Alfalfa, corn, and soybeans are growing on this farmland in · PDF file · 2016-03-15Humans and other living organisms depend on organic compounds—and ... Describe the role of chlorophylls

Alfalfa, corn, and soybeans are growing on thisfarmland in Pennsylvania. Through photosynthesis,these plants obtain energy from the sun and storeit in organic compounds. Humans and other livingorganisms depend on organic compounds—andtherefore on photosynthesis—to obtain the energynecessary for living.

SECTION 1 The Light Reactions

SECTION 2 The Calvin Cycle

Unit 2—PhotosynthesisTopics 1–6

C H A P T E R 6112

6CHAPTER PHOTOSYNTHESISPHOTOSYNTHESIS

Copyright © by Holt, Rinehart and Winston. All rights reserved.

113P H O T O S Y N T H E S I S

T H E L I G H T R E A C T I O N SAll organisms use energy to carry out the functions of life.

Where does this energy come from? Directly or indirectly,

almost all of the energy in living systems comes from the sun.

Energy from the sun enters living systems when plants, algae,

some unicellular protists, and some prokaryotes absorb sunlight

and use it to make organic compounds.

OBTAINING ENERGYOrganisms can be classified according to how they get energy.Organisms that use energy from sunlight or from chemical bonds ininorganic substances to make organic compounds are calledautotrophs (AWT-oh-TROHFS). Most autotrophs use the process ofphotosynthesis to convert light energy from the sun into chemicalenergy in the form of organic compounds, primarily carbohydrates.

The tree in Figure 6-1 is an autotroph because it converts lightenergy from the sun into organic compounds. The tree uses thesecompounds for energy. The caterpillar also depends on the treefor energy, as it cannot manufacture organic compounds itself.Animals and other organisms that must get energy from foodinstead of directly from sunlight or inorganic substances arecalled heterotrophs (HEHT-uhr-oh-TROHFS). The bird is also a het-erotroph. The food that fuels the bird originates with an autotroph(the tree), but it passes indirectly to the bird through the cater-pillar. In similar ways, almost all organisms ultimately depend onautotrophs to obtain the energy necessary to carry out theprocesses of life.

Photosynthesis involves a complex series of chemical reactionsin which the product of one reaction is consumed in the next reac-tion. A series of chemical reactions linked in this way is referred toas a biochemical pathway.

SECTION 1

O B J E C T I V E S● Explain why almost all organisms

depend on photosynthesis.● Describe the role of chlorophylls

and other pigments inphotosynthesis.

● Summarize the main events of thelight reactions.

● Explain how ATP is made duringthe light reactions.

V O C A B U L A R Yautotrophphotosynthesisheterotrophlight reactionschloroplastthylakoidgranumstromapigmentchlorophyllcarotenoidphotosystemprimary electron acceptorelectron transport chainchemiosmosis

Lightenergy

Plants convert lightenergy to chemical energy.

Caterpillars get energyby eating plants.

Birds get energy byeating caterpillars.

The tree depends on the sun for energy.Like all heterotrophs, the caterpillar andbird depend on an autotroph (the treeand its leaves) for energy.

FIGURE 6-1


CHLOROPLAST

PLANT CELLLEAF

Thylakoid

Inner membrane

Granum

StromaOuter membrane

C H A P T E R 6114

Carbondioxide

and water

Organic compoundsand oxygen

Lightenergy

PHOTOSYNTHESISby autotrophs

CELLULARRESPIRATION

by autotrophs and heterotrophs

Many autotrophs produce organiccompounds and oxygen throughphotosynthesis. Both autotrophs andheterotrophs produce carbon dioxidethrough cellular respiration.

FIGURE 6-2

Photosynthesis in eukaryotes occursinside the chloroplast. The lightreactions of photosynthesis take place in the thylakoids, which arestacked to form grana.

FIGURE 6-3

OVERVIEW OFPHOTOSYNTHESIS

Figure 6-2 shows how autotrophs use photosynthesis to produceorganic compounds from carbon dioxide (CO2) and water. The oxy-gen (O2) and some of the organic compounds produced are thenused by cells in a process called cellular respiration. During cellularrespiration, CO2 and water are produced. Thus, the products ofphotosynthesis are reactants in cellular respiration. Conversely, theproducts of cellular respiration are reactants in photosynthesis.

Photosynthesis can be divided into two stages:1. Light Reactions Light energy (absorbed from the sun) is con-

verted to chemical energy, which is temporarily stored in ATPand the energy carrier molecule NADPH.

2. Calvin Cycle Organic compounds are formed using CO2 andthe chemical energy stored in ATP and NADPH.

Photosynthesis can be summarized by the following equation:

6CO2 ! 6H2O C6H12O6 ! 6O2

This equation, however, does not explain how photosynthesisoccurs. It is helpful to examine the two stages separately in orderto better understand the overall process of photosynthesis.

CAPTURING LIGHT ENERGYThe first stage of photosynthesis includes the light reactions, sonamed because they require light to happen. The light reactionsbegin with the absorption of light in chloroplasts, organelles foundin the cells of plants and algae. Most chloroplasts are similar instructure. As shown in Figure 6-3, each chloroplast is surroundedby a pair of membranes. Inside the inner membrane is another sys-tem of membranes called thylakoids (THIE-luh-koydz) that arearranged as flattened sacs. The thylakoids are connected and layeredto form stacks called grana (GRAY-nuh) (singular, granum).Surrounding the grana is a solution called the stroma (STROH-muh).

light energy



Sun

White light

Visible spectrum

Increasing wavelength400 nm 700 nm

Prism

Light and PigmentsTo understand how chloroplasts absorb light in photosynthesis, itis important to understand some of the properties of light. Lightfrom the sun appears white, but it is actually made of a variety ofcolors. As shown in Figure 6-4, white light can be separated into itscomponent colors by passing the light through a prism. The result-ing array of colors, ranging from red at one end to violet at theother, is called the visible spectrum.

When white light strikes an object, its component colors can bereflected, transmitted, or absorbed by the object. Many objectscontain pigments, compounds that absorb light. Most pigmentsabsorb certain colors more strongly than others. By absorbing cer-tain colors, a pigment subtracts those colors from the visible spec-trum. Therefore, the light that is reflected or transmitted by thepigment no longer appears white. For example, the lenses in green-tinted sunglasses contain a pigment that reflects and transmitsgreen light and absorbs the other colors. As a result, the lenseslook green.

Chloroplast PigmentsLocated in the membrane of the thylakoids are several pigments,the most important of which are called chlorophylls (KLAWR-uh-FILZ).There are several different types of chlorophylls. The two mostcommon types are known as chlorophyll a and chlorophyll b. AsFigure 6-5 shows, chlorophyll a absorbs less blue light but morered light than chlorophyll b absorbs. Neither chlorophyll a norchlorophyll b absorbs much green light. Instead, they allow greenlight to be reflected or transmitted. For this reason, leaves andother plant structures that contain large amounts of chlorophylllook green.

Only chlorophyll a is directly involved in the light reactions ofphotosynthesis. Chlorophyll b assists chlorophyll a in capturinglight energy, and therefore chlorophyll b is called an accessory pig-ment. Other compounds found in the thylakoid membrane, includ-ing the yellow, orange, and brown carotenoids (kuh-RAHT’n-OYDZ),also function as accessory pigments. Looking again at Figure 6-5,notice that the pattern of light absorption of one of the carotenoidsdiffers from the pattern of either type ofchlorophyll. By absorbing colors that chloro-phyll a cannot absorb, the accessory pig-ments enable plants to capture more of theenergy in light.

In the leaves of a plant, the chlorophylls aregenerally much more abundant and thereforemask the colors of the other pigments. But inthe nonphotosynthetic parts of a plant, such asfruits and flowers, the colors of the other pig-ments may be quite visible. During the fall,many plants lose their chlorophylls, and theirleaves take on the rich hues of the carotenoids.

Absorption Spectra of Photosynthetic Pigments

Wavelength (nm)

Carotenoid

Chlorophyll bChlorophyll a

400 500 600 700

Rela

tive

abso

rptio

n


White light contains a variety of colorscalled the visible spectrum. Each colorhas a different wavelength, measured innanometers.

FIGURE 6-4

The three curves on this graph showhow three pigments involved inphotosynthesis differ in the colors oflight they absorb. Where a curve has a peak, much of the light at thatwavelength is absorbed. Where a curvehas a trough, much of the light at thatwavelength is reflected or transmitted.

FIGURE 6-5

1 23

45

Primary electronacceptorLight

Light

STROMA

e–e– e–

e–

Primary electronacceptor

Electrontransport chain

H+

Photosystem II Photosystem I Electrontransport chain

NADP+

+H+

NADPH

CHLOROPLAST

THYLAKOID

Thylakoidmembrane

INSIDE OF THYLAKOID

C H A P T E R 6116

CONVERTING LIGHT ENERGYTO CHEMICAL ENERGY

Once the pigments in the chloroplast have captured light energy,the light energy must then be converted to chemical energy. Thechemical energy is temporarily stored in ATP and NADPH. Duringthese reactions, O2 is given off. The chlorophylls and carotenoidsare grouped in clusters of a few hundred pigment molecules in thethylakoid membrane. Each cluster of pigment molecules and theproteins that the pigment molecules are embedded in are referredto collectively as a photosystem. Two types of photosystems areknown: photosystem I and photosystem II. They contain similar kindsof pigments, but they have different roles in the light reactions.

The light reactions begin when accessory pigment moleculesin both photosystems absorb light. By absorbing light, thosemolecules acquire some of the energy carried by the light. Ineach photosystem, the acquired energy is passed quickly to theother pigment molecules until it reaches a specific pair of chloro-phyll a molecules. Chlorophyll a can also absorb light. Theevents that occur next can be divided into five steps, as shownin Figure 6-6.

In step , light energy forces electrons to enter a higher energylevel in the two chlorophyll a molecules of photosystem II. Theseenergized electrons are said to be “excited.”

1


The light reactions, which take place inthe thylakoid membrane, use energyfrom sunlight to produce ATP, NADPH,and oxygen.

FIGURE 6-6

thylakoid

from the Greek thylakos,meaning “pocket”

Word Roots and Origins


The excited electrons have enough energy to leave the chlorophylla molecules. Because they have lost electrons, the chlorophyll a mol-ecules have undergone an oxidation reaction. Remember that eachoxidation reaction must be accompanied by a reduction reaction.So, some substance must accept the electrons that the chlorophylla molecules have lost, as shown in step . The acceptor of theelectrons lost from chlorophyll a is a molecule in the thylakoidmembrane called the primary electron acceptor.

In step , the primary electron acceptor donates the electronsto the first of a series of molecules located in the thylakoid mem-brane. These molecules are called an electron transport chainbecause they transfer electrons from one molecule to the next. Asthe electrons pass from molecule to molecule in the chain, theylose most of the energy that they acquired when they were excited.The energy they lose is used to move protons (H!) into the thylakoid.

In step , light is absorbed by photosystem I. This happens atthe same time that light is absorbed by photosystem II. Electronsmove from a pair of chlorophyll a molecules in photosystem I toanother primary electron acceptor. The electrons lost by thesechlorophyll a molecules are replaced by the electrons that havepassed through the electron transport chain from photosystem II.

The primary electron acceptor of photosystem I donates elec-trons to a different electron transport chain. This chain brings theelectrons to the side of the thylakoid membrane that faces thestroma. There the electrons combine with a proton and NADP!, anorganic molecule that accepts electrons during oxidation/reduc-tion reactions. As you can see in step , this reaction causesNADP! to be reduced to NADPH.

Replacing Electrons in Light ReactionsIn step , electrons from chlorophyll molecules in photosystem IIreplace electrons that leave chlorophyll molecules in photosystem I.If this replacement did not occur, both electron transport chainswould stop, and photosynthesis would not happen. The replace-ment electrons for photosystem II are provided bywater molecules. As Figure 6-7 shows, an enzymeinside the thylakoid splits water molecules intoprotons, electrons, and oxygen. The followingequation summarizes the reaction:

2H2O 4H! ! 4e" ! O2

For every two molecules of water that are split,four electrons become available to replace thoselost by the chlorophyll molecules in photosystemII. The protons that are produced are left insidethe thylakoid, and the oxygen diffuses out of thechloroplast and can then leave the plant. Thus, oxy-gen is not needed for photosynthesis to occur but isessential for cellular respiration in most organisms,including plants themselves.

4

5

4

3

2


www.scilinks.orgTopic: PhotosynthesisKeyword: HM61140

Primaryelectron acceptor

Light

Water-splittingenzyme

Photosystem II

Thylakoidmembrane

e–

4H+

2H2O O2

STROMA

INSIDE OFTHYLAKOID

The splitting of water inside thethylakoid releases electrons, whichreplace the electrons that leavephotosystem II.

FIGURE 6-7

C H A P T E R 6118

Making ATP in Light ReactionsATP is the main energy currency of cells. An important part of thelight reactions is the synthesis of ATP through a process calledchemiosmosis (KEM-i-ahs-MOH-sis). Chemiosmosis relies on a concen-tration gradient of protons across the thylakoid membrane. Recallthat some protons are produced from the splitting of water mole-cules inside the thylakoid. Other protons are pumped from the

stroma to the interior of the thylakoid. The energyrequired to pump these protons is supplied by theexcited electrons as they pass along the electrontransport chain of photosystem II. Both of thesemechanisms act to build up a concentration gradi-ent of protons. That is, the concentration of protonsis higher inside the thylakoid than in the stroma.

The concentration gradient of protons representspotential energy. That energy is harnessed by anenzyme called ATP synthase, which is located in thethylakoid membrane, as Figure 6-8 shows. ATP syn-thase makes ATP by adding a phosphate group toadenosine diphosphate, or ADP. The energy drivingthis reaction is provided by the movement of pro-tons from inside the thylakoid to the stromathrough the ATP synthase complex. Thus, ATP syn-thase converts the potential energy of the protonconcentration gradient into chemical energy storedin ATP. As you learned earlier, some of the protonsin the stroma are used to make NADPH from NADP!

at the end of the electron transport chain of photo-system I. Together, NADPH and ATP provide energyfor the second set of reactions in photosynthesis,which are described in the next section.

H+

(low concentration)

H+ (high concentration)

ATP

ADP +phosphate

Thylakoidmembrane

ATPsynthase

STROMA

INSIDE OFTHYLAKOID

Thylakoid

Thylakoidspace

Thylakoidmembrane

During chemiosmosis, the movement of protons into the stroma of thechloroplast releases energy, which isused to produce ATP.

FIGURE 6-8

1. Explain why both autotrophs and heterotrophsdepend on photosynthesis to obtain the energythey need for life processes.

2. Describe the role of chlorophylls in the biochem-ical pathways of photosynthesis.

3. List the three substances that are producedwhen water molecules are broken down duringthe light reactions.

4. Explain why the splitting of water is importantto the continuation of the light reactions.

5. Name the product of the process known aschemiosmosis.

CRITICAL THINKING6. Making Comparisons Thinking about the main

role of pigments in photosynthesis, explain howthe pigments in colored objects such as clothesdiffer from plant pigments.

7. Analyzing Information The molecule thatprecedes the electron transport chains of bothphotosystem I and photosystem II is an electronacceptor. What is the original molecule that isthe electron donor for both of these systems?

8. Predicting Results Explain how the light reac-tions would be affected if there were no concen-tration gradient of protons across the thylakoidmembrane.

SECTION 1 REVIEW


HYPOTHESIS: Synthetic Compounds CanImitate a Photosynthetic System

In nature, a photosystem contains pigments andproteins embedded in a linear sequence in the mem-brane of a chloroplast. This arrangment allows foreffective capture and orderly transfer of electronsfrom one photosystem to another and eventually to a final electron acceptor for powering synthesisof sugar.

Dr. Darius Kuciauskas (koo-si-AHS-kuhs) of VirginiaCommonwealth University and his colleaguesthought they could produce an artificial photosys-tem that carried out the initial steps of photosyn-thesis. Kuciauskas and his team predicted that whenthey shined light on four pigment molecules withenergy-conducting chemical bonds, the pigmentmolecules would capture and channel light energythrough the “wirelike” bonds. The electrons travelingthrough the bonds would then pass to, and electricallycharge, an acceptor molecule that was placed at thecenter of the pigment complex.

Science in ActionHow Can Scientists Replicate Photosynthesis in the Lab?Life on Earth is powered by the sun’s energy. The chloroplasts found inplants and some other organisms can efficiently capture and convertsolar energy to sugars through a process known as photosynthesis.Now, scientists have developed an artificial photosystem that may oneday allow for solar-powered food or fuel production in the lab. Dr. Darius

Kuciauskas

METHODS: Design and Build aPigment-Protein Light-Harvesting Complex

Kuciauskas and his team set out to create thephotosystem. The team started with purple pig-ment molecules called porphyrins. These moleculesabsorb light energy and are related to naturallyoccurring chlorophyll. The scientists joined four ofthe porphyrins to each other. To the center-mostporphyrin, they attached a ball-shaped cluster of 60 carbon atoms called C60, or fullerene.

RESULTS: The Artificial Photosystem Capturedand Transferred Electrons

The four porphyrin pigments captured light energy,and the bonds passed electrons to the C60 acceptormolecule. The acceptor held the charge for only onemicrosecond—about one millionth of a second.

CONCLUSION: Artificial Photosystems Are Possible

The Kuciauskas team concluded that their complexdoes act as an artificial photosystem, at least for afleeting instant. Kuciauskas and his colleagueshave continued their work, aiming to modify thecomplex further so it can hold its energy longer.Ultimately, they hope their photosystem can powerchemical reactions for making important organicmolecules that serve as food or fuel.

Kuciauskas and his colleagues use laser systems such as thisone to study light absorption in molecules that might one daybe used in an artificial photosystem.

R E V I E W

1. What are the essential parts of a photosystem?2. Describe the method by

which the Kuciauskas teambuilt an artificial photosystem.

3. Critical Thinking Review themethods that Kuciauskas andhis team used to test theirhypothesis. Determine onevariable the scientists mightchange to modify their results.

www.scilinks.orgTopic: Plant

PhotosyntheticPigments

Keyword: HM61164

119Copyright © by Holt, Rinehart and Winston. All rights reserved.

C H A P T E R 6120

T H E C A L V I N C Y C L EIn the second set of reactions in photosynthesis, plants use the

energy that was stored in ATP and NADPH during the light

reactions to produce organic compounds in the form of sugars.

These organic compounds are then consumed by autotrophs

and heterotrophs alike for energy. The most common way that

plants produce organic compounds is called the Calvin cycle.

CARBON FIXATIONThe Calvin cycle is a series of enzyme-assisted chemical reactionsthat produces a three-carbon sugar. In the Calvin cycle, carbonatoms from CO2 in the atmosphere are bonded, or “fixed,” intoorganic compounds. This incorporation of CO2 into organic com-pounds is called carbon fixation. A total of three CO2 moleculesmust enter the Calvin cycle to produce each three-carbon sugarthat will be used to make the organic compounds. The Calvin cycleoccurs within the stroma of the chloroplast. Figure 6-9 shows theevents that occur when three CO2 molecules enter the Calvin cycle.

In step , CO2 diffuses into the stroma from the surroundingcytosol. An enzyme combines each CO2 molecule with a five-carbonmolecule called ribulose bisphosphate (RuBP). The six-carbon mole-cules that result are very unstable, and they each immediately splitinto two three-carbon molecules. These three-carbon molecules arecalled 3-phosphoglycerate (3-PGA).

1

SECTION 2

O B J E C T I V E S● Summarize the main events of the

Calvin cycle.● Describe what happens to the

compounds that are made in theCalvin cycle.

● Distinguish between C3, C4, andCAM plants.

● Summarize how the light reactionsand the Calvin cycle work togetherto create the continuous cycle ofphotosynthesis.

● Explain how environmental factorsinfluence photosynthesis.

V O C A B U L A R YCalvin cyclecarbon fixationstomataC4 pathwayCAM pathway


C

6 moleculesof 3-PGA

6 moleculesof G3P

3 moleculesof RuBP

3 CO2

6 ATP

3 ATP

3 ADP

6 ADP

6 NADPH

6 NADP+

6 Phosphate

CHLOROPLAST

Stroma

1 moleculeof G3P

Organiccompounds

Each of the three CO2molecules combines witha molecule of RuBP. Each resulting six-carbon moleculeimmediately splits into two molecules of 3-PGA.

1

4 The rest of theG3P is convertedback into RuBP.

3 One molecule ofG3P is used to makeorganic compounds.

2 Each molecule of3-PGA is convertedinto a moleculeof G3P.

C C C6 -P

C C C6 -P

C C C1 -P

C C C C C3 P- -P

The Calvin cycle, in which carbon isfixed into organic compounds, takesplace in the stroma of the chloroplast.

FIGURE 6-9



In step , each molecule of 3-PGA is converted into anotherthree-carbon molecule, glyceraldehyde 3-phosphate (G3P), in atwo-part process. First, each 3-PGA molecule receives a phosphategroup from a molecule of ATP. The resulting compound thenreceives a proton (H!) from NADPH and releases a phosphategroup, producing G3P. The ADP, NADP!, and phosphate that arealso produced can be used again in the light reactions to makemore ATP and NADPH.

In step , one of the G3P molecules leaves the Calvin cycle andis used to make organic compounds (carbohydrates) in whichenergy is stored for later use.

In step , the remaining G3P molecules are converted back intoRuBP through the addition of phosphate groups from ATP molecules.The resulting RuBP molecules then enter the Calvin cycle again.

The Calvin cycle (named for Melvin Calvin, the American bio-chemist who worked out the chemical reactions in the cycle) is themost common pathway for carbon fixation. Plant species that fixcarbon exclusively through the Calvin cycle are known as C3 plantsbecause of the three-carbon compound that is initially formed inthis process.

ALTERNATIVE PATHWAYSMany plant species that evolved in hot, dry climates fix carbonthrough alternative pathways. Under hot and dry conditions,plants can rapidly lose water to the air through small pores calledstomata (STOH-muh-tuh). Stomata (singular, stoma), shown in Figure6-10, are usually located on the undersurface of the leaves. Plantscan reduce water loss by partially closing their stomata when theair is hot and dry.

Stomata are the major passageways through which CO2 entersand O2 leaves a plant. When a plant’s stomata are partly closed, thelevel of CO2 in the plant falls as CO2 is consumed in the Calvincycle. At the same time, the level of O2 in the plant rises as the lightreactions generate O2. Both a low CO2 level and a high O2 levelinhibit carbon fixation by the Calvin cycle. Alternative pathwaysfor carbon fixation help plants deal with this problem.

4

3

2

stomata

from the Greek stoma, meaning“mouth”

Word Roots and Origins

(a) OPEN STOMA (b) CLOSED STOMA

These photos show stomata in the leafof a tobacco plant, Nicotiana tabacum.(a) When a stoma is open, water, carbondioxide, and other gases can passthrough it to enter or leave a plant(814"). (b) When a stoma is closed,passage through it is greatly restricted(878").

FIGURE 6-10


The C4 PathwayOne alternative pathway enables certain plants to fix CO2 into four-carbon compounds. This pathway is thus called the C4 pathway,and plants that use it are known as C4 plants. During the hottestpart of the day, C4 plants have their stomata partially closed.However, certain cells in C4 plants have an enzyme that can fix CO2into four-carbon compounds even when the CO2 level is low andthe O2 level is high. These compounds are then transported toother cells, where CO2 is released and enters the Calvin cycle. C4plants include corn, sugar cane, and crab grass. Such plants loseonly about half as much water as C3 plants when producing thesame amount of carbohydrates. Many plants that use the C4 path-way evolved in tropical climates.

The CAM PathwayCactuses, pineapples, and certain other plants have a differentadaptation to hot, dry climates. Such plants fix carbon through apathway called the CAM pathway. CAM is an abbreviation forcrassulacean acid metabolism, because this water-conserving path-way was first discovered in plants of the family Crassulaceae, suchas the jade plant. Plants that use the CAM pathway open theirstomata at night and close them during the day—just the oppositeof what other plants do. At night, CAM plants take in CO2 and fix itinto a variety of organic compounds. During the day, CO2 isreleased from these compounds and enters the Calvin cycle.Because CAM plants have their stomata open at night, when thetemperature is lower, they grow fairly slowly. However, they loseless water than either C3 or C4 plants.

A SUMMARY OFPHOTOSYNTHESIS

Photosynthesis happens in two stages, both of which occur insidethe chloroplasts of plant cells and algae.

1. The light reactions—Energy is absorbed from sunlight and con-verted into chemical energy, which is temporarily stored inATP and NADPH.

2. The Calvin cycle—Carbon dioxide and the chemical energystored in ATP and NADPH are used to form organic compounds.

Photosynthesis itself is an ongoing cycle: the products of the lightreactions are used in the Calvin cycle, and some of the products ofthe Calvin cycle are used in the light reactions. The other products ofthe Calvin cycle are used to produce a variety of organic compounds,such as amino acids, lipids, and carbohydrates. Many plants producemore carbohydrates than they need. These extra carbohydrates canbe stored as starch in the chloroplasts and in structures such asroots and fruits. These stored carbohydrates provide the chemicalenergy that both autotrophs and heterotrophs depend on.

C H A P T E R 6122

ConnectionConnectionEcoEcoEcoPhotosynthesis and theGlobal GreenhouseWith the beginning of the IndustrialRevolution around 1850, theatmospheric concentration of CO2started to increase. This increasehas resulted largely from the burn-ing of fossil fuels, which releasesCO2 as a byproduct. You mightexpect plants to benefit from thebuildup of CO2 in the atmosphere.In fact, the rise in CO2 levels mayharm photosynthetic organismsmore than it helps them. CO2 andother gases in the atmosphereretain some of the Earth’s heat,causing the Earth to becomewarmer. This warming could reducethe amount of worldwide precipita-tion, creating deserts that would beinhospitable to most plants.


NADPHATP

NADP+ADP+P

H2O

O2

CO2

Light

LIGHTREACTIONS

Organiccompounds

CALVINCYCLE

CHLOROPLAST

PLANT CELL

Photosynthesis is an ongoing cycle.FIGURE 6-11

Analyzing PhotosynthesisMaterials disposable gloves, labapron, safety goggles, 250 mLErlenmeyer flasks (3), bromothymolblue, 5 cm sprigs of elodea (2),water, drinking straw, plastic wrap,100 mL graduated cylinder

Procedure

1. Put on your disposable gloves,lab apron, and safety goggles.

2. Label the flasks “1,” “2,” and“3.” Add 200 mL of water and20 drops of bromothymol blue toeach flask.

3. Put the drinking straw in flask 1,and blow into the blue solutionuntil the solution turns yellow.Repeat this step with flask 2.

4. Put one elodea sprig in flask 1.Do nothing to flask 2. Put theother elodea sprig in flask 3.

5. Cover all flasks with plasticwrap. Place the flasks in a well-lighted location, and leavethem overnight. Record yourobservations.

Analysis Describe your results.Explain what caused one of thesolutions to change color. Why didthe other solutions not changecolor? Which flask is the controlin this experiment?

Quick Lab

Figure 6-11 above illustrates how the light reactions in the thylakoid and the Calvin cycle in the stroma actually worktogether as one continuous cycle—photosynthesis. Amazingly,this complicated process can occur in every one of the thousandsof chloroplasts present in a plant if the conditions are right.

Recall that water is split during the light reactions, yieldingelectrons, protons, and oxygen as a byproduct. Thus, the simplestoverall equation for photosynthesis, including both the light reactions and the Calvin cycle, can be written as follows:

CO2 ! H2O (CH2O) ! O2

In the equation above, (CH2O) represents the general formulafor a carbohydrate. It is often replaced in this equation by the car-bohydrate glucose, C6H12O6, giving the following equation:

6CO2 ! 6H2O C6H12O6 ! 6O2

However, glucose is not actually a direct product of photosyn-thesis. Glucose is often included in the equation mainly to empha-size the relationship between photosynthesis and cellularrespiration, in which glucose plays a key role. Just as the light reac-tions and the Calvin cycle together create an ongoing cycle, pho-tosynthesis and cellular respiration together also create anongoing cycle.

The light reactions are sometimes referred to as light-dependentreactions, because the energy from light is required for the reac-tions to occur. The Calvin cycle is sometimes referred to as thelight-independent reactions or the dark reactions, because theCalvin cycle does not require light directly. But the Calvin cycleusually proceeds during daytime, when the light reactions are pro-ducing the materials that the Calvin cycle uses to fix carbon intoorganic compounds.

light energy

light energy


C H A P T E R 6124

FACTORS THAT AFFECTPHOTOSYNTHESIS

Photosynthesis is affected by the environment. Three factors withthe most influence are light intensity, CO2 level, and temperature.

Light Intensity As shown in Figure 6-12a, the rate of photosynthesis increases aslight intensity increases. Higher light intensity excites more elec-trons in both photosystems. Thus, the light reactions occur morerapidly. However, at some point all of the available electrons areexcited, and the maximum rate of photosynthesis is reached. Therate stays level regardless of further increases in light intensity.

Carbon Dioxide Levels Increasing levels of CO2 also stimulate photosynthesis until the rateof photosynthesis levels off. Thus, a graph of the rate of photosyn-thesis versus CO2 concentration would resemble Figure 6-12a.

TemperatureIncreasing temperature accelerates the chemical reactions involvedin photosynthesis. As a result, the rate of photosynthesis increases

as temperature increases, over a cer-tain range. This effect is illustrated bythe left half of the curve in Figure 6-12b.The rate peaks at a certain tempera-ture, at which many of the enzymesthat catalyze the reactions becomeineffective. Also, the stomata begin toclose, limiting water loss and CO2 entryinto the leaves. These conditionscause the rate of photosynthesis todecrease when the temperature is fur-ther increased, as shown by the righthalf of the curve in Figure 6-12b.

1. Name the part of the chloroplast where theCalvin cycle takes place.

2. Describe what can happen to the three-carbonmolecules made in the Calvin cycle.

3. Distinguish between C3, C4, and CAM plants.

4. Explain why the light reactions and the Calvincycle are dependent on each other.

5. Explain why increased light intensity might notresult in an increased rate of photosynthesis.

CRITICAL THINKING6. Predicting Results What would happen to

photosynthesis if all of the three-carbon sugarsproduced in the Calvin cycle were used to makeorganic compounds?

7. Inferring Relationships Explain how a globaltemperature increase could affect plants.

8. Evaluating Differences Explain how the worldwould be different if C4 plants and CAM plantshad not evolved.

Environmental Influences on Photosynthesis

Rate

of p

hoto

synt

hesi

s

(b) Temperature(a) Light intensity

Rate

of p

hoto

synt

hesi

s

SECTION 2 REVIEW


Environmental factors affect the rate ofphotosynthesis in plants. (a) As lightintensity increases, the rate ofphotosynthesis increases and then levelsoff at a maximum. (b) As temperatureincreases, the rate of photosynthesisincreases to a maximum and thendecreases with further rises intemperature.

FIGURE 6-12

The Light ReactionsSECTION 1

CHAPTER HIGHLIGHTS

The Calvin CycleSECTION 2

● Photosynthesis converts light energy into chemicalenergy through series of reactions known as biochemicalpathways. Almost all life depends on photosynthesis.

● Autotrophs use photosynthesis to make organiccompounds from carbon dioxide and water. Heterotrophscannot make their own organic compounds frominorganic compounds and therefore depend onautotrophs.

● White light from the sun is composed of an array ofcolors called the visible spectrum. Pigments absorbcertain colors of light and reflect or transmit the other colors.

● The light reactions of photosynthesis begin with theabsorption of light by chlorophyll a and accessorypigments in the thylakoids.

● Excited electrons that leave chlorophyll a travel alongtwo electron transport chains, resulting in the productionof NADPH. The electrons are replaced when water is splitinto electrons, protons, and oxygen in the thylakoid.Oxygen is released as a byproduct of photosynthesis.

● As electrons travel along the electron transport chains,protons move into the thylakoid and build up aconcentration gradient. The movement of protons downthis gradient of protons and through ATP synthase resultsin the synthesis of ATP through chemiosmosis.

● The ATP and NADPH produced in the light reactions drivethe second stage of photosynthesis, the Calvin cycle. Inthe Calvin cycle, CO2 is incorporated into organiccompounds, a process called carbon fixation.

● The Calvin cycle produces a compound called G3P. MostG3P molecules are converted into RuBP to keep theCalvin cycle operating. However, some G3P molecules areused to make other organic compounds, including aminoacids, lipids, and carbohydrates.

● Plants that fix carbon using only the Calvin cycle areknown as C3 plants. Some plants that evolved in hot, dryclimates fix carbon through alternative pathways—the C4and CAM pathways. These plants carry out carbonfixation and the Calvin cycle either in different cells or atdifferent times.

● Photosynthesis occurs in two stages. In the lightreactions, energy is absorbed from sunlight andconverted into chemical energy; in the Calvin cycle,carbon dioxide and chemical energy are used to formorganic compounds.

● The rate of photosynthesis increases and then reaches aplateau as light intensity or CO2 concentration increases.Below a certain temperature, the rate of photosynthesisincreases as temperature increases. Above thattemperature, the rate of photosynthesis decreases astemperature increases.


autotroph (p. 113)photosynthesis (p. 113)heterotroph (p. 113)light reactions (p. 114)chloroplast (p. 114)

thylakoid (p. 114)granum (p. 114)stroma (p. 114)pigment (p. 115)chlorophyll (p. 115)

carotenoid (p. 115)photosystem (p. 116)primary electron

acceptor (p. 117)

electron transport chain (p. 117)

chemiosmosis (p. 118)

Vocabulary

Calvin cycle (p. 120)carbon fixation (p. 120)

stomata (p. 121)C4 pathway (p. 122)

CAM pathway (p. 122)Vocabulary


CHAPTER REVIEW

C H A P T E R 6126

USING VOCABULARY1. For each pair of terms, explain the relationship

between the terms.a. electron transport chain and primary

electron acceptorb. photosystem and pigmentc. thylakoid and stromad. carbon fixation and C4 pathway

2. Use the following terms in the same sentence:photosynthesis, light reactions, pigment, andphotosystem.

3. Choose the term that does not belong in the fol-lowing group, and explain why it does not belong:electron transport chain, chemiosmosis, Calvincycle, and photosystem II.

4. Word Roots and Origins The prefix chloro- isderived from the Greek word that means “green.”Using this information, explain why the chloro-phylls are well named.

UNDERSTANDING KEY CONCEPTS5. Distinguish between autotrophs and heterotrophs.6. Identify the primary source of energy for humans.7. Relate the types of pigments involved in photo-

synthesis and their roles.8. Compare the different roles of photosystem I and

photosystem II in photosynthesis.9. Describe what happens to the extra energy in

excited electrons as they pass along an electrontransport chain in a chloroplast.

10. Explain how oxygen is generated in photo-synthesis.

11. Summarize the events of chemiosmosis.12. State the three major steps of the Calvin cycle.13. Explain what happens to the 3-carbon com-

pounds that do not leave the Calvin cycle to bemade into organic compounds.

14. Propose what might happen to photosynthesis ifATP were not produced in the light reactions.

15. Relate the rate of photosynthesis to carbon dioxide levels.

16. Unit 2–PhotosynthesisMany plants have stomata thattake in CO2 at night and release itduring the day. Write a report

about these types of plants, and summarize whythis adaptation is an advantage for plants living ina hot, dry climate.

17. CONCEPT MAPPING Create a concept map that shows the steps of photosyn-

thesis. Include the following terms in your concept map: light reactions, Calvin cycle,photosystem I, photosystem II, carbon fixation,accessory pigment, chlorophyll, electron trans-port chain, ATP synthase, chemiosmosis, andphotosynthesis.

CRITICAL THINKING18. Predicting Results When the CO2 concentration in

the cells of a C3 plant is low compared with theO2 concentration, an enzyme combines RuBPwith O2 rather than with CO2. What effect wouldthis enzymatic change have on photosynthesis?Under what environmental conditions would it bemost likely to occur?

19. Evaluating Information All of the major compo-nents of the light reactions, including the pigmentmolecules clustered in photosystems I and II, arelocated in the thylakoid membrane. What is theadvantage of having these components confinedto the same membrane rather than dissolved inthe stroma or the cytosol?

20. Inferring Relationships When the sun’s rays areblocked by a thick forest, clouds, or smoke froma large fire, what effect do you think there will beon photosynthesis? How might it affect the levelsof atmospheric carbon dioxide and oxygen? Whatexperiments could scientists conduct in the labo-ratory to test your predictions?

21. Interpreting Graphics The graph below showshow the percentage of stomata that are openvaries over time for two different plants. Onecurve represents the stomata of a geranium, andthe other curve represents the stomata of apineapple. Which curve corresponds to thepineapple stomata? Explain your reasoning.

Noon Midnight Noon Midnight

50

100 A

B

0

Perc

enta

ge o

fop

en s

tom

ata

Daily Cycle of Stomatal Opening



Standardized Test PreparationDIRECTIONS: Choose the letter of the answer choicethat best answers the question.

1. Which of the following is a reactant in the Calvincycle?A. O2B. CO2C. H2OD. C6H12O6

2. Which of the following statements is correct?F. Accessory pigments are not involved in

photosynthesis.G. Accessory pigments add color to plants but

do not absorb light energy.H. Accessory pigments absorb colors of light

that chlorophyll a cannot absorb.J. Accessory pigments receive electrons

from the electron transport chain of photo-system I.

3. Oxygen is produced at what point during photosynthesis?A. when CO2 is fixedB. when water is splitC. when ATP is converted into ADPD. when 3-PGA is converted into G3P

INTERPRETING GRAPHICS: The diagram belowshows a portion of a chloroplast. Use the diagram toanswer the question that follows.

4. Which of the following correctly identifies thestructure marked X and the activities that takeplace there?F. stroma—Calvin cycleG. stroma—light reactionsH. thylakoid—Calvin cycleJ. thylakoid—light reactions

DIRECTIONS: Complete the following analogy.5. light reactions : ATP :: Calvin cycle :

A. H!

B. O2C. G3PD. H2O

INTERPRETING GRAPHICS: The diagram belowshows a step in the process of chemiosmosis. Use thediagram to answer the question that follows.

6. What is the substance identified as Y in theimage?F. H!

G. NAD!

H. NADPHJ. ADP synthase

SHORT RESPONSEChloroplasts are organelles with areas that conductdifferent specialized activities.

Where in the chloroplast do the light reactions andthe Calvin cycle occur?

EXTENDED RESPONSEThe reactions of photosynthesis make up a biochemi-cal pathway.

Part A What are the reactants and products for boththe light reactions and the Calvin cycle?

Part B Explain how the biochemical pathway of pho-tosynthesis recycles many of its own reac-tants, and identify the recycled reactants.

For a question involving a bio-chemical pathway, write out all of the reactants andproducts of each step before answering the question.

Y

Y

ATP ADP +phosphate

Thylakoidmembrane

STROMA

INSIDE OFTHYLAKOID

X


C H A P T E R 6128

Measuring the Rate of Photosynthesis

■ Measure the amount of oxygen produced by anelodea sprig.

■ Determine the rate of photosynthesis for elodea.

■ making observations■ measuring■ collecting data■ graphing

■ 500 mL of 5% baking-soda-and-water solution■ 600 mL beaker■ 20 cm long elodea sprigs (2–3)■ glass funnel■ test tube■ metric ruler

Background1. Summarize the main steps of photosynthesis.2. State the source of the oxygen produced during

photosynthesis.3. Identify factors that can affect the rate of

photosynthesis.

ProcedureSetting Up the Experiment

CAUTION Always wearsafety goggles and a lab

apron to protect your eyes and clothing. Glasswareis fragile. Notify the teacher of broken glass or cuts.Do not clean up broken glass or spills with brokenglass unless the teacher tells you to do so. Wear disposable polyethylene gloves when handling anyplant. Do not eat any part of a plant or plant seedused in the lab. Wash hands thoroughly after han-dling any part of a plant.

1. Add 450 mL of baking-soda-and-water solution to a beaker.

2. Put two or three sprigs of elodea in the beaker. Thebaking soda will provide the elodea with the carbondioxide it needs for photosynthesis.

3. Place the wide end of the funnel over the elodea.The end of the funnel with the small opening shouldbe pointing up. The elodea and the funnel should becompletely under the solution.

4. Fill a test tube with the remaining baking-soda-and-water solution. Place your thumb over the end ofthe test tube. Turn the test tube upside down, takingcare that no air enters. Hold the opening of the testtube under the solution, and place the test tubeover the small end of the funnel. Try not to let anysolution leak out of the test tube as you do this.

5. Place the beaker setup in a well-lighted area near alamp or in direct sunlight.

PART A

MATERIALS

PROCESS SKILLS

OBJECTIVES

EXPLORATION LAB



AMOUNT OF GAS PRESENT IN THE TEST TUBE

Days of exposure to light Total amount of gas present (mm) Amount of gas produced per day (mm)

0

1

2

3

4

5

Collecting Data6. Create a data table like the one below.7. Record that there was 0 mm gas in the test tube on

day 0. (If you were unable to place the test tube withoutgetting air in the tube, measure the height of the columnof air in the test tube in millimeters. Record this value forday 0.) In this lab, change in gas volume is indicated bya linear measurement expressed in millimeters.

8. For days 1 through 5, measure the amount of gas in the test tube. Record the measurements in yourdata table under the heading “Total amount of gaspresent (mm).”

9. Calculate the amount of gas produced each day bysubtracting the amount of gas present on the previousday from the amount of gas present today. Recordthese amounts under the heading “Amount of gas produced per day (mm).”

Analysis and Conclusions1. Using the data from your table, prepare a graph.2. Using information from your graph, describe what

happened to the amount of gas in the test tube.3. How much gas was produced in the test tube

after day 5?4. Write the equation for photosynthesis. Explain each

part of the equation. For example, which ingredientsare necessary for photosynthesis to take place? Whichsubstances are produced by photosynthesis? Whichgas is produced that we need in order to live?

5. What may happen to the oxygen level if an animal,such as a snail, were put in the beaker with the elodeasprig while the elodea sprig was making oxygen?

Further Inquiry Design an experiment for predicting the effects of tempera-ture on the amount of oxygen produced or rate of photo-synthesis by elodea.

PART B


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