptions and Concerns 1. 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. 2. Students may not connect the growth in plant mass to the fixation of carbon during the Calvin cycle. It can be difficult for many students to appreciate that molecules in air can contribute significantly to the mass of plants. Teaching Tips 1. When introducing the diverse ways that plants impact our lives, consider challenging your students to come up with a list of products made from plants that they encounter regularly. Perhaps you might only list those encountered in a single day of college life. The list can be surprising and help to build up your “catalog of examples.” 2. Within the living world, there are many examples of adaptations to increase surface area. Some examples are the many folds of the inner mitochondrial membrane, the highly branched surfaces of fish gills and human lungs, and the highly branched system of capillaries in the tissues of our bodies. Consider relating this broad principle seen elsewhere to the extensive folding of the thylakoid membranes. 3. 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. 4. You might wish to discuss the evolution of chloroplasts from photosynthetic prokaryotes if you will not address this subject elsewhere in your course. 5. Students who have not read all of chapter 7 may not realize that glucose is not the direct product of photosynthesis. Although glucose is shown as a product of photosynthesis, a three-carbon sugar is directly produced (G3P). A plant can use G3P to make many types of organic molecules, including glucose. (The authors address the production of G3P under the section “The Calvin Cycle” later in this chapter.) 6. Figure 7.3 is an important visual organizer that notes the key structures and functions of the two stages of photosynthesis. This figure reminds students where 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 generate carbohydrates.

H2O Light Light reactions O2 ATP NADPH Chloroplast Figure 7.3-1 Figure 7.3 Photosynthesis roadmap (Step 1)

H2O CO2 Light Calvin Light cycle reactions O2 NADP+ ADP P ATP NADPH Chloroplast NADP+ ADP P Calvin cycle Light reactions ATP NADPH O2 Sugar (C6H12O6) Figure 7.3-2 Figure 7.3 Photosynthesis roadmap (Step 2)

THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY Chloroplasts: Are chemical factories powered by the sun Convert solar energy into chemical energy © 2010 Pearson Education, Inc. Student Misconceptions and Concerns 1. 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. 2. The authors note that electromagnetic energy travels through space in waves that are like those 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 as a wave and particle may need to be discussed further, if students are to do more than just accept the definitions. 3. The light reactions are not solely responsible for all of the products in the general chemical equation of photosynthesis. Point out that oxygen is generated in the light reactions but that glucose results from products of the Calvin cycle. Teaching Tips 1. Consider bringing a prism to class and demonstrating the spectrum of visible light. Depending on what you have available, it can be a dramatic reinforcement of this key concept. You might show an image of a rainbow if you are willing to explain how it is produced. 2. Consider bringing a glow stick to class just to help illustrate the authors’ note about the light-generating reactions in glow sticks. Small demonstrations break up lectures and stir attention. 3. Here is a fundamental explanation for 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 to appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. At high school, they may have learned about light energy and 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. 4. The authors develop a mechanical analogy to the energy levels and movement of electrons in the light reaction. Figure 7.12 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. 5. The authors note 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 the relevant parts of chapter 6 (including Figure 6.11) to show the similarities of these processes.

The Nature of Sunlight Sunlight is a type of energy called radiation, or electromagnetic energy. The full range of radiation is called the electromagnetic spectrum. The distance between the crests of two adjacent waves is called a wavelength. The full range of radiation, from the very short gamma ray wavelengths to the very long radio signals, is called electromagnetic spectrum. Visible light is only a small fraction of the spectrum; it consists of the wavelengths that our eyes see as different colors. Student Misconceptions and Concerns 1. 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. 2. The authors note that electromagnetic energy travels through space in waves that are like those 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 as a wave and particle may need to be discussed further, if students are to do more than just accept the definitions. 3. The light reactions are not solely responsible for all of the products in the general chemical equation of photosynthesis. Point out that oxygen is generated in the light reactions but that glucose results from products of the Calvin cycle. Teaching Tips 1. Consider bringing a prism to class and demonstrating the spectrum of visible light. Depending on what you have available, it can be a dramatic reinforcement of this key concept. You might show an image of a rainbow if you are willing to explain how it is produced. 2. Consider bringing a glow stick to class just to help illustrate the authors’ note about the light-generating reactions in glow sticks. Small demonstrations break up lectures and stir attention. 3. Here is a fundamental explanation for 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 to appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. At high school, they may have learned about light energy and 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. 4. The authors develop a mechanical analogy to the energy levels and movement of electrons in the light reaction. Figure 7.12 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. 5. The authors note 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 the relevant parts of chapter 6 (including Figure 6.11) to show the similarities of these processes.

When sunlight shines on a pigmented material, certain wavelengths (colors) of the visible light are absorbed and disappear from the light that is reflected by the material. For example, we see a pair of jeans as blue because pigments in the fabric absorb the other colors, leaving only light in the blue part of the spectrum to be reflected from the fabric to our eyes.

Micro- waves Radio waves Infrared Increasing wavelength 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m Micro- waves Radio waves Gamma rays X-rays UV Infrared Visible light 380 400 500 600 700 750 Wavelength (nm) Wavelength = 580 nm Figure 7.4 Figure 7.4 The electromagnetic spectrum

The selective absorption of light by leaves explains why they appear green to us; light of that color is poorly absorbed by chloroplasts (chlorophyll) and is thus reflected toward the observer. Since energy cannot be destroyed, the absorbed energy must be converted to other forms, such as chemical energy.

Light Reflected light Chloroplast Absorbed light Transmitted light Figure 7.6 Figure 7.6 Why are leaves green?

Light Reflected light Chloroplast Absorbed light Transmitted light Figure 7.6a Figure 7.6a Why are leaves green?

Chloroplast Pigments Chloroplasts contain several pigments: Chlorophyll a: Absorbs mostly blue-violet and red light Participates directly in the light reactions Chlorophyll b: Absorbs mostly blue and orange light Participates indirectly in the light reactions, but it broadens the range of light that a plant can use by conveying absorbed energy to chlorophyll a, which then puts the energy to work in the light reactions. Student Misconceptions and Concerns 1. 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. 2. The authors note that electromagnetic energy travels through space in waves that are like those 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 as a wave and particle may need to be discussed further, if students are to do more than just accept the definitions. 3. The light reactions are not solely responsible for all of the products in the general chemical equation of photosynthesis. Point out that oxygen is generated in the light reactions but that glucose results from products of the Calvin cycle. Teaching Tips 1. Consider bringing a prism to class and demonstrating the spectrum of visible light. Depending on what you have available, it can be a dramatic reinforcement of this key concept. You might show an image of a rainbow if you are willing to explain how it is produced. 2. Consider bringing a glow stick to class just to help illustrate the authors’ note about the light-generating reactions in glow sticks. Small demonstrations break up lectures and stir attention. 3. Here is a fundamental explanation for 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 to appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. At high school, they may have learned about light energy and 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. 4. The authors develop a mechanical analogy to the energy levels and movement of electrons in the light reaction. Figure 7.12 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. 5. The authors note 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 the relevant parts of chapter 6 (including Figure 6.11) to show the similarities of these processes.

Absorb mainly blue-green light Chloroplasts also contain a family of yellow-orange pigments called Carotenoids: Absorb mainly blue-green light Participate indirectly in the light reactions, passing energy to chlorophyll a Protective function: Absorb and dissipate excessive light energy that might damage chlorophyll The spectacular colors of fall foliage are due partly to the yellow-orange light reflected from carotenoids. The falling autumn temps cause a decrease in the levels of chlorophyll, allowing the colors of the longer-lasting carotenoids to show through. All of these chloroplast pigments are built into the thykaloid membranes. There the pigments are organized into light-harvesting complexes called photosystems. Student Misconceptions and Concerns 1. 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. 2. The authors note that electromagnetic energy travels through space in waves that are like those 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 as a wave and particle may need to be discussed further, if students are to do more than just accept the definitions. 3. The light reactions are not solely responsible for all of the products in the general chemical equation of photosynthesis. Point out that oxygen is generated in the light reactions but that glucose results from products of the Calvin cycle. Teaching Tips 1. Consider bringing a prism to class and demonstrating the spectrum of visible light. Depending on what you have available, it can be a dramatic reinforcement of this key concept. You might show an image of a rainbow if you are willing to explain how it is produced. 2. Consider bringing a glow stick to class just to help illustrate the authors’ note about the light-generating reactions in glow sticks. Small demonstrations break up lectures and stir attention. 3. Here is a fundamental explanation for 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 to appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. At high school, they may have learned about light energy and 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. 4. The authors develop a mechanical analogy to the energy levels and movement of electrons in the light reaction. Figure 7.12 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. 5. The authors note 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 the relevant parts of chapter 6 (including Figure 6.11) to show the similarities of these processes.

Figure 7.7 Figure 7.7 Photosynthetic pigments

How Photosystems Harvest Light Energy The theory of light as waves explains most of light’s properties. However, Light also behaves as photons, discrete packets of energy. A photon is a fixed quantity of light energy. The shorter the wavelength of light, the greater the energy of a photon. A photon of violet light packs twice as much energy as a photon of red light. Chlorophyll molecules absorb photons. Electrons in the pigment gain energy, electrons have become “excited”, that is, the electrons has been raised from a ground to an excited state. Excited e- are unstable, so then electrons fall back to their ground state, energy is released as heat or light. (Black gives off heat) But some pigments emit light after absorbing photons. Ex. Fluorescence. In the thykaloid membrane, chlorophyll is organized with other molecules into photosystems. A photosystem is a group of chlorophyll and other molecules that function as a light-gathering antenna. Student Misconceptions and Concerns 1. 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. 2. The authors note that electromagnetic energy travels through space in waves that are like those 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 as a wave and particle may need to be discussed further, if students are to do more than just accept the definitions. 3. The light reactions are not solely responsible for all of the products in the general chemical equation of photosynthesis. Point out that oxygen is generated in the light reactions but that glucose results from products of the Calvin cycle. Teaching Tips 1. Consider bringing a prism to class and demonstrating the spectrum of visible light. Depending on what you have available, it can be a dramatic reinforcement of this key concept. You might show an image of a rainbow if you are willing to explain how it is produced. 2. Consider bringing a glow stick to class just to help illustrate the authors’ note about the light-generating reactions in glow sticks. Small demonstrations break up lectures and stir attention. 3. Here is a fundamental explanation for 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 to appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. At high school, they may have learned about light energy and 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. 4. The authors develop a mechanical analogy to the energy levels and movement of electrons in the light reaction. Figure 7.12 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. 5. The authors note 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 the relevant parts of chapter 6 (including Figure 6.11) to show the similarities of these processes.

(a) Absorption Excited state of a photon Light Heat Light (fluorescence) Photon Ground state Chlorophyll molecule (b) Fluorescence of a glow stick Figure 7.8 Figure 7.8 Excited electrons in pigments

When a photon strikes one of the pigment molecules, the energy jumps from molecule to molecule until it arrives at the reaction center of the photosystem. The reaction center consists of a chlorophyll a that sits next to another molecule called primary electron acceptor. This primary e- acceptor traps the light-excited e- from chlorophyll a in the reaction center. Another team of molecules built into the thykaloids then uses that trapped energy to make ATP and NADPH

Chloroplast Figure 7.9-1 Figure 7.9 A photosystem (Step 1)

Chloroplast Thylakoid membrane Figure 7.9-2 Pigment molecules Figure 7.9 A photosystem (Step 2)

Chloroplast Thylakoid membrane Photosystem Transfer of energy Pigment molecules Photon Primary electron acceptor Reaction center Electron transfer Reaction- center chlorophyll a Antenna pigment molecules Transfer of energy Thylakoid membrane Photosystem Figure 7.9-3 Figure 7.9 A photosystem (Step 3)

How the Light Reactions Generate ATP and NADPH Two types of photosystems cooperate in the light reactions: The water-splitting photosystem The NADPH-producing photosystem Student Misconceptions and Concerns 1. 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. 2. The authors note that electromagnetic energy travels through space in waves that are like those 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 as a wave and particle may need to be discussed further, if students are to do more than just accept the definitions. 3. The light reactions are not solely responsible for all of the products in the general chemical equation of photosynthesis. Point out that oxygen is generated in the light reactions but that glucose results from products of the Calvin cycle. Teaching Tips 1. Consider bringing a prism to class and demonstrating the spectrum of visible light. Depending on what you have available, it can be a dramatic reinforcement of this key concept. You might show an image of a rainbow if you are willing to explain how it is produced. 2. Consider bringing a glow stick to class just to help illustrate the authors’ note about the light-generating reactions in glow sticks. Small demonstrations break up lectures and stir attention. 3. Here is a fundamental explanation for 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 to appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. At high school, they may have learned about light energy and 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. 4. The authors develop a mechanical analogy to the energy levels and movement of electrons in the light reaction. Figure 7.12 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. 5. The authors note 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 the relevant parts of chapter 6 (including Figure 6.11) to show the similarities of these processes.

Figure 7.10-1 Primary electron acceptor Light Reaction- center – Light Reaction- center chlorophyll H2O 2e – Water-splitting photosystem 2 H + + O2 Figure 7.10-1 Figure 7.10 Light reactions (Step 1)

Electron transport chain Energy to make ATP Primary electron acceptor 2e – Electron transport chain Light Reaction- center chlorophyll H2O 2e – Water-splitting photosystem 2 H + + O2 Figure 7.10-2 Figure 7.10 Light reactions (Step 2)

Electron transport chain Primary electron acceptor NADP 2e – Energy to make ATP Primary electron acceptor 2e – NADPH 2e – Light Electron transport chain Light Reaction- center chlorophyll Reaction- center chlorophyll NADPH-producing photosystem H2O 2e – Water-splitting photosystem 2 H + + O2 Figure 7.10-3 Figure 7.10 Light reactions (Step 3)

The light reactions are located in the thylakoid membrane. An electron transport chain: Connects the two photosystems Releases energy that the chloroplast uses to make ATP and NADPH Student Misconceptions and Concerns 1. 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. 2. The authors note that electromagnetic energy travels through space in waves that are like those 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 as a wave and particle may need to be discussed further, if students are to do more than just accept the definitions. 3. The light reactions are not solely responsible for all of the products in the general chemical equation of photosynthesis. Point out that oxygen is generated in the light reactions but that glucose results from products of the Calvin cycle. Teaching Tips 1. Consider bringing a prism to class and demonstrating the spectrum of visible light. Depending on what you have available, it can be a dramatic reinforcement of this key concept. You might show an image of a rainbow if you are willing to explain how it is produced. 2. Consider bringing a glow stick to class just to help illustrate the authors’ note about the light-generating reactions in glow sticks. Small demonstrations break up lectures and stir attention. 3. Here is a fundamental explanation for 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 to appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. At high school, they may have learned about light energy and 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. 4. The authors develop a mechanical analogy to the energy levels and movement of electrons in the light reaction. Figure 7.12 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. 5. The authors note 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 the relevant parts of chapter 6 (including Figure 6.11) to show the similarities of these processes.

Figure 7.11 – To Calvin cycle Light Light Stroma ATP Thylakoid NADPH ATP ADP  P NADP H Stroma Electron transport chain Thylakoid membrane Photosystem Photosystem ATP synthase Inside thylakoid Electron flow 2e – H H H2O H H H+ O2 Figure 7.11 Figure 7.11 How the thylakoid membrane converts light energy to the chemical energy of NADPH and ATP

– To Calvin cycle Light Light Stroma ATP Thylakoid synthase membrane NADPH ATP ADP  P NADP H Stroma Electron transport chain Thylakoid membrane Photosystem Photosystem ATP synthase Inside thylakoid Electron flow 2e – H H H2O H H H O2 Figure 7.11a Figure 7.11a How the thylakoid membrane converts light energy to the chemical energy of NADPH and ATP

– – – – – – – ATP NADPH Water-splitting photosystem NADPH-producing Photon e – Photon Water-splitting photosystem NADPH-producing photosystem Figure 7.12 Figure 7.12 A hard-hat analogy for the light reactions

THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE Functions like a sugar factory within the stroma of a chloroplast Regenerates the starting material with each turn © 2010 Pearson Education, Inc. Student Misconceptions and Concerns 1. Glucose is not the direct product of the Calvin cycle, as might be expected from the general equation for photosynthesis. Instead, G3P, as noted in the text, is the main product. Clarify for students the diverse uses of G3P in the production of many important plant molecules and the advantages of producing a molecule with this flexibility. Teaching Tips 1. If you can find examples of potted C3, C4, and CAM plants, consider bringing them to class. Referring to living plants in class 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 are grown. Students might generally understand that crops have specific requirements, but may not specifically relate these physiological differences to their geographic sites of production.

Three-carbon molecule CO2 (from air) P RuBP sugar Three-carbon molecule P P Calvin cycle Figure 7.13-1 Figure 7.13 The Calvin cycle (Step 1)

Three-carbon molecule CO2 (from air) P RuBP sugar Three-carbon molecule ATP P P ADP  P Calvin cycle NADPH NADP G3P sugar P Figure 7.13-2 Figure 7.13 The Calvin cycle (Step 2)

Three-carbon molecule CO2 (from air) P RuBP sugar Three-carbon molecule ATP P P ADP  P Calvin cycle NADPH NADP G3P sugar G3P sugar P P G3P sugar Glucose (and other organic compounds) P Figure 7.13-3 Figure 7.13 The Calvin cycle (Step 3)

Three-carbon molecule CO2 (from air) P RuBP sugar Three-carbon molecule ATP P P ADP  P ADP  P Calvin cycle NADPH ATP NADP G3P sugar G3P sugar P P G3P sugar Glucose (and other organic compounds) P Figure 7.13-4 Figure 7.13 The Calvin cycle (Step 4)

Evolution Connection: Solar-Driven Evolution C3 plants: Use CO2 directly from the air Are very common and widely distributed © 2010 Pearson Education, Inc. Student Misconceptions and Concerns 1. Glucose is not the direct product of the Calvin cycle, as might be expected from the general equation for photosynthesis. Instead, G3P, as noted in the text, is the main product. Clarify for students the diverse uses of G3P in the production of many important plant molecules and the advantages of producing a molecule with this flexibility. Teaching Tips 1. If you can find examples of potted C3, C4, and CAM plants, consider bringing them to class. Referring to living plants in class 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 are grown. Students might generally understand that crops have specific requirements, but may not specifically relate these physiological differences to their geographic sites of production.

C4 plants: Close their stomata to save water during hot and dry weather Can still carry out photosynthesis CAM plants: Are adapted to very dry climates Open their stomata only at night to conserve water Student Misconceptions and Concerns 1. Glucose is not the direct product of the Calvin cycle, as might be expected from the general equation for photosynthesis. Instead, G3P, as noted in the text, is the main product. Clarify for students the diverse uses of G3P in the production of many important plant molecules and the advantages of producing a molecule with this flexibility. Teaching Tips 1. If you can find examples of potted C3, C4, and CAM plants, consider bringing them to class. Referring to living plants in class 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 are grown. Students might generally understand that crops have specific requirements, but may not specifically relate these physiological differences to their geographic sites of production.

C4 Pathway (example: sugarcane) CAM Pathway (example: pineapple) ALTERNATIVE PHOTOSYNTHETIC PATHWAYS C4 Pathway (example: sugarcane) CAM Pathway (example: pineapple) Cell type 1 CO2 CO2 Night Four-carbon compound Four-carbon compound CO2 CO2 Cell type 2 Calvin cycle Calvin cycle Sugar Sugar Day C4 plant CAM plant Figure 7.14 Figure 7.14 C4 and CAM photosynthesis

Light energy 6 CO2 6 H2O C6H12O6 6 O2 Photo- synthesis Carbon dioxide Water Glucose Oxygen gas Figure 7.UN1 Figure 7.UN1 Photosynthesis equation

Calvin cycle Light reactions CO2 H2O Light NADP ADP P Calvin cycle Light reactions ATP NADPH Sugar O2 (C6H12O6) Figure 7.UN2 Figure 7.UN2 Orientation diagram: light reactions

Calvin cycle Light reactions CO2 Light H2O NADP ADP P Calvin cycle Light reactions ATP NADPH Sugar O2 (C6H12O6) Figure 7.UN3 Figure 7.UN3 Orientation diagram: Calvin cycle

Light energy Photosynthesis Carbon dioxide Water Glucose Oxygen gas C6H12O6 6 O2 Photosynthesis Carbon dioxide Water Glucose Oxygen gas Figure 7.UN4 Figure 7.UN4 Summary: photosynthesis equation

H2O Chloroplast CO2 Light Stack of thylakoids NADP+ Stroma ADP Calvin cycle Light reactions ATP NADPH O2 Sugar (C6H12O6) Figure 7.UN5 Figure 7.UN5 Summary: light reactions and Calvin cycle

– – NADP+ ADP ATP NADPH Photon Photon H2O NADPH-producing photosystem acceptor e – 2e – acceptor 2e NADPH – Photon Electron transport chain Photon Chlorophyll H2O Chlorophyll NADPH-producing photosystem 2e – Water-splitting photosystem 2 H 2 1 + O2 + Figure 7.UN6 Figure 7.UN6 Summary: light reactions

CO2 ATP ADP P Calvin cycle NADPH NADP G3P Glucose and other compounds Figure 7.UN7 Figure 7.UN7 Summary: Calvin cycle