Photosynthesis: Using Light to Make Food

Photosynthesis: Using Light to Make Food
Chapter 7 Photosynthesis: Using Light to Make Food © 2016 Pearson Education, Inc.

Why Photosynthesis Matters
Figure 7.0-1 Why Photosynthesis Matters Figure Why photosynthesis matters

Biology and Society: A Greasy Crime Wave
In September 2013, police in Ocala, Florida, arrested two men and charged them with organized fraud and grand theft for stealing more than 700 gallons of used cooking oil pilfered from a variety of local eateries. As fossil fuel supplies dwindle and prices rise, the need for reliable, renewable sources of energy increases. Scientists are researching better ways to harness energy from biofuels, energy obtained from living material. Some focus on burning plant matter directly (wood pellet boilers, for example). Others focus on using plant material to produce biofuels that can be burned. 3

Chapter Thread: Biofuels
Figure 7.0-2 Figure Biofuels: using biofuels

There are several types of biofuels. Bioethanol is a type of alcohol that is made from wheat, corn, sugar beets, and other food crops and can be used directly as a fuel source in specially designed vehicles or as a gasoline additive. Cellulosic ethanol is a form of bioethanol made from cellulose found in nonedible plant material such as wood, grass, or scraps from crops. Biodiesel, the most common biofuel in Europe, is made from plant oils such as recycled frying oil. Today, only about 2.7% of the world’s fuel used for driving is provided by biofuels, but the International Energy Agency has set of goal of 25% by 2050. © 2016 Pearson Education, Inc. 5

Whatever its form, when we derive energy from biofuels, we are actually tapping into the energy of the sun, which drives the process of photosynthesis in plants. Photosynthesis is the process by which plants use light to make sugars from carbon dioxide—sugars that are food for the plant and the starting point for most of our own food. © 2016 Pearson Education, Inc. 6

The Basics of Photosynthesis
is used by plants, algae (protists), and certain bacteria, transforms light energy into chemical energy, uses carbon dioxide and water as starting materials, and releases oxygen gas as a by-product. The chemical energy produced via photosynthesis is stored in the bonds of sugar molecules. © 2016 Pearson Education, Inc. 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. 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. But nearly 50% of the carbohydrates produced by plant cells are used for cellular respiration (involving mitochondria). 3. 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. 4. 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. 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.) Teaching Tips 1. 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 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. 2. 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. 3. You might wish to discuss the evolution of chloroplasts from photosynthetic prokaryotes if you will not address this subject elsewhere in your course. 4. 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 sugar. 5. The thylakoid space and the intermembrane space of a mitochondrion have analogous roles. Students might be encouraged to create a list of the similarities in structure and function of mitochondria and chloroplasts through these related chapters. Active Lecture Tips 1. When introducing the diverse ways that plants impact our lives, challenge your students to work with others seated nearby to come up with a list of products made from plants that they encounter on a regular basis. The collective lists from your students can be surprisingly long and might help you build your own catalog of examples. 2. See the activity Photosynthesis and Respiration: Are They Similar? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 3. 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

Photosynthetic Protists Photosynthetic Bacteria
Photosynthetic autotrophs: Producer of most ecosystem Organisms that use photosynthesis are: photosynthetic autotrophs – photoautotrophs the producers for most ecosystems Plants (mostly on land) Photosynthetic Protists (aquatic) DIVERSITY OF PHOTOSYNTHETIC AUTOTROPHS Photosynthetic Bacteria Micrograph of cyanobacteria Kelp, a large alga Forest plants LM Figure 7.1 A diversity of photosynthetic autotrophs

Journey into a leaf (step 1)
Chloroplasts are light-absorbing organelles, the site of photosynthesis, and found mostly in the interior cells of leaves. Their green color is from chlorophyll, a pigment (light-absorbing molecule) that plays a central role in converting solar energy to chemical energy. Photosynthetic cells Mesophyll Vein CO2 O2 Stomata Leaf cross section Figure 7.2-s1 Journey into a leaf (step 1)

Mesophyll cells are specialized for photosynthesis, contain many chloroplasts. Stomata are tiny pores in leaves where carbon dioxide enters and oxygen exits. Photosynthetic cells Mesophyll LM Vein Interior cell Chloroplast CO2 O2 Stomata Leaf cross section Figure 7.2-s2 Journey into a leaf (step 2)

Chloroplasts: Sites of Photosynthesis
Like a mitochondrion, a chloroplast has a double-membrane envelope. The inner membrane encloses a compartment filled with stroma, a thick fluid. Suspended in the stroma are interconnected membranous sacs called thylakoids. The thylakoids are concentrated in stacks called grana (singular, granum). The chlorophyll molecules that capture light energy are built into the thylakoid membranes. © 2016 Pearson Education, Inc. 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. 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. But nearly 50% of the carbohydrates produced by plant cells are used for cellular respiration (involving mitochondria). 3. 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. 4. 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. 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.) Teaching Tips 1. 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 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. 2. 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. 3. You might wish to discuss the evolution of chloroplasts from photosynthetic prokaryotes if you will not address this subject elsewhere in your course. 4. 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 sugar. 5. The thylakoid space and the intermembrane space of a mitochondrion have analogous roles. Students might be encouraged to create a list of the similarities in structure and function of mitochondria and chloroplasts through these related chapters. Active Lecture Tips 1. When introducing the diverse ways that plants impact our lives, challenge your students to work with others seated nearby to come up with a list of products made from plants that they encounter on a regular basis. The collective lists from your students can be surprisingly long and might help you build your own catalog of examples. 2. See the activity Photosynthesis and Respiration: Are They Similar? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 3. 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. 11

Inner and outer membranes Vein (transports water and nutrients) Photosynthetic cells LM Stroma Thylakoid Interior cell Thylakoid space Chloroplast Granum CO2 O2 Stomata Colorized TEM Leaf cross section Figure 7.2-s3 Figure 7.2-s3 Journey into a leaf (step 3)

Energy Transformations: An Overview of Photosynthesis
In the overall equation for photosynthesis, notice that the reactants of photosynthesis, carbon dioxide (CO2) and water (H2O), are the same as the waste products of cellular respiration, and photosynthesis produces what respiration uses—glucose (C6H12O6) and oxygen (O2). Carbon dioxide 6 O2 6 CO2 6 H2O C6H12O6 Water Glucose Photosynthesis Oxygen gas Light energy 6 H2O C6H12O6 6 O2 This simple chemical equation highlights the relationship between photosynthesis and cellular respiration. The reactants of photosynthesis are the waste products of cellular respiration – PS takes the exhaust of CR and rearrange its atoms to produce food and oxygen. During respiration, a fall of e- from food molecules to O2 to form water release the energy that mitochondria can use it to make ATP. The opposite occurs in PS. E- are boosted uphill and added to CO2 to produce sugar. This process requires lot of energy which is supplied by the light energy and e- from hydrogen atom of water molecule 13

Energy Transformations: An Overview of Photosynthesis
Photosynthesis, a chemical transformation requires a lot of energy, Sunlight absorbed by chlorophyll provides that energy. Recall, cellular respiration is a process of electron transfer. A “fall” of electrons from food molecules to oxygen to form water releases the energy that mitochondria can use to make ATP. The opposite occurs in photosynthesis: Electrons are boosted “uphill” and added to carbon dioxide to produce sugar. water is split into hydrogen and Oxygen Hydrogen are transferred along with electrons and added to CO2 to produce sugar. Oxygen escapes through stomata into 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. 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. But nearly 50% of the carbohydrates produced by plant cells are used for cellular respiration (involving mitochondria). 3. 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. 4. 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. 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.) Teaching Tips 1. 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 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. 2. 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. 3. You might wish to discuss the evolution of chloroplasts from photosynthetic prokaryotes if you will not address this subject elsewhere in your course. 4. 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 sugar. 5. The thylakoid space and the intermembrane space of a mitochondrion have analogous roles. Students might be encouraged to create a list of the similarities in structure and function of mitochondria and chloroplasts through these related chapters. Active Lecture Tips 1. When introducing the diverse ways that plants impact our lives, challenge your students to work with others seated nearby to come up with a list of products made from plants that they encounter on a regular basis. The collective lists from your students can be surprisingly long and might help you build your own catalog of examples. 2. See the activity Photosynthesis and Respiration: Are They Similar? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 3. 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. 14

The role of oxygen in harvesting food energy
Electrons from food NAD+ e– e– NAD+ NADH Stepwise release of energy used to make ATP ATP 2 e   Electron transport chain 2 e 1 O2 2 H2O Hydrogens, electrons, and oxygen combine to form water 2 H+ Figure 6.9 Figure 6.9 The role of oxygen in harvesting food energy

Predicting Photosynthetic Process
Photosynthesis is a multistep chemical pathway, with each step in the path producing products that are used as reactants in the next step If photosynthesis is the reverse of cellular respiration, then what processes are needed? Remember that cellular respiration consisted of glycolysis, the citric acid cycle, and the electron transport chain If reversed, there should be a process handling electrons, a cycle that builds organic molecules from CO2, and a reaction that assembles the organic molecules into sugars 1- ETC 2- Cycle 3- Make glucose

A Photosynthesis Road Map
The initial incorporation of carbon from the atmosphere into organic compounds is called carbon fixation. This lowers the amount of carbon in the air. Deforestation, which removes a lot of photosynthetic plant life, thereby reduces the ability of the biosphere to absorb carbon. Planting new forests can have the opposite effect of fixing carbon from the atmosphere, potentially reducing the effect of the gases that contribute to global climate change. 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 (energized electrons are added to the CO2 to make sugar) 17

THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY
The light reactions convert solar energy to chemical energy chlorophyll in the thylakoid membranes absorbs solar energy, which is then converted to the chemical energy of ATP and NADPH, and Water is split, providing a source of electrons and giving off O2 gas as a by-product Chloroplast H2O Light NADP+ ADP  P Light reactions ATP   NADPH O2 Figure 7.3-s1 A road map for photosynthesis (step 1) In LR chlorophyll in thylakoid membrane absorbs solar energy, convert them to chemical energy of ATP and NADPH (e- carrier) Water is split, provides of source of e- and releases O2 gas In DR Uses the products of LR to power the production of sugar from CO2 Enzymes in stroma are used NADPH provides the high energy e- for the reduction of CO2 to glucose

The Calvin Cycle The Calvin cycle uses the products of the light reactions to make sugar from carbon dioxide. ATP generated by the light reactions provides the energy for sugar synthesis. The NADPH produced by the light reactions provides the high-energy electrons that drive the synthesis of glucose from carbon dioxide. Thus, the Calvin cycle indirectly depends on light to produce sugar because it requires the supply of ATP and NADPH produced by the light reactions. © 2016 Pearson Education, Inc. 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. 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. But nearly 50% of the carbohydrates produced by plant cells are used for cellular respiration (involving mitochondria). 3. 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. 4. 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. 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.) Teaching Tips 1. 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 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. 2. 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. 3. You might wish to discuss the evolution of chloroplasts from photosynthetic prokaryotes if you will not address this subject elsewhere in your course. 4. 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 sugar. 5. The thylakoid space and the intermembrane space of a mitochondrion have analogous roles. Students might be encouraged to create a list of the similarities in structure and function of mitochondria and chloroplasts through these related chapters. Active Lecture Tips 1. When introducing the diverse ways that plants impact our lives, challenge your students to work with others seated nearby to come up with a list of products made from plants that they encounter on a regular basis. The collective lists from your students can be surprisingly long and might help you build your own catalog of examples. 2. See the activity Photosynthesis and Respiration: Are They Similar? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 3. 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. 19

  H2O CO2 Chloroplast Light NADP+ ADP  P Calvin Light cycle
reactions Calvin cycle ATP   NADPH O2 Sugar Figure 7.3-s2 Figure 7.3-s2 A road map for photosynthesis (step 2) In LR chlorophyll in thylakoid membrane absorbs solar energy, convert them to chemical energy of ATP and NADPH (e- carrier) Water is split, provides of source of e- and releases O2 gas In DR Uses the products of LR to power the production of sugar from CO2 Enzymes in stroma are used NADPH provides the high energy e- for the reduction of CO2 to glucose

BioFlix Animation: Photosynthesis
© 2016 Pearson Education, Inc.

Chloroplasts are solar-powered sugar factories.
The Light Reactions: Converting Solar Energy to Chemical Energy CO2 Chloroplasts are solar-powered sugar factories. H2O Light NADP+ ADP + P Calvin cycle Light reactions ATP   NADPH O2 Sugar Figure 7.UN01 Figure 7.UN01 In-text figure, light reactions, p. 110

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 gamma rays [shortest] to radio signals [longest]) is called the electromagnetic spectrum. Visible light Wavelength (nm) Radio waves 500 600 750 700 Wavelength = 580 nm Micro- Gamma rays Infrared UV X-rays 10–5 nm 10–3 nm 103 nm 1 nm 106 nm 1 m 103 m Increasing wavelength Electromagnetic energy travels as rhythmic waves, oscillates between electric and magnetic field. The distance between the crests of 2 adjacent waves is called a wavelength. The full range of radiation of very short wavelength of gamma rays to very long wavelength of radiowaves is called the electromagnetic spectrum

Why are leaves green? Light Reflected light Chloroplast Absorbed light Transmitted light (detected by your eye) 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. Figure 7.6 Why are leaves green?

The Process of Science: What Colors of Light Drive Photosynthesis?
In the 19th century, botanists discovered that only certain wavelengths of light are used by plants Observation: In 1883, German biologist Theodor Engelmann saw that certain bacteria living in water tend to cluster in areas with higher oxygen concentrations. Question: Could this information determine which wavelengths of light work best for photosynthesis? Hypothesis: Oxygen-seeking bacteria will congregate near regions of algae performing the most photosynthesis. © 2016 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. 4. 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 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. The authors discuss a phenomenon that most students have noticed: Dark surfaces heat up faster in the sun than 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. 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. Active Lecture Tips 1. 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. 2. 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. 25

The Process of Science: What Colors of Light Drive Photosynthesis?
Experiment: Engelmann laid a string of freshwater algal cells in a drop of water on a microscope slide, added oxygen-sensitive bacteria to the drop, and used a prism to create a spectrum of light shining on the slide. Results: Most bacteria congregated around algae exposed to red-orange and blue-violet light and Very few moved to areas of green light. Conclusion: Other experiments have since verified that chloroplasts absorb light mainly in the blue-violet and red-orange part of the spectrum And those wavelengths of light are the ones mainly responsible for photosynthesis. © 2016 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. 4. 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 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. The authors discuss a phenomenon that most students have noticed: Dark surfaces heat up faster in the sun than 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. 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. Active Lecture Tips 1. 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. 2. 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. 26

Investigating how light wavelength affects photosynthesis
Prism Microscope slide Bacteria Bacteria Number of bacteria Algal cells 400 500 600 700 Wavelength of light (nm) Figure 7.5 Figure 7.5 Investigating how light wavelength affects photosynthesis

Animation: Light and Pigments
© 2016 Pearson Education, Inc.

Chloroplast Pigments The selective absorption of light by leaves explains why they appear green to us. Light of that color is poorly absorbed by chloroplasts and is thus reflected or transmitted toward the observer. Energy cannot be destroyed, so the absorbed energy must be converted to other forms. © 2016 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. 4. 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 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. The authors discuss a phenomenon that most students have noticed: Dark surfaces heat up faster in the sun than 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. 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. Active Lecture Tips 1. 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. 2. 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. 29

How Sunlight and Chloroplast Pigments Work in Photosynthesis
Chloroplasts contain several different pigments that absorb light of different wavelengths. The most important is chlorophyll Chlorophyll a participates directly in the light reactions and absorbs mainly blue-violet and red light. Chlorophyll b is very similar to chlorophyll a, absorbs mainly blue and orange light, and participates indirectly in the light reactions conveys absorbed energy to chlorophyll a. © 2016 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. 4. 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 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. The authors discuss a phenomenon that most students have noticed: Dark surfaces heat up faster in the sun than 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. 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. Active Lecture Tips 1. 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. 2. 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. 30

Carotenoids absorb mainly blue-green light, absorb and dissipate excessive light energy that might damage chlorophyll. Some carotenoids are human nutrients. Beta-carotene is a bright orange/red pigment found in pumpkins, sweet potatoes, and carrots and is converted to vitamin A in the body. Lycopene is a bright red pigment found in tomatoes, watermelon, and red peppers and is an antioxidant that is being studied for potential anti-cancer properties. 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. 4. 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 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. The authors discuss a phenomenon that most students have noticed: Dark surfaces heat up faster in the sun than 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. 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. Active Lecture Tips 1. 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. 2. 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. 31

The spectacular colors of fall foliage are due partly to the yellow-orange light reflected from carotenoids. The decreasing temperatures in autumn cause a decrease in the levels of chlorophyll. This allows the colors of the longer-lasting carotenoids to be seen in all their fall glory. 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. 4. 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 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. The authors discuss a phenomenon that most students have noticed: Dark surfaces heat up faster in the sun than 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. 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. Active Lecture Tips 1. 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. 2. 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. 32

How Photosystems Harvest Light Energy
Light behaves as waves and discrete packets of energy called photons, fixed quantities of light energy. Photons like electrons have no weight, but travelled very fast, and have a lot of energy Photon collide violently with the pigment molecules inside the chloroplast. The shorter the wavelength of light, the greater the energy of a photon. © 2016 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. 4. 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 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. The authors discuss a phenomenon that most students have noticed: Dark surfaces heat up faster in the sun than 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. 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. Active Lecture Tips 1. 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. 2. 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. 33

When a pigment molecule (Chlorophyll) absorbs a photon, one of the pigment’s electrons in the covalent bounds gains energy. This electron is now said to be “excited.” This energy splits H2O into ½ O2 + 2H+ + 2e- The excited state is highly unstable. An excited electron usually loses its excess energy and falls back to its ground state almost immediately. as the electrons fall back to their ground state, most pigments release energy as heat or light 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. 4. 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 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. The authors discuss a phenomenon that most students have noticed: Dark surfaces heat up faster in the sun than 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. 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. Active Lecture Tips 1. 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. 2. 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. 34

Excited electrons in pigments (part 1: absorption)
A pigment molecule can absorb a photon, causing the pigment's electrons to gain energy Most pigments only release heat energy as their light-executed electrons fall back to ground state Some pigments emit light along with heat after absorbing photons Excited state The electron falls to its ground state. Absorption of a photon excites an electron. e– Heat Light Light (fluorescence) Photon Ground state Chlorophyll molecule (a) Absorption of a photon Figure 7.8 Excited electrons in pigments 35

Excited electrons in pigments (part 2: fluorescence)
Breaking a glass vial starts a chemical reactions that excites electrons of a fluorescent dye The fluorescent light emitted by a glow stick is caused by a chemical reaction that excites electrons of a fluorescent dye. The excited electrons quickly fall back down to their ground state, releasing energy in the form of fluorescent light Figure 7.8 Excited electrons in pigments The fluorescent light in a glow stick is due to chemical reaction that excites electrons of a fluorescent dye. The excited e- falls back to ground state after releasing the energy in the form of fluorescent light

In the thylakoid membrane, chlorophyll molecules are organized with other molecules into photosystems. Each photosystem has a cluster of a few hundred pigment molecules, including chlorophylls a and b and some carotenoids that function as a light-gathering antenna. 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 chlorophyll a molecules that sit next to a primary electron acceptor, which traps the light-excited electron from the chlorophyll a in the reaction center. Another team of molecules built into the thylakoid membrane then uses that trapped energy 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. 4. 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 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. The authors discuss a phenomenon that most students have noticed: Dark surfaces heat up faster in the sun than 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. 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. Active Lecture Tips 1. 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. 2. 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. 37

A photosystem A photosystem is a group of chlorophyll and other molecules that function as a light-gathering antenna. Chloroplast Cluster of pigment molecules Photon Primary electron acceptor Reaction center Electron transfer e Reaction- center chlorophyll a Antenna Pigment molecules Transfer of energy Thylakoid membrane Photosystem Figure 7.9 A photosystem: light-gathering molecules that focus light energy onto a reaction center

How the Light Reactions Generate ATP & NADPH
The light reactions occurred in the thylakoid membrane Two photosystems cooperate in the light reactions: Photons excite electrons in the chlorophyll of water- splitting photosystem (WSP, first photosystem) . These photons are trapped by the primary electron acceptor. The WSP hen replaces the lost electrons by extracting new ones from water. This is the step that releases O2 during photosynthesis. Energized electrons from the WSP pass down an electron transport chain to the NADPH-producing photosystem (second photosystem). The chloroplast uses the energy released by this electron “fall” to make ATP. The second photosystem transfers its light-excited electrons to NADP+ ,(oxidized form) reducing it to NADPH (reduced form ) 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. 4. 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 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. The authors discuss a phenomenon that most students have noticed: Dark surfaces heat up faster in the sun than 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. 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. Active Lecture Tips 1. 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. 2. 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. 39

Light reactions (part 1: first photosystem)
Photons excite electrons (e-) in the chlorophyll of Water-splitting photosystem (WSP). The electrons are then trapped by the Primary electron acceptor. The WSP replaces its light –exited electrons by extracting electrons from water (release of O2) Primary electron acceptor Water-splitting photosystem Light H2O 2 H Reaction- center chlorophyll 2e – O2 + Figure 7.10 Light reactions (Step 1)

Light reactions (part 2: Electron transport chain)
Energy to make Those energized e- from WSP pass down the ETC to the NADPH-producing photosystem. The chloroplasts uses the energy released by this electron “fall” to make ATP ATP Primary electron acceptor 2e- Electron transport chain Light Reaction- center chlorophyll H2O 2e – Water-splitting photosystem 2 H + + O2 Figure 7.10 Light reactions (Step 2) 2. Energised e- from PS II pass down the ETC to the NADPH-producing PS. The chloroplast uses the energy released by this e- fall to make ATP

Light reactions (part 3: Second photosystem)
Primary electron acceptor NADP 2e – Energy to make ATP Primary electron acceptor 2e – NADPH 2e – Light e- boosted to Electron transport chain Light Series of Redox Reaction Reaction- center chlorophyll Reaction- center chlorophyll NADPH-producing photosystem H2O 2e – The NADPH-producing photosystem transfers its light-exited e- to NADP+, reducing it to NADPH. Water-splitting photosystem 2 H + + O2 Two places where energy is made Figure 7.10 Light reactions (Step 3) 3. The NADPH-producing PS transfers its light-excited e- to NADP+ to, reducing it to NADPH

How the Light Reactions Generate ATP and NADPH
The light reactions are located in the thylakoid membrane. The two photosystems and the electron transport chain that connects them transfer electrons from H2O to NADP+, producing 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. 4. 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 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. The authors discuss a phenomenon that most students have noticed: Dark surfaces heat up faster in the sun than 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. 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. Active Lecture Tips 1. 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. 2. 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. 43

How the thylakoid membrane converts light energy to the chemical energy of NADPH and ATP
Calvin cycle Light Light   H+ NADPH ATP ADP  P NADP+ H+ Stroma Electron transport chain Thylakoid membrane Photosystem Photosystem ATP synthase Inside thylakoid Electron flow 2e H+ H+ H2O H+ H+ H+ 1 2 O2 Thylakoid membrane This figure shows the location of light reaction in the thylakoid membrane Figure 7.11 Figure 7.11 How the thylakoid membrane converts light energy to the chemical energy of NADPH and ATP

Electron transport chain
Figure Light H+ Stroma Electron transport chain Thylakoid membrane Photosystem Inside thylakoid 2e H2O H+ 1 2 O2 Thylakoid membrane Figure How the thylakoid membrane converts light energy to the chemical energy of NADPH and ATP (part 1: first photosystem)

Calvin cycle Light Electron transport chain Photosystem Electron flow
Figure Calvin cycle Light   H+ NADPH ATP ADP + P NADP+ H+ Electron transport chain Photosystem ATP synthase Electron flow H+ H+ H+ H+ H+ Figure How the thylakoid membrane converts light energy to the chemical energy of NADPH and ATP (part 2: second photosystem)

A hard-hat analogy for the light reactions
– ATP e – e – – – NADPH e – e – e – Photon e – Photon Water-splitting photosystem NADPH-producing photosystem Figure 7.12 Figure 7.12 A hard-hat analogy for the light reactions 47

How the Light Reactions Generate ATP and NADPH
The mechanism of ATP production during the light reactions is very similar to the mechanism we saw in cellular respiration. In both cases, an electron transport chain pumps hydrogen ions across a membrane and ATP synthases use the energy stored by the H+ gradient to make ATP. The main difference is that food provides the high-energy electrons in cellular respiration, whereas light-excited electrons flow down the transport chain during photosynthesis. © 2016 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. 4. 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 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. The authors discuss a phenomenon that most students have noticed: Dark surfaces heat up faster in the sun than 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. 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. Active Lecture Tips 1. 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. 2. 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. 48

Electron transport chain ATP synthase Matrix
Figure Space between membranes H+ H+ H+ H+ H+ H+ H+ H+ Electron carrier H+ H+ H+ 3 H+ 5 H+ Protein complex Inner mitochondrial membrane   FADH2 FAD H+ Electron flow 2 1 O2 + 2 H+ H2O 6 2   4 NADH NAD+ ADP + P ATP 1 H+ H+ H+ H+ H+ Electron transport chain ATP synthase Matrix Figure How electron transport drives ATP synthase machines (part 1: detail)

In summary, light reactions take place in the thylakoid membranes
Products are NADPH ATP Oxygen.

The Calvin Cycle: Making Sugar from Carbon Dioxide
functions like a sugar factory within a chloroplast and regenerates the starting material with each turn. With each turn of the cycle, there are chemical inputs and outputs. The inputs are Three CO2 from the air and energy from ATP and NADPH produced by the light reactions. © 2016 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, 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. Teaching Tips 1. 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. Nearly 50% of the carbohydrates produced by plant cells are used for cellular respiration (involving mitochondria). 51

The Calvin Cycle: Making Sugar from Carbon Dioxide
The Calvin cycle constructs an energy-rich sugar molecule called glyceraldehyde 3-phosphate (G3P) using carbon from CO2, energy from ATP, and high-energy electrons from NADPH. The plant cell can then use G3P as the raw material to make glucose and other organic compounds. © 2016 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, 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. Teaching Tips 1. 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. Nearly 50% of the carbohydrates produced by plant cells are used for cellular respiration (involving mitochondria). 52

Calvin cycle (Step 1) Calvin cycle
Three-carbon molecule RuBP sugar CO2 (from air) Carbon dioxide (CO2) enters the cycle an enzyme adds each Co2 to a 5-C sugar called RuBP and the resulting molecule breaks into two 3-C molecules An enzyme adds each Co2 to a 5-C sugar called RuBP and the resulting molecule breaks into two 3-C molecules

Calvin cycle (Step 2) Calvin cycle
CO2 (from air) P RuBP sugar Three-carbon molecule ATP P P ADP  P Calvin cycle NADPH NADP G3P sugar P Using energy from ATP and NADPH produced by the LR, enzymes convert each 3-C molecule to the 3-C sugar (G3P) The Calvin cycle (Step 2) Using energy from ATP and NADPH produced by the LR, enzymes convert each 3-C molecule to the 3-C sugar (G3P)

Calvin cycle (Step 3) CO2 (from air) P For every 3 molecules of CO2 that enters the cycle, the Net output is one G3P sugar as a raw material for making glucose and other organic compounds . The other G3P sugars continue in the cycle 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 The Calvin cycle (Step 3) For every 3 molecules of CO2 that enters the cycle, the net output is one G2P sugar. The other G3P sugars continue in the cycle

CO2 (from air) Calvin cycle (Step 4) P RuBP sugar Three-carbon molecule ATP P P ADP  P ADP  P Calvin cycle NADPH ATP NADP G3P sugar G3P sugar P P Using energy from ATP, enzymes rearrange the remaining G3P sugars to regenerate RuBP G3P sugar Glucose (and other organic compounds) P Figure The Calvin cycle (Step 4) Using energy from ATP, enzymes rearrange the remaining G3P sugars to regenerate RuBP

Summary: Calvin cycle NADPH Calvin cycle ADP P NADP ATP G3P CO2 Glucose and other compounds It takes two turns of Calvin cycle to produce one molecule of glucose Figure 7.UN7 Summary: Calvin cycle

Evolution Connection: Creating a Better Biofuel Factory
The production of biofuels is highly inefficient. It is usually far more costly to produce biofuels than to extract the equivalent amount of fossil fuels. Biomechanical engineers are working to solve this dilemma by turning to an obvious example: evolution by natural selection. In nature, organisms with genes that make them better suited to their local environment will, on average, more often survive and pass those genes to the next generation. Repeated over many generations, genes that enhance survival within that environment will become more common, and the species evolves. 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, 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. Teaching Tips 1. 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. Nearly 50% of the carbohydrates produced by plant cells are used for cellular respiration (involving mitochondria). 59

When trying to solve an engineering problem, scientists can impose their own desired outcomes using a process called directed evolution, in which scientists in the laboratory determine which organisms are the fittest. Directed evolution of biofuel production often involves microscopic algae rather than plants because algae are easier to manipulate and maintain within the laboratory. © 2016 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, 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. Teaching Tips 1. 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. Nearly 50% of the carbohydrates produced by plant cells are used for cellular respiration (involving mitochondria). 60

Figure 7.14 Figure 7.14 Microscopic biofuel factories

The researcher starts with a large collection of individual alga, sometimes naturally occurring species and sometimes transgenic algae that have been engineered to carry useful genes, such as fungal genes for enzymes that break down cellulose. The algae are exposed to mutation-promoting chemicals. This produces a highly varied collection of algae that can be screened for the desired outcome: the ability to produce the most useful biofuel in the largest quantity. The tiny fraction of total algae that can best perform this task is grown and subjected to another round of mutation and selection. Repeated many times, the algae may slowly improve their ability to efficiently produce biofuels 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, 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. Teaching Tips 1. 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. Nearly 50% of the carbohydrates produced by plant cells are used for cellular respiration (involving mitochondria). 62

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

Review: light reaction
Figure 7.UN2 Orientation diagram: light reactions

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

Summary of Key Concepts: A Photosynthesis Road Map
Photosynthesis is the older process evolutionary compared to Cellular respiration (WHY?) Chloroplast CO2 Light H2O Stroma Stack of thylakoids NADP RuBP ADP 3-PGA P Calvin cycle Light reactions ATP – – G3P sugar NADPH Sugar used for cellular respiration cellulose starch other organic compounds O2 Sugar (C6H12O6) Figure 7-UN05 Summary of Key Concepts: A Photosynthesis Road Map 66

Summary: light reaction
NADPH-producing photosystem Water-splitting photosystem Figure 7.UN6 Summary: light reactions

  2e 2e 2e 2e NADP+ e acceptor ADP ATP e acceptor NADPH
Figure 7.UN05 NADP+ e acceptor 2e ADP ATP e acceptor 2e   2e NADPH Photon Electron transport chain Photon Chlorophyll H2O Chlorophyll Second photosystem 2e First photosystem 1 2 2 H+ + O2 Figure 7.UN05 Summary of key concepts: photosystems

Absorption of light by chloroplast pigments
Figure 7.UN07 Chloro- phyll a Chlorophyll b Absorption of light by chloroplast pigments Carotenoids 400 500 600 700 Wavelength of light (nm) Figure 7.UN07 Process of science, question 12 (absorption spectrum)

Photosynthesis: Using Light to Make Food

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