Key Concepts • What did the experiments of

van Helmont, Priestley, andIngenhousz reveal about howplants grow?

• What is the overall equationfor photosynthesis?

• What is the role of light andchlorophyll in photosynthesis?

Vocabularyphotosynthesispigmentchlorophyll

Reading Strategy:Summarizing As you read,find the key ideas under eachblue head. Write down a few key words from each key idea.Then, use the key words in yoursummary.

Understanding PhotosynthesisMany scientists have contributed to understanding how plants carryout photosynthesis. Early research focused on the overall process.Later researchers investigated the detailed chemical pathways.

1600 1700 1800

1643Jan van HelmontAfter careful measurementsof a plant’s water intakeand mass increase,van Helmont concludesthat trees gain most oftheir mass from water.

1779JanIngenhouszIngenhousz findsthat aquaticplants produceoxygen bubblesin the light butnot in the dark.He concludes thatplants needsunlight toproduce oxygen.

1771Joseph PriestleyUsing a bell jar, a candle, anda plant, Priestley finds thatthe plant releases oxygen.

8–2 Photosynthesis: An Overview

The key cellular process identified with energy production isphotosynthesis. In the process of plants

use the energy of sunlight to convert water and carbon dioxideinto high-energy carbohydrates—sugars and starches—andoxygen, a waste product. The investigations of many scientistshave contributed to the current understanding of the process ofphotosynthesis.

Investigating PhotosynthesisResearch into photosynthesis began centuries ago with a simplequestion: When a tiny seedling grows into a tall tree with amass of several tons, where does the tree’s increase in masscome from? From the soil? From the water? From the air?

Van Helmont’s Experiment In the 1600s, the Belgianphysician Jan van Helmont devised an experiment to find out if plants grew by taking material out of the soil. Van Helmontdetermined the mass of a pot of dry soil and a small seedling.Then, he planted the seedling in the pot of soil. He watered it regularly. At the end of five years, the seedling, which by then had grown into a small tree, had gained about 75 kg.

photosynthesis,

204 Chapter 8

1 FOCUSObjectives8.2.1 Explain what the experiments

of van Helmont, Priestley, andIngenhousz reveal about howplants grow.

8.2.2 State the overall equation forphotosynthesis.

8.2.3 Describe the role of light andchlorophyll in photosynthesis.

Vocabulary PreviewHave students write the Vocabularywords, dividing each into its separatesyllables as best they can. Remindstudents that each syllable usuallyhas only one vowel sound. The cor-rect syllabications arepho•to•syn•the•sis, pig•ment,chlo•ro•phyll.

Reading StrategyStudents’ summaries should describethe findings of van Helmont, Priestley,and Ingenhousz and explain thatplants use the energy of sunlight toconvert water and carbon dioxideinto oxygen and high-energy sugars.They should also describe the role of light and chlorophyll in photo-synthesis.

2 INSTRUCT

InvestigatingPhotosynthesisBuild Science SkillsApplying Concepts To help stu-dents grasp the basic problem facedby researchers centuries ago, on theboard draw a large tree with leavesand roots. Point to the roots and ask:What could trees obtain fromunderground that could help themgrow? (Minerals, water) Point to theleaves and ask: What could treesobtain from the air that could helpthem grow? (Students might suggestoxygen or other gases. Some maymention light.) Ask for their opinionsabout answers to the questions inthe text about where a tall tree getsits mass.

PROGRAM RESOURCES

Section 8–2

SECTION RESOURCES

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eeSSaavveerr

Print:

• Laboratory Manual A, Chapter 8 Lab• Laboratory Manual B, Chapter 8 Lab• Teaching Resources, Lesson Plan 8–2,

Adapted Section Summary 8–2, SectionSummary 8–2, Worksheets 8–2, SectionReview 8–2, Enrichment

• Reading and Study Workbook A, Section 8–2

Technology:

• iText, Section 8–2• Animated Biological Concepts DVD, 9

Photosynthesis• Transparencies Plus, Section 8–2• Lab Simulations CD-ROM, Photosynthesis• Virtual Labs, Lab 6, Lab 7

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The mass of the soil, however, was almost unchanged. He con-cluded that most of the gain in mass had come from water,because that was the only thing that he had added.

Van Helmont’s experiment accounts for the “hydrate,” orwater, portion of the carbohydrate produced by photosynthesis.But where does the carbon of the “carbo-” portion come from?Although van Helmont did not realize it, carbon dioxide in theair made a major contribution to the mass of his tree. Thecarbon in carbon dioxide is used to make sugars and othercarbohydrates in photosynthesis. Van Helmont had only part ofthe story, but he had made a major contribution to science.

Priestley’s Experiment More than 100 years after vanHelmont’s experiment, the English minister Joseph Priestleyperformed an experiment that would give another insight intothe process of photosynthesis. Priestley took a candle, placed aglass jar over it, and watched as the flame gradually died out.Something in the air, Priestley reasoned, was necessary to keepa candle flame burning. When that substance was used up, thecandle went out. That substance was oxygen.

Priestley then found that if he placed a live sprig of mintunder the jar and allowed a few days to pass, the candle couldbe relighted and would remain lighted for a while. The mintplant had produced the substance required for burning. In otherwords, it released oxygen.

What did Priestley discover about photosynthesis?

Photosynthesis comes from theGreek words photo, meaning“light,” and synthesis, meaning“putting together.” Therefore,photosynthesis means “using lightto put something together,”specifically, carbohydrates. Chemomeans “having to do with chemi-cals or chemical reactions.” Whatdo you think chemosynthesismeans?

1845Julius RobertMayerMayer proposesthat plantsconvert lightenergy intochemical energy.

1948Melvin CalvinCalvin traces thechemical path thatcarbon follows toform glucose. Thesereactions are alsoknown as the Calvincycle.

1992Rudolph MarcusMarcus wins the NobelPrize in chemistry fordescribing the processby which electrons aretransferred from onemolecule to another inthe electron transportchain.

Use the Internet or libraryresources to research theexperiments conducted byone of these scientists. Then,write a summary describinghow the scientist contributedto the modern understand-ing of photosynthesis.

1800 1900 2000

Chemosynthesis means “using chemi-cal reactions to put somethingtogether.”

Build Science SkillsDesigning an Experiment Dividethe class into small groups and chal-lenge them to design an experimentsimilar to the experiment that JosephPriestley did in the 1700s. Theyshould keep this concept in mind:Plant photosynthesis produces oxy-gen that animals need to breathe,while animal and plant respirationproduces carbon dioxide that plantsneed in photosynthesis. Ask studentsto list the materials they would need and to write a step-by-stepprocedure.

Photosynthesis 205

Answer to . . . Priestley discovered that

a plant releases a substance that keepsa candle burning. This substance, oxy-gen, is released during photosynthesis.

Invite student volunteers to readaloud to the class the annotationson the timeline. Encourage stu-dents to add to the timeline bysuggesting historical events thattook place near in time to one ofthe discoveries mentioned. Foreach contribution to the under-standing of photosynthesismentioned in the timeline, chal-lenge students to suggest howthat discovery might have provid-ed the basis for the next.

Encourage students to look fordescriptions of these experimentsin books about photosynthesis,books about the history of biolo-gy, and college-level textbooks, aswell as at Internet sites that spe-cialize in biology. Provide severalexamples of news articles aboutmedical and other scientificbreakthroughs from a large dailynewspaper.

riting in Science

Less Proficient ReadersFocus students’ attention onpages 204–206. Help themanalyze the discoveries of thethree scientists and develop atable to show what each onelearned. Direct them to theword equation on page 206,and ask which part of theprocess each scientist discov-ered.

English Language LearnersHave students compare thetwo equations on page 206.Relate each formula to its writ-ten name. Students who havestudied the periodic table canuse element symbols (such as Cfor carbon) to recognize eachname. Help students restate thephotosynthesis equation as asentence.

Advanced LearnersWith students, review theWriting Activity on this page.Have each student research ascientist and write a featurestory on him or her. Stress thatstudents must find more infor-mation than is given in thetextbook. Emphasize that theirstories should be written for theaverage citizen.

UNIVERSAL ACCESS

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Jan Ingenhousz Later, the Dutch scientist JanIngenhousz showed that the effect observed by Priestleyoccurred only when the plant was exposed to light. Theresults of both Priestley’s and Ingenhousz’s experimentsshowed that light is necessary for plants to produceoxygen. The experiments performed by vanHelmont, Priestley, and Ingenhousz led to work byother scientists who finally discovered that in thepresence of light, plants transform carbon dioxideand water into carbohydrates, and they alsorelease oxygen.

The Photosynthesis EquationBecause photosynthesis usually produces 6-carbon sugars(C6H12O6) as the final product, the overall equation forphotosynthesis can be shown as follows:

Photosynthesis uses the energy of sunlightto convert water and carbon dioxide into high-energy sugars and oxygen. Plants then use the sugarsto produce complex carbohydrates such as starches. Plantsobtain carbon dioxide from the air or water in which theygrow. The process of photosynthesis is shown in Figure 8–4.

Light energy

Light-dependentreactions

(thylakoids)

ADP,NADP+

H2O

ATP,NADPH

O2

Calvin cycle(stroma)

SugarCO2

+H2O

G Figure 8–4 Photosynthesisis a series of reactions that useslight energy from the sun toconvert water and carbon dioxideinto sugars and oxygen.

6CO2 + 6H2O C6H12O6 6O2

carbon dioxide water sugars oxygen

light

light

+

++

What waste material is producedduring photosynthesis?

Materials large clear plastic cup, sodiumbicarbonate solution, elodea plant, large test tube

Procedure

1. Fill a large clear plastic cup about half full withsodium bicarbonate solution. The sodium bicar-bonate solution is a source of carbon dioxide.

2. Place an elodea plant in a large test tube with thecut stem at the bottom. Fill the tube with sodiumbicarbonate solution. CAUTION: Handle the testtube carefully.

3. Hold your thumb over the mouth of the tube. Turnthe tube over, and lower it to the bottom of thecup. Make sure there is no air trapped in the tube.

4. Place the cup in bright light.5. After at least 20 minutes, look closely at the elodea

leaves. Record your observations.

Analyze and Conclude1. Observing What did you observe on the elodea

leaves?2. Inferring What substance accumulated in the

leaves? Should that substance be considered awaste product? Explain.

3. Applying Concepts What plant organelle carriesout photosynthesis and produces the gas?

Elodea

Sodiumbicarbonatesolution

The PhotosynthesisEquation

Priestley’s experiment “purifies” airJoseph Priestley (1733–1804), a BritishUnitarian minister, never formally studied sci-ence. His interest in science began when hemet Benjamin Franklin in London in 1766. Forone of his many experiments, Priestley devisedan apparatus that consisted of enclosed con-tainers of air sealed at the bottom by a troughof water. He discovered that a burning candlein one of the closed containers caused the air

to become “impure,” eventually putting outthe flame. He also found that a mouse placedinside the container of “impure” air died. Heexpected the same to happen to a sprig ofspearmint. Much to his surprise, instead ofdying, the plant flourished. Furthermore, hediscovered that the plant “purified” the air,since after leaving the plant in the space forseveral weeks, a candle would burn or a mousecould live in the same enclosed space.

HISTORY OF SCIENCE

BioInsights

8–2 (continued)

Objective Students will be able toconclude that oxygen is producedby plants during photosynthesis.

Skills Focus Observing, InferringMaterials large clear plastic cup,sodium bicarbonate solution, elodeaplant, large test tubeTime 10 minutes for setup, 5 min-utes for observationsAdvance Prep Prepare the sodi-um bicarbonate solution by mixing5 g of sodium bicarbonate intoeach liter of water. Obtain theelodea plants. If they are not avail-able, any small water plants will do.Safety If students break a test tube,warn them not to handle the glass.Strategies• It will take up to 30 minutes foroxygen to appear, so you may wantto set up this lab at the beginningof the class and return to it at theend.• To show that the gas is oxygen,you may want to demonstrate theglowing splint test. CAUTION: Weargoggles and heat-resistant gloves dur-ing this demonstration. Remove oneof the test tubes, keeping it upsidedown. Remove the elodea and letthe water drain out. Light a splint,blow it out, and then turn the tubesideways next to the splint. Thesplint will glow brighter and pos-sibly reignite.• Students may need to repeat step3 several times to make sure no airis trapped.Expected Outcome Studentsshould observe a gas forming onthe elodea leaves and conclude thatit is oxygen.Analyze and Conclude1. Students should see bubbles ofgas. An answer of “oxygen” is aninference, not an observation.2. Photosynthesis produces sugarsand oxygen. Because sugars are notgases, the gas must be oxygen.Oxygen is a waste product becauseit is released into the environment.3. The chloroplast

206 Chapter 8

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Light and PigmentsAlthough the equation tells you that water andcarbon dioxide are required for photosynthesis,it does not tell you how plants use these low-energy raw materials to produce high-energysugars. To answer that question, you have toknow how plants capture the energy of sunlight.

In addition to water and carbondioxide, photosynthesis requires light andchlorophyll, a molecule in chloroplasts.

Energy from the sun travels to Earth in theform of light. Sunlight, which your eyes perceiveas “white” light, is actually a mixture of differ-ent wavelengths of light. Many of these wave-lengths are visible to your eyes and make upwhat is known as the visible spectrum. Youreyes see the different wavelengths of the visiblespectrum as different colors.

Plants gather the sun’s energy with light-absorbing molecules called The plants’principal pigment is (KLAWR-uh-fil). There are twomain types of chlorophyll: chlorophyll a and chlorophyll b.

As Figure 8–5 shows, chlorophyll absorbs light very well in theblue-violet and red regions of the visible spectrum. However,chlorophyll does not absorb light well in the green region of thespectrum. Green light is reflected by leaves, which is why plantslook green. Plants also contain red and orange pigments such ascarotene that absorb light in other regions of the spectrum.

Because light is a form of energy, any compound that absorbslight also absorbs the energy from that light. When chlorophyllabsorbs light, much of the energy is transferred directly to elec-trons in the chlorophyll molecule, raising the energy levels of theseelectrons. These high-energy electrons make photosynthesis work.

chlorophyllpigments.

100

80

60

40

20

0650600500 550400 450 700 750

G Figure 8–5 Photosynthesisrequires light and chlorophyll.In the graph above, notice howchlorophyll a absorbs light mostly inthe blue-violet and red regions ofthe visible spectrum, whereaschlorophyll b absorbs light in theblue and red regions of the visiblespectrum.

1. Key Concept What didvan Helmont, Priestley, andIngenhousz discover aboutplants?

2. Key Concept Describe theprocess of photosynthesis,including the reactants andproducts.

3. Key Concept Why arelight and chlorophyll needed forphotosynthesis?

4. Describe the relationshipbetween chlorophyll and thecolor of plants.

5. Critical Thinking PredictingHow well would a plant growunder pure yellow light? Explainyour answer.

Descriptive WritingWrite a summary paragraphdescribing either van Helmont’s,Priestley’s, or Ingenhousz’sexperiments with plants andlight. Hint: Use the firstboldface key sentence onpage 206 to give you an ideafor the topic sentence.

8–2 Section Assessment

Light and PigmentsBuild Science SkillsUsing Tables and Graphs Afterstudents have examined the graph inFigure 8–5, ask: How does the colorspectrum at the bottom relate tothe graph itself? (Each of the colorsof the visible spectrum has a character-istic range of wavelengths, asdesignated on the horizontal axis ofthe graph.) In what region of thespectrum does chlorophyll babsorb light best? (In the blueregion) Challenge students to convertthe data in the graph into a datatable that shows, for example, theestimated absorption of chlorophylla and chlorophyll b at 550 nm.

3 ASSESSEvaluate UnderstandingHave students write a paragraph,using their own words, that explainshow plants produce high-energysugars through the process of photo-synthesis. Call on students atrandom to read their paragraphs.

ReteachAsk students to make a labeled draw-ing based on Figure 8–4 but withmore realistic objects, including aleafy tree, the sun, and clouds in thesky (visually representing the atmos-phere). Ask that they use arrows andsymbols in their drawing to show thesame equation for photosynthesisthat Figure 8–4 does.

Photosynthesis 207

If your class subscribes to the iText,use it to review the Key Concepts inSection 8–2.

8–2 Section Assessment1. Van Helmont discovered that water was

involved in increasing the mass of a plant.Priestley discovered that a plant producesthe substance in air required for burning.Ingenhousz discovered that light is necessaryfor plants to produce oxygen.

2. Photosynthesis uses the energy of sunlight toconvert water and carbon dioxide into oxy-gen and high-energy sugars.

3. Light provides the energy needed to pro-duce high-energy sugars. Chlorophyllabsorbs light, and the energy of thatabsorbed light makes photosynthesis work.

4. Plants are green because green light isreflected by the chlorophyll in leaves.

5. The plant would not grow well becausechlorophyll does not absorb much light inthe yellow region of visible light.

Students should write a paragraphsummarizing the experiments ofone of the three scientists dis-cussed in the section. A goodparagraph should have a topicsentence derived from the firstboldface sentence on page 206,details about the experiments per-formed, and a concludingsentence that explains the signifi-cance of the scientist’s work.

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