PHOTOSYNTHESIS

Review Your Understanding

AP Biology

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Overview• This interactive presentation will review

the major concepts involved in photosynthesis allowing you to focus on areas that are least clear.

• At the end of the presentation there will be a quiz to test your understanding.

• Click the following image now (and at anytime) to go to the main menu.

Main Menu• What is Photosynthesis• Photosynthetic Pigments• Site of Photosynthesis• Photosystems – (Capture)• Light-Dependent Reactions – (Convert)• Light-Independent Reactions – (Store)• AP Biology Lab 4• Quiz

Photosynthesis• Autotrophic organisms are able to manufacture

their own organic food molecules from inorganic molecules. Most autotrophs use sunlight as their energy source but some use inorganic chemical bond energy in a process call chemosynthesis.

• Autotrophs which contain the green pigment chlorophyll (located in their chloroplast) are able to convert solar energy into stored chemical bond energy in a process called photosynthesis.

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• The following reaction (in the presence of light) summarizes the chemical process of photosynthesis.

– Carbon dioxide + water glucose + oxygen + water– 6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

• This process can be broken down into 3 stages– The capture of light energy– The conversion of light energy into chemical energy– The storage of chemical energy in sugar (carbon fixation)

• The first 2 stages are referred to as the light-dependent reactions while the last stage refers to the light–independent reactions.

Photosynthesis

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Overview

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Photosynthetic Pigments

• The sun radiates light energy in individual packets called photons. The energy of a photon corresponds to its wavelength.

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» Wavelength of Visible Light

• The first step in photosynthesis is the capture of photons by pigments in the chloroplast. NEXT

• A pigment absorbs all colors that it does not reflect.

• The chloroplast contains several types of pigment molecules that absorb differentwavelengths of light.

• Chlorophyll is the key light capturing pigment (containing > 120 atoms) found in the thylakoid membrane of the chloroplast. It strongly absorbs violet, blue, and red light but reflects green, and therefore appears green. See graph

Photosynthetic Pigments

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Figure 1. Different types of pigments selectively absorb certain colors; the height of each line represents the ability of each pigment to absorb light of each color. Photosynthesis is driven to some extent by all of the colors of light, owing to the absorption of light by several thylakoid pigments. BACK

• Thylakoids also contain other molecules called antenna/accessory pigments, that capture light energy and transfer it to chlorophyll, thus expanding the spectrum of light that can be used in photosynthesis.

• Carotenoids and xanthophylls absorb blue and green light and appear yellow, orange or red.

• Phycocyanins absorb green and yellow light and appear blue or purple.

– See graph

Photosynthetic Pigments

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Where Photosynthesis Happens

• The main photosynthetic structure of a plant is the leave.

• - Mesophyll cells contain the chloroplasts used in photosynthesis.

• - Necessary CO2 and O2

exchange occurs via the stoma.

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Cross section of a leafCross section of a leaf

Chloroplast• Here we see a single mesophyll cell

and some of its major features including a nucleus, vacuole, and the green chloroplast

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Structure of a Chloroplast Refer to diagram on preceding page

• The chloroplast is surrounded by a double membrane enclosing a semifluid medium know as stroma.

• Embedded in the stroma are disk-shaped, interconnected membranous sacs called thylakoids.

• A stack of thylakoids is referred to as granum (grana, plural).

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Photosystems• In the thylakoid membrane, chlorophyll,

antenna pigments, and electron-carrier molecules form highly organized assemblies called photosystems which capture light.

• Each thylakoid contains thousands of copies of two different kinds of photosystems.

• Each is made up of 2 parts, a light harvesting complex and an electron transport system.

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• These light-harvesting complexes consist of a reaction center containing chlorophyll plus antenna pigments.

• The reaction center is located next to the electron transport system – a series of electron carriers in the thylakoid membrane.

• The 2 photosystems are referred to as PS 1 and PS II or P700 and P680, respectively .

Photosystems

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• PS I (P700) absorbs light, on the average in the 700 nm range.

• PS II(P680) absorbs light, on the average in the 680 nm range.

• PS II activates before PS I

Photosystems

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Photosystem II• Lets take a closer look

• In the following video you will see:– The processes that take place in the PS II complex– How chlorophyll pigments become excited by

light energy– How an electron is transferred to the electron

transport chain– How oxygen is generated form water

• VIDEO

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Light-Dependent Reaction

• Occurs in clusters of molecules called photosystems (in the thylakoid membrane)

• Energy from the sun is converted into chemical energy in the form of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH).

• This energy is needed to later fuel the light

independent (Calvin Cycle) in the stroma.NEXT NEXT

Light-Dependent Reaction

• The reaction begins when light is absorbed by chlorophyll pigments in PS II located within the thylakoid membrane.

• An excited electron will then pass through the electron transport chain until it reaches PS I

• Through this process energy (ATP and NADPH) is made.

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• Use the following animation to study the steeps of the light-dependent reaction.

• Animation – click the play button (►) at the bottom right to begin. Place your mouse over each object to reveal the name of that object.

• When you have finished close the webpage and return to this presentation. Click ‘next’ to see the steeps in the light-dependent reaction.

Light-Dependent Reaction

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1. Light is harvested in photosystem II2. This energy ejects electrons out of the reaction center3. Electrons pass to the adjacent electron transport system4. The transport system passes the electrons along, some of their

energy is used to pump hydrogen ions into the thylakoid interior. A hydrogen gradient results and drives ATP synthesis

5. Light strikes photosystem I6. PS I emits electrons7. These electrons are captures by PS I electron transport system.

Electrons lost from PS I reaction center are replaced by those coming from the transport system of PS II.

8. High energy electrons from PS I are captured by NADP+, which becomes NADPH

9. The electrons lost from the reaction center of PS II are replaced by electrons obtained from the splitting of water (photolysis). This liberates oxygen (O2)

Light-Dependent Reaction

Process *See diagram**

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Lets go to the video!

• To see a video of the light reaction in progress click the following link– VIDEO

• As the page loads, study the diagram again– **See diagram**

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• ATP is produced: The flow of electrons through electron transport chains is coupled with the phosphorylation of ADP to ATP via chemiosmosis– ATP is needed to fuel the light independent reactions

(Calvin Cycle)

• Oxygen is produced: Water is split apart via Photolysis to provide electrons for photosystem II and protons to reduce NADP in photosystem I.

• NADPH is produced: NADP+ carries H2 to the light-independent reactions (Calvin Cycle).

Light-Dependent ReactionSummary

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• The main process occurring during the light-dependent reaction.

• Light energy (‘photo’) is used to ‘phosphorylate’ ADP, forming ATP.

• Process begins at PS II and continues in PS I• Electrons supplied by photolysis enter two

electron transport chains and ATP and NADPH are formed

• Chemiosmosis powers the production of ATP

Photophosphorylation

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Light-Independent Reactions

• Storing chemical energy in sugar molecules, occurs in the stoma.

• ATP and NADPH (synthesized during the light-dependent reactions) are dissolve in the stoma, providing the energy to power the synthesis of glucose from CO2 and H20.

• Reaction occurs independently of light as long as ATP and NADPH are available.

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Light-Independent Reactions

• CO2 capture occurs in a set of reactions know as the Calvin cycle or C3 cycle.

• The Calvin/C3 cycle requires;

– CO2 (normally from the air)

– A CO2 capturing sugar, ribulose bisphosphate (RuBP)

– Enzymes to catalyze all the reactions– Energy in the form of ATP and NADPH (from the light-

dependent reactions)

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• Use the following animation to study the steeps of the Calvin Cycle.

• Animation – click the 5 carbon molecule RuBP to begin. Use the flow chart on the right to proceed steep by steep through the cycle.

• When you have finished close the webpage and return to this presentation. Click ‘next’ to see the parts of the Calvin cycle.

Calvin Cycle

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Calvin Cycle• CO2 (from the atmosphere) is chemically

reduced by hydrogen ions (produced in the light-dependent reactions) to produce carbohydrates

CO2 + H CH2O (unbalanced)

• The Calvin/C3 cycle is best understood in three parts (be sure to look at each before continuing)

1. Carbon fixation2. Synthesis of phosphoglyceraldehyde (PGAL)

– A 3 carbon sugar

3. The regeneration of ribulose biphosphate (RuBP)

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Carbon FixationSection #1 of diagram

• The Calvin cycle begins and ends with a 5-carbon sugar, RuBP.

• RuBP combines with CO2 from the atmosphere to form an extreamly unstabe 6-carbon compound.

• This spontaneously reacts with H2O to form two 3-carbon molecules of phosphoglyceric acid PGA (hence C3 cycle)

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Synthesis of PGALSection #2 of diagram

• A series of enzyme-catalyzed reactions occur.

• Energy donated by ATP and NADPH, generated in the light reactions, is used to convert PGA to phosphoglyceraldehyde (PGAL).

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Regeneration of RuBPSection #3 of diagram

• A complex series of reactions requiring ATP energy occurs.

• Ten molecules of PGAL (10 x 3-carbon) can regenerate 6 molecules of RuBP (6 x 5-carbon).

• These six RuBP molecules are used to start the cycle again.

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Click to go back to Step #1

Click to go back to Step #2

Click to go back to Step #3

Calvin CycleEnd Results

• The Calvin Cycle starts with 6 molecules of RuBP, adds CO2 to each, and ends with 6 molecules of RuBP again.

• The fixed carbon (from atmospheric CO2) has been transformed and two molecules of phosphoglyceraldehyde (PGAL) are left over.

• These two PGAL molecules (3-carbon each) combine to form one glucose (6-carbon)

• Glucose can later be broken down (respiration) or linked together to form starch (storage) or cellulose (cell walls), or modified into other cellular contents.

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Lab Activity • Go to the following link to review Plant

Pigments and Photosynthesis.

• Be sure to look at part II of the lab as we do not have the materials at the school to do this part of the lab.

• Click me

Quiz• The following is a short quiz covering

some of the important ideas of photosynthesis.

• If you select a wrong answer, you will be given an explanation of why it is incorrect.

• You can not move to the next question until you choose the correct answer

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QUESTION #1• Which of the following are products of

photosynthesis?

• A) Carbon dioxide and water

• B) Carbon dioxide and glucose

• C) Glucose and oxygen

• D) Oxygen and caclcium

Q #1 A Incorrect• A is incorrect because carbon dioxide and

water are the products of aerobic respiration

Return to Question #1

Q #1 B Incorrect• B is incorrect because only one of these is

a product of photosynthesis!

Return to Question #1

Q #1 C CORRECT

• C is correct because the products of photosynthesis are oxygen and glucose.

– Carbon dioxide + water glucose + oxygen + water

• 6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

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Q #1 D Incorrect• D is incorrect because only one of these is

a product of photosynthesis!

Return to Question #1

QUESTION #2• What is the name of the pigment used to

trap light energy during the process of photosynthesis?

• A) PGAL

• B) ATP

• C) Chlorophyll

• D) Stoma

Q #2 A Incorrect• A is not correct because PAGL is the first

organic compound produced during photosynthesis

Return to Question #2

Q #2 B Incorrect• B is not correct because ATP is a

chemical found in living organisms used to store and release energy.

Return to Question #2

Q #2 C CORRECT

• C is the correct answer because the chemical used to trap light energy during photosynthesis is chlorophyll.

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Q #2 D Incorrect• D is incorrect because the stoma is the

region inside the chloroplast of the cell.

Return to Question #2

QUESTION #3• A water molecule is split into hydrogen

and hydroxide ions during the process of what?

• A) Chemosynthesis

• B) Hydrolysis

• C) Photolysis

• D) Carbon fixation

Q #3 A Incorrect• A is not correct because chemosynthesis

is the manufacturing of organic molecules by using chemical energy located in inorganic molecules.

Return to Question #3

Q #3 B Incorrect• B is not correct because hydrolysis is the

splitting of two molecules with the addition of water.

Return to Question #3

Q #3 C CORRECT

• C is correct because photolysis is the splitting of water molecules.

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Q #3 D Incorrect• D is not correct because carbon fixation is

the chemical process where atmospheric carbon dioxide is captured by plants.

Return to Question #3

QUESTION #4• What energy conversion occurs during the

process of photosynthesis?

• A) Light energy to nuclear energy

• B) Chemical bond energy to light energy

• C) Light energy to chemical bond energy

• D) Mechanical energy to radiant energy

Q #4 A Incorrect• A is not correct because this type of

energy conservation does not occur in photosynthesis.

Return to Question #4

Q #4 B Incorrect• B is not correct because this type of

energy conservation does not occur in photosynthesis.

Return to Question #4

Q #4 C Correct • C is the correct answer because the

energy conservation that occurs during photosynthesis is light energy to chemical bond energy.

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Q #4 D Incorrect• B is not correct because this type of

energy conservation does not occur in photosynthesis.

Return to Question #4

QUESTION #5• The coenzyme that carries hydrogen

atoms from the light reaction to the dark reaction is?

• A) NADP

• B) Chlorophyll

• C) ATP

• D) RNA

Q #5 A CORRECT

• A is the correct answer because NADP is the coenzyme that carries hydrogen atoms from the light reaction to the dark reaction.

End of Presentation

Q #5 B Incorrect• B is not the correct answer because

chlorophyll is the chemical in photosynthetic plants used to trap light energy for food production.

Return to Question #5

Q #5 C Incorrect• C is not the correct answer because ATP

is a chemical found in living organisms used to store and release energy.

Return to Question #5

Q #5 D Incorrect• D is not the correct answer because RNA

is a nucleic acid that aids in protein synthesis.

Return to Question #5

THE ENDMr. Young

AP Biology 2007/2008