6____________________________________________________________________________________________

WHERE IT STARTS—PHOTOSYNTHESISChapter Outline

6.1 BIOFUELS6.2 SUNLIGHT AS AN ENERGY SOURCE

Properties of LightPigments: The Rainbow Catchers

6.3 EXPLORING THE RAINBOW6.4 OVERVIEW OF PHOTOSYNTHESIS6.5 LIGHT-DEPENDENT REACTIONS

The Noncyclic PathwayThe Cyclic Pathway

6.6 THE LIGHT-INDEPENDENT REACTIONS

Energy Flow in PhotosynthesisLight-Independent Reactions

6.7 ADAPTATIONS: ALTERNATIVE CARBON-FIXING PATHWAYS

6.8 BIOFUELS (REVISITED)SUMMARYSELF-QUIZ DATA ANALYSIS ACTIVITIESCRITICAL THINKING

Learning Objectives6.1 Examine the importance of biofuels.6.2 Examine the characteristics of sunlight.6.3 Examine how pigments capture light.6.4 Examine how the wavelength of light affects photosynthesis.6.5 Examine the role of pigments in photosynthesis.6.6 Describe the process of photosynthesis using a flowchart.6.7 Outline the importance of light-dependent reactions in the process of photosynthesis.6.8 Outline the cyclic and noncyclic pathways involved in energy transportation in photosynthetic

organisms.6.9 Examine the stages in the light-independent reactions of photosynthesis.6.10 Examine the different methods used by photosynthetic organisms to fix carbon.

Key Termsautotrophs C3 plantsC4 plants Calvin-Benson cycleCAM plantscarbon fixation

chlorophyll aelectron transfer phosphorylation

heterotrophslight-dependent reactions

light-independent reactions

photolysisphotorespirationphotosystempigment

rubiscostomatastromathylakoid membranewavelength

Where It Starts—Photosynthesis

Lecture Outline 6.1 Biofuels

A. Photosynthesis uses sunlight as an energy source and stores the energy in chemical bonds of organic molecules.1. Autotrophs make their own food via photosynthesis.

2. Heterotrophs obtain energy by breaking down molecules made by autotrophs or by consuming other heterotrophs. 3. As organic material dies and decays it compacts and becomes fossil fuels. These fuels —coal, petroleum, and natural gas—are limited. 4. Biomass (organic matter that is not fossilized) is a renewable source of energy converted from sunlight.

6.2 Sunlight as an Energy SourceA. Properties of Light

1. Organisms use only a small range of wavelengths for photosynthesis, vision, and other processes.2. Most of these wavelengths are the ones we see as visible light, a small part of the electromagnetic spectrum from the sun.3. Light energy is packaged as photons, which vary in energy as a function of wavelength. (The shortest are gamma rays, with the highest energy; the longest are radio waves, with the lowest energy.)4. Only a small range (380–750 nm) of wavelengths is used for photosynthesis.

B. Pigments: The Rainbow Catchers1. The light reflected from each pigment gives the pigment its color. For example, red pigment absorbs all colors of visible light except red light. 2. There is more than one kind of chlorophyll. Chlorophyll a is the most common pigment used in photosynthesis by plants, photosynthetic protists, and cyanobacteria.3. Carotenoid pigments absorb blue-violet and blue-green but reflect yellow, orange, and red.4. Xanthophylls reflect yellow, brown, purple, or blue light; anthocyanins reflect red and purple light in fruit and flowers; phycobilins reflect red or blue-green light and are accessory pigments found in red algae and cyanobacteria.5. The chlorophylls in green leaves mask the accessory pigments until autumn, when the chlorophyll content declines.

6. A pigment absorbs light of specific wavelengths by acting as an antenna for receiving photon energy.

7. Photosynthesis begins when photosynthetic pigments absorb a photon of light.

6.3 Exploring the RainbowA. Photosynthetic pigments work together to harvest light of different wavelengths.

B. In an elegant experiment with photosynthesizing algae and aerobic bacteria, Engelmann determined that violet and red light were the wavelengths best suited for photosynthesis.

6.4 Overview of PhotosynthesisA. A Look Inside the Chloroplast

1. Both stages of photosynthesis occur in the chloroplast.a. The semifluid interior (stroma) is the site for the second series of photosynthesis reactions.

Chapter Six

b. Flattened sacs, thylakoids, interconnected by channels weave through the stroma; the first reactions occur here.

2. In the thylakoid membranes, pigments are organized in clusters called photosystems, each consisting of 200–300 pigment molecules capable of trapping energy from the sun.

B. Two Stages of Reactions1. Light-dependent reactions convert light energy to chemical bond energy of ATP.

a. Water is split to release oxygen.b. NADP+ picks up electrons to become NADPH to be used later.

2. The light-independent reactions assemble sugars and other organic molecules using ATP, NADPH, and CO2.3. Overall, the equation for glucose formation is written:

light energy & enzymes6H2O + 6CO2

——————————> 6O2 + C6H12O6

6.5 Light-Dependent ReactionsA. The Noncyclic Pathway

1. Light energy causes electrons to exit from a photosystem II.2. Electrons are pulled from water, which separates into oxygen and hydrogen ions. a. The oxygen is released.3. The electrons enter the electron transfer chain in the membranes of the thylakoids.

4. The energy lost by the electrons moves hydrogen ions from the stroma into the thylakoid compartment.

a. There is the formation of a hydrogen ion gradient across the thylakoid membrane.5. Light energy forces electrons out of photosystem I.

a. An electron chain in the thylakoid membrane serves as a source of replacement electrons.

6. Electrons from step 5 move through another electron transport chain. a. The electrons then combine with NADP+ and H+ to form NADPH.

7. Hydrogen ions in the thylakoid compartment travel through the interior of ATP synthases because of the gradient across the thylakoid membrane.

8. This movement of hydrogen ions makes ATP synthases attach phosphate to ADP, forming ATP.

B. The Cyclic Pathway1. The cyclic pathway occurs when oxygen levels are high, or when NADPH accumulates in

the stroma.2. This process deals with photosystem I and an electron transfer chain that feeds electrons

back into itself.3. In this method, NADPH and oxygen are not formed.

6.6 The Light-Independent ReactionsA. These reactions constitute a pathway known as the Calvin-Benson cycle.

1. The participants and their roles in the synthesis of carbohydrates are: a. ATP, which provides energy.

b. NADPH, which provides hydrogen atoms and electrons.c. Atmospheric air, which provides the carbon and oxygen from carbon dioxide.

2. The reactions take place in the stroma of chloroplasts and are not dependent on sunlight.B. Carbon dioxide diffuses into a leaf across the plasma membrane of a photosynthetic cell.

Where It Starts—Photosynthesis

1. Taking carbon from an inorganic source and incorporating that carbon into an organic molecule is called carbon fixation. 2. Rubisco joins carbon dioxide to RuBP to produce an unstable intermediate that splits to form two molecules of PGA.3. Each PGA then receives a Pi from ATP plus H+ and electrons from NADPH to form PGAL (phosphoglyceraldehyde).4. Most of the PGAL molecules continue in the cycle to fix more carbon dioxide, but two PGAL join to form a sugar-phosphate, which will be modified to sucrose, starch, and cellulose.

6.8 Adaptations: Alternative Carbon-Fixing PathwaysA. Photorespiration

1. Plants in hot, dry environments close their stomata to conserve water, but in so doing retard carbon dioxide entry and permit oxygen buildup inside the leaves.

2. C3 plants are named so because the three-carbon PGA is the first stable intermediate of the Calvin-Benson cycle. Thus, oxygen—not carbon dioxide—becomes attached to RuBP to yield one PGA (instead of two) and one phosphoglycolate (not useful), and photorespiration dominates.3. C3 plants such as beans do not grow well without irrigation in hot, dry climates.

B. C4 Plants1. To overcome this fate, crabgrass, sugarcane, corn, and other C4 plants fix carbon twice (in mesophyll cells, then in bundle-sheath cells) to produce oxaloacetate (a four-carbon compound), which can then donate the carbon dioxide to the Calvin-Benson cycle.2. Photorespiration hampers growth in C3 plants, but natural selection has not eliminated it from the population, perhaps because the gene coding for rubisco structure is linked to carbon fixation—rubisco’s primary role.

C. CAM Plants1. CAM (Crassulacean Acid Metabolism) plants, such as cacti, open their stomata and fix CO2 only at night, storing the intermediate product for use in photosynthesis the next day.2. CAM plants use the C4 cycle to break down crassulacean acid to CO2 once the stomata close. The CO2 then enters the Calvin-Benson cycle.

6.9 Biofuels (Revisited)A. The first cells on Earth were chemoautotrophs that took energy and molecules from the

environment.1. Then cyclic photophosphorylation evolved in the first species that directly used the

energy from the sun.2. Organisms that utilized the cyclic pathway were successful.

a. Oxygen became plentiful in the atmosphere.B. The atmosphere on Earth is changing.

1. The level of carbon dioxide in the atmosphere is rising.a. Over 8,000 years ago, humans burned the forests.b. Today we have combustion engines in cars and factories.

C. Man has upset the natural balance between carbon dioxide and oxygen.1. By studying layers of snow in Antarctica, scientists can compare the atmosphere in the

past with conditions that are present today.a. The percentage of carbon dioxide in the atmosphere was consistent until the

industrial revolution.

Chapter Six

2. The pattern of global warming mirrors the rise of carbon dioxide in the atmosphere.a. There is an emphasis on reducing the use of fossil fuels as a power source.b. The use of biofuels is a better option since they simply recycle the carbon that is

already in the atmosphere. Suggestions for Presenting the Material• Many students think that the only autotrophic organisms are those with green pigments, which

are, of course, capable of using light for food manufacture. Point out the existence of the many chemoautotrophs, which derive their energy from chemicals. Use deep-sea thermal vents as an example of where chemoautotrophs can be found.

• Most students have heard of geothermal vents. Discuss where this energy source originates and how it is ultimately tied to photosynthesis.

• Students may have previously learned to refer to the two divisions of photosynthesis as the “light” and “dark” reactions. Starr and Taggart use the more accurate and currently acceptable terms light-dependent and light-independent, respectively. You may wish to explain why “dark reactions” is a poor designator and no longer used (since the reactions can occur in both the dark and the light).

• A discussion of the reason why colored objects appear the color they do may facilitate the students’ understanding of why, for example, green light is ineffective for photosynthesis (not absorbed).

• Note that different species perceive colors differently. For example, honey bees see extremely bright colors where we see white and blue. This is the basis for insect-pollinating flowers. Show students images that reveal the difference between the way we see flowers and the way insects do. Scientists believe that these patterns on flowers serve as a signal to attract the same species of pollinator.

• ATP and NADPH were introduced earlier as carrier molecules. Their connection between the light-dependent and light-independent reactions is an excellent opportunity for reinforcement of these molecules as bridges.

• The term granum dates from the days when microscopes revealed “grains” within the chloroplast. Now we realize that these grains consist of complex thylakoid membranes.

• Even though cyclic photophosphorylation is presented prior to the noncyclic pathway in the text, note that this is done in deference to its position as the first to evolve. Point out that most existing plants use the noncyclic pathway.

• Tell the students to master each phase of photosynthesis before going to the next. Compartmentalizing the overall process will make the material less overwhelming to students and allow them to “connect the dots” when each phase is mastered.

• You may wish to note the similarities and differences between the ATP synthase mechanism in chloroplasts and mitochondria.

• Although the diagram of the Calvin-Benson cycle (Figure 6.10) is intimidating, the most important features are the entry of carbon dioxide and the production of the sugar phosphate, driven by ATP and NADPH from the light-dependent reactions.

• Bring in plant examples of C3, C4, and CAM plants. Note which plants are typical in the area where you are teaching and how that ties to climate. Have students explain how the features of C3, C4 and CAM plants are adapted to the environment in which they are found.

Illustrate the mechanism by which guard cells open and close the stomata to regulate moisture loss.

Where It Starts—Photosynthesis

Discuss the idea of examining your “carbon footprint.” The following website helps you calculate your own “footprint”: http://www.nature.org/greenliving/carboncalculator/

Classroom and Laboratory Enrichment

• You can demonstrate the production of oxygen by plants with the following:a. Place Elodea (an aquarium plant) in a bowl and expose it to bright light.b. Invert a test tube over the plant and collect the bubbles.c. Remove the tube and immediately thrust a glowing wood splint into the tube.d. Result: The splint burns brightly in the high-oxygen air.

• Separate the pigments in green leaves by using paper chromatography. (Consult a botany or biology laboratory manual for the correct procedure.)

• When teaching in the fall, bring in leaves of different fall colors. Have the students identify the pigments that remain once chlorophyll has been degraded. Have the student speculate as to whether these pigments are present when leaves are green in the summer? If so, why can’t you see them?

• Many students have never seen the action of a prism of separating white light into its component colors; a demonstration would most likely be appreciated.

• If a greenhouse facility is readily accessible, a brief tour and explanation of the devices used to con-trol light, water, air, and heat are very instructive.

• Use a light microscope to view living Euglena. Note the chlorophyll in this photosynthetic protist.• Show a video on photosynthesis.• Provide a model of a chloroplast.• Show an electron micrograph of a chloroplast and indicate where light-dependent reactions and

light-independent reactions occur.• Cut out separated leaf pigments from a paper chromatogram and elute each pigment from the pa-

per with a small amount of alcohol. Using a spectrometer, determine the absorption spectrum for each pigment, and graph the results. (Consult a biology or botany laboratory manual for the procedure.)

Remove two leaves from a plant and put each in a separate flask. Add sodium hydrogen carbonate to one flask and potassium hydroxide solution to the other. Seal each flask so it is air tight. Wait several days and examine the leaves. The leaf with the sodium hydrogen carbonate looks okay, but the leaf with the potassium hydroxide solution appears to be very weak. This is because the potassium hydroxide solution removed all of the carbon dioxide, so photosynthesis could not occur.

Have a student make large posters of both phases of photosynthesis and explain the processes to the class.

Classroom Discussion Ideas• C3 plants grow very quickly, where C4 plants grow more slowly. CAM plants grow even more

slowly still. Discuss the trade-off of slower growth for high temperature and drought tolerance in these three categories of plant.

• There is so little moisture in the air of desert environments that temperatures drop suddenly at night. In the winter, particularly, how does this affect CAM plants?

Chapter Six

• How might altitude affect what kind of plants predominate an area? Look into the plants found at each level of Mt. Washington in New Hampshire. You will see a vast difference in the plants that exist at each altitude level of this tall mountain!

• Use a map to draw a 10 mile circle around your location. What differences in plant-life do you see? How about in at 100 mile circle? 300 miles? What accounts for these differences?

Additional Ideas for Classroom Discussion• In what ways could the “greenhouse effect” hurt agriculture? In what ways could it possibly help,

especially in Canada and the countries of the former Soviet Union?• Suppose you could purchase light bulbs that emitted only certain wavelengths of visible light.

What wavelengths would promote the most photosynthesis? The least?• Assume you have supernatural powers and can stop and start the two sets of photosynthesis reac-

tions. Will stopping the light-independent affect the light-dependent, or vice versa?• What conflicting “needs” confront a plant living in a hot, dry environment? What is the frequent

result in C3 plants? How is the problem avoided in C4 plants?

• What colors of the visible spectrum are absorbed by objects that are black? What about white objects? Does it really matter if you wear light colors when it is hot outside?

• The website http://photoscience.la.asu.edu/photosyn/study.html has great ideas for discussing the importance of photosynthesis with students. The article “Why Study Photosynthesis” provides good argument for calling photosynthesis the most important biological process on Earth. The website also contains explanations for using reaction centers of chloroplasts as microscopic models for exploring technologies that use light as an energy source.

• Global warming has caused a rise in the world’s oceans. How is this affecting world coastlines? How is this affecting the coastline closest to you? Can you envision some popular tourist locations that might be endangered by a rising ocean?

• What effect is global warming having on wildlife in the Arctic and in Antarctica?• Are “greening” efforts like using biofuels and low-carbon imprint practices too little, too late? If global warming continues, will the areas where tropical diseases are prevalent expand

northward? Investigate the original research performed in Antarctica, where the layers of snow tell the story of

the increase in carbon dioxide in the atmosphere over time.

How Would You Vote? Classroom Discussion Ideas• Monitor the voting for the online question. Poll your students: would they pay more for a vehicle

that runs on biofuels?

Term Paper Topics, Library Activities, and Special Projects• Full-spectrum lights are used to grow plants indoors. It’s suggested that humans are less prone to

illness when exposed to full-spectrum lights when indoors. Investigate this discovery.• Prepare a detailed diagram of the Calvin-Benson cycle showing the introduction of radioactively

labeled carbon dioxide and its subsequent journey through several “turns” of the cycle.• Of the total amount of sunlight energy impinging on a green plant, what percentage of the energy

is actually converted into glucose? Use this as a gateway to discussing how energy becomes ‘less’ available the farther up the food chain you eat.

Where It Starts—Photosynthesis

• Compare the mechanisms of C3 and C4 photosynthesis and give examples of plants that fit into each category.

• Discuss why radiation with greater or lesser wavelengths than visible light is not generally used in biological processes.

• Investigate “green” practices being used in your city or town. Research what practices might be used in the future. Are “green” practices being mandated in your state? Have students come up with fifty things that each could do to reduce their carbon footprint.

Look up the original research performed by Calvin, Benson, and Bassham at the University of California. How did they discover the mechanisms of the Calvin-Benson cycle?

Are there any fuel stations in your area that sell biofuel? If so, find out about how it is manufactured.

Investigate the research performed by Engelmann to determine the colors of light that are most useful. Do you think his experiment could be reproduced easily today?

What species might be endangered by global warming?

Possible Responses to Critical Thinking Questions1. Let’s look at the possibilities. The gain in weight of the tree might have been water simply absorbed

through the roots and retained unchanged in the plant tissues, but not 164 pounds of it! Furthermore, we know that the water molecules are not going to combine directly with any other molecules in the tree. Of course we now know much more about the reactions of photosynthesis than van Helmont did, so we can conclude that the gain in weight was from the synthesis of carbohydrate (specifically, cellulose) in the chloroplasts of the leaf cells due to the reacting of carbon dioxide from the air and water from the soil, in the presence of light energy.

2. If you notice bubbles coming from an aquatic plant, the bubbles would contain oxygen, an end-product of photosynthesis.

3. The first step in the Calvin cycle in C3 plants is the formation of two molecules of 3-phosphoglycerate (3-PGA) from CO2 and ribulose bisphosphate (RuBP):

RuBP + CO2 -----> 2 3-PGASo, the first compound that would incorporate the labeled C is 3-PGA.For C4 plants, the first step in carbon fixation is the formation of oxaloacetate from CO2 and phosphoenolpyruvate (PEP):

CO2 + PEP ----> oxaloacetateSo, the first compound that would incorporate the labeled C is oxaloacetate.

4. Since photosynthetic prokaryotes do not have membrane-bound organelles, including chloroplasts, the proteins necessary for the light-dependent reactions (including pigments such as chlorophyll) are located in the plasma membrane.

Possible Responses to Data Analysis Exercise QuestionsIt is helpful when growing crops for use as biofuel to measure the net energy output, which is equal to the energy output minus the energy input.

1. The ethanol from one hectare of corn produced approximately 23 kcal X 106. It took approximately 18 kcal X 106 to grow the corn used to make ethanol.

2. The grass grown for synfuel had the highest ratio of energy output to energy input.3. Corn would require the least amount of land to produce a given amount of biofuel energy.

Chapter Six