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Copyright Pearson Prentice Hall
8-3 The Reactions of 8-3 The Reactions of PhotosynthesisPhotosynthesis
History of the Study of History of the Study of PhotosynthesisPhotosynthesis
The experiments performed by van Helmont, Priestley, and Ingenhousz led to work by other scientists who finally discovered that, in the presence of light, plants transform carbon dioxide and water into carbohydrates, and they also release oxygen.
In plants, photosynthesis takes place inside In plants, photosynthesis takes place inside chloroplasts.chloroplasts.
Plant
Plant cells
Chloroplast
Where does Photosynthesis Occur?
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Inside a ChloroplastInside a Chloroplast
Chloroplasts contain thylakoids—saclike Chloroplasts contain thylakoids—saclike photosynthetic membranes.photosynthetic membranes.
Chloroplast
Singlethylakoid
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Inside a ChloroplastInside a ChloroplastThylakoids are arranged in stacks known as Thylakoids are arranged in stacks known as
grana. A singular stack is called a granum.grana. A singular stack is called a granum.Granum
Chloroplast
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Inside a ChloroplastInside a ChloroplastProteins in the thylakoid membrane organize Proteins in the thylakoid membrane organize
chlorophyll and other pigments into clusters chlorophyll and other pigments into clusters called photosystems, which are the light-called photosystems, which are the light-collecting units of the chloroplast.collecting units of the chloroplast.
Chloroplast
Photosystems
Overview: PhotosynthesisOverview: Photosynthesis
Chloroplast
LightH2O
O2
CO2
Sugars
NADP+
ADP + P
Calvin Cycle
Light- dependent reactions
Calvin cycle
Photosynthesis EquationPhotosynthesis Equation
6CO2 + 6H2O + Sunlight C6H12O6 + 6O2
Carbon Water Energy Glucose OxygenDioxide
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Electron CarriersElectron CarriersWhen electrons in chlorophyll absorb When electrons in chlorophyll absorb sunlight, the electrons gain a great deal sunlight, the electrons gain a great deal of of energyenergy..Cells use Cells use electron carrierselectron carriers to transport to transport these high-energy electrons from these high-energy electrons from chlorophyll to other molecules.chlorophyll to other molecules.Example of electron carrier: Example of electron carrier: NADP+NADP+
NADPNADP++ + H + H++ NADPH NADPH
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Light-Dependent ReactionsLight-Dependent Reactions
The light-dependent reactions require light.The light-dependent reactions require light. The light-dependent reactions produce The light-dependent reactions produce
oxygen gas and convert ADP and NADPoxygen gas and convert ADP and NADP++ into into the energy carriers the energy carriers ATPATP and and NADPHNADPH..
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Light-Dependent ReactionsLight-Dependent Reactions
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Photosystem II
Light-Dependent ReactionsLight-Dependent Reactions
1) Photosynthesis begins when pigments in 1) Photosynthesis begins when pigments in photosystem II absorb light, increasing their photosystem II absorb light, increasing their energy level.energy level.
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Light-Dependent ReactionsLight-Dependent Reactions
Photosystem II
2) These high-energy electrons are passed on to the 2) These high-energy electrons are passed on to the electron transport chain.electron transport chain.
Electroncarriers
High-energy electron
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Light-Dependent ReactionsLight-Dependent Reactions
Photosystem II
2H2O
Enzymes on the thylakoid membrane break water Enzymes on the thylakoid membrane break water molecules into:molecules into:
Electroncarriers
High-energy electron
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Light-Dependent ReactionsLight-Dependent Reactions
Photosystem II
2H2O
hydrogen ionshydrogen ions oxygen atomsoxygen atoms energized electronsenergized electrons
+ O2
Electroncarriers
High-energy electron
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Light-Dependent ReactionsLight-Dependent Reactions
Photosystem II
2H2O
+ O2
3) The energized electrons from water replace the high-energy electrons that chlorophyll lost to the electron transport chain.
High-energy electron
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Light-Dependent ReactionsLight-Dependent Reactions
Photosystem II
2H2O
4) As plants remove electrons from water, oxygen is left behind and is released into the air.
+ O2
High-energy electron
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Light-Dependent ReactionsLight-Dependent Reactions
Photosystem II
2H2O
The hydrogen ions left behind when water is broken apart are released inside the thylakoid membrane.
+ O2
High-energy electron
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Light-Dependent ReactionsLight-Dependent Reactions
Photosystem II
2H2O
Energy from the electrons is used to transport H+ ions from the stroma into the inner thylakoid space.
+ O2
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Light-Dependent ReactionsLight-Dependent Reactions
Photosystem II
2H2O
5) High-energy electrons move through the electron transport chain from photosystem II to photosystem I.
+ O2
Photosystem I
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Light-Dependent ReactionsLight-Dependent Reactions
2H2O
Pigments in photosystem I use energy from light to re-energize the electrons.
+ O2
Photosystem I
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Light-Dependent ReactionsLight-Dependent Reactions
2H2O
6) NADP+ then picks up these high-energy electrons, along with H+ ions, and becomes NADPH.
+ O2
2 NADP+
2 NADPH2
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Light-Dependent ReactionsLight-Dependent Reactions
2H2O
As electrons are passed from chlorophyll to NADP+, more H+ ions are pumped across the membrane.
+ O2
2 NADP+
2 NADPH2
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Light-Dependent ReactionsLight-Dependent Reactions
2H2O
7) Soon, the inside of the membrane fills up with positively charged hydrogen ions, which makes the outside of the membrane negatively charged.
+ O2
2 NADP+
2 NADPH2
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Light-Dependent ReactionsLight-Dependent Reactions
2H2O
The difference in charges across the membrane provides the energy to make ATP
+ O2
2 NADP+
2 NADPH2
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Light-Dependent ReactionsLight-Dependent Reactions
2H2O
H+ ions cannot cross the membrane directly.
+ O2
ATP synthase
2 NADP+
2 NADPH2
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Light-Dependent ReactionsLight-Dependent Reactions
2H2O
8) The cell membrane contains a protein called ATP synthase that allows H+ ions to pass through it
+ O2
ATP synthase
2 NADP+
2 NADPH2
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Light-Dependent ReactionsLight-Dependent Reactions
2H2O
As H+ ions pass through ATP synthase, the protein rotates.
+ O2
ATP synthase
2 NADP+
2 NADPH2
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Light-Dependent ReactionsLight-Dependent Reactions
2H2O
9) As it rotates, ATP synthase binds ADP and a phosphate group together to produce ATP.
+ O2
2 NADP+
2 NADPH2
ATP synthase
ADP
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Light-Dependent ReactionsLight-Dependent Reactions
2H2O
Because of this system, light-dependent electron transport produces not only high-energy electrons but ATP as well.
+ O2
ATP synthase
ADP2 NADP+
2 NADPH2
Light ReactionsLight Reactions
Key Events:Key Events: Absorption of light energy.Absorption of light energy. Excitation of electrons by Excitation of electrons by solar energy.solar energy.
Formation of ATP and Formation of ATP and NADPH via ETC.NADPH via ETC.
Dark Rxn OverviewDark Rxn Overview Occurs in the Occurs in the stromastroma of the of the
chloroplast.chloroplast.
Obtains Obtains CO CO2 2 ( (Use CarbonUse Carbon)) From surrounding AirFrom surrounding Air
Use Use ATPATP ( (Need Energy)Need Energy) Use Use NADPH (NADPH (Need High Energy Need High Energy
Electrons) Electrons) Both Obtained from Both Obtained from Light ReactionsLight Reactions
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The Calvin cycle uses ATP and The Calvin cycle uses ATP and NADPH from the light-dependent NADPH from the light-dependent reactions to produce high-reactions to produce high-energy sugars.energy sugars.
Because the Calvin cycle does not Because the Calvin cycle does not require light, these reactions are require light, these reactions are also called the also called the light-independentlight-independent reactions.reactions.
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The Calvin CycleThe Calvin Cycle
Six carbon dioxide molecules enter the cycle from the Six carbon dioxide molecules enter the cycle from the atmosphere and combine with six 5-carbon atmosphere and combine with six 5-carbon molecules.molecules.
CO2 Enters the Cycle
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The Calvin CycleThe Calvin Cycle
The result is twelve 3-carbon molecules, which are then The result is twelve 3-carbon molecules, which are then converted into higher-energy forms. converted into higher-energy forms.
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The Calvin CycleThe Calvin Cycle
The energy for this conversion comes from ATP and high-The energy for this conversion comes from ATP and high-energy electrons from NADPH.energy electrons from NADPH.
12 NADPH
12
12 ADP
12 NADP+
Energy Input
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The Calvin Cycle The Calvin Cycle
Two of twelve 3-carbon molecules are removed from the Two of twelve 3-carbon molecules are removed from the cycle.cycle.
Energy Input
12 NADPH
12
12 ADP
12 NADP+
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The Calvin CycleThe Calvin Cycle
The molecules are used to produce sugars, lipids, amino The molecules are used to produce sugars, lipids, amino acids and other compounds.acids and other compounds.
12 NADPH
12
12 ADP
12 NADP+
6-Carbon sugar produced
Sugars and other compounds
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The Calvin CycleThe Calvin Cycle
The 10 remaining 3-carbon molecules are converted The 10 remaining 3-carbon molecules are converted back into six 5-carbon molecules, which are used to back into six 5-carbon molecules, which are used to begin the next cycle.begin the next cycle.
12 NADPH
12
12 ADP
12 NADP+
5-Carbon MoleculesRegenerated
Sugars and other compounds
6
6 ADP
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The two sets of The two sets of photosynthetic reactions work photosynthetic reactions work togethertogetherThe The light-dependentlight-dependent reactions reactions
trap sunlight energy in chemical trap sunlight energy in chemical form. form.
The The light-independentlight-independent reactions reactions use that chemical energy to use that chemical energy to produce stable, high-energy produce stable, high-energy sugars from carbon dioxide and sugars from carbon dioxide and water.water.