1. 8-3 The Reactions of Photosynthesis
Photo Credit: ©Stone
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2. History of the Study of Photosynthesis
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.

3. Where does Photosynthesis Occur?
In plants, photosynthesis takes place inside chloroplasts.
Plant
Chloroplast
Plant cells

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Inside a Chloroplast
Chloroplasts contain thylakoids—saclike photosynthetic membranes.
Single
thylakoid
Chloroplast
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Inside a Chloroplast
Thylakoids are arranged in stacks known as grana. A singular stack is called a granum.
Granum
Chloroplast
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Inside a Chloroplast
Proteins in the thylakoid membrane organize chlorophyll and other pigments into clusters called photosystems, which are the light-collecting units of the chloroplast.
Photosystems
Chloroplast
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7. Overview: Photosynthesis
CO2
Light
NADP+
ADP + P
Light- dependent reactions
Calvin Cycle
Calvin cycle
The process of photosynthesis includes the light-dependent reactions as well as the Calvin cycle.
Chloroplast O2
Sugars

8. Photosynthesis Equation
6CO2 + 6H2O + Sunlight  C6H12O6 + 6O2
Carbon Water Energy Glucose Oxygen
Dioxide

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Electron Carriers
When electrons in chlorophyll absorb sunlight, the electrons gain a great deal of energy.
Cells use electron carriers to transport these high-energy electrons from chlorophyll to other molecules.
Example of electron carrier: NADP+
NADP+ + H+  NADPH
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Light-Dependent Reactions
The light-dependent reactions require light.
The light-dependent reactions produce oxygen gas and convert ADP and NADP+ into the energy carriers ATP and NADPH.
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11. Light-Dependent Reactions
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. The light-dependent reactions take place within the thylakoid membranes of chloroplasts.
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12. Light-Dependent Reactions
1) Photosynthesis begins when pigments in photosystem II absorb light, increasing their energy level.
Photosystem II
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. The light-dependent reactions take place within the thylakoid membranes of chloroplasts.
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13. Light-Dependent Reactions
2) These high-energy electrons are passed on to the electron transport chain.
Photosystem II
Electron carriers
High-energy electron
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14. Light-Dependent Reactions
Enzymes on the thylakoid membrane break water molecules into:
Photosystem II
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
Electron carriers
High-energy electron
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15. Light-Dependent Reactions
hydrogen ions
oxygen atoms
energized electrons
Photosystem II
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
Electron carriers
High-energy electron
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16. Light-Dependent Reactions
3) The energized electrons from water replace the high-energy electrons that chlorophyll lost to the electron transport chain.
Photosystem II
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
High-energy electron
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17. Light-Dependent Reactions
4) As plants remove electrons from water, oxygen is left behind and is released into the air.
Photosystem II
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. The light-dependent reactions take place within the thylakoid membranes of chloroplasts.
High-energy electron
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18. Light-Dependent Reactions
The hydrogen ions left behind when water is broken apart are released inside the thylakoid membrane.
Photosystem II
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
High-energy electron
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19. Light-Dependent Reactions
Energy from the electrons is used to transport H+ ions from the stroma into the inner thylakoid space.
Photosystem II
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
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20. Light-Dependent Reactions
5) High-energy electrons move through the electron transport chain from photosystem II to photosystem I.
Photosystem II
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
Photosystem I
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21. Light-Dependent Reactions
Pigments in photosystem I use energy from light to re-energize the electrons.
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
Photosystem I
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22. Light-Dependent Reactions
6) NADP+ then picks up these high-energy electrons, along with H+ ions, and becomes NADPH.
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH
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23. Light-Dependent Reactions
As electrons are passed from chlorophyll to NADP+, more H+ ions are pumped across the membrane.
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH
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24. Light-Dependent Reactions
7) Soon, the inside of the membrane fills up with positively charged hydrogen ions, which makes the outside of the membrane negatively charged.
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH
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25. Light-Dependent Reactions
The difference in charges across the membrane provides the energy to make ATP
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH
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26. Light-Dependent Reactions
H+ ions cannot cross the membrane directly.
ATP synthase
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH
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27. Light-Dependent Reactions
8) The cell membrane contains a protein called ATP synthase that allows H+ ions to pass through it
ATP synthase
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH
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28. Light-Dependent Reactions
As H+ ions pass through ATP synthase, the protein rotates.
ATP synthase
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH
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29. Light-Dependent Reactions
9) As it rotates, ATP synthase binds ADP and a phosphate group together to produce ATP.
ATP synthase
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
ADP
2 NADP+ 2 2
NADPH
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30. Light-Dependent Reactions
Because of this system, light-dependent electron transport produces not only high-energy electrons but ATP as well.
ATP synthase
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
ADP
2 NADP+ 2 2
NADPH
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31. Light Reactions Key Events: Absorption of light energy.
Excitation of electrons by solar energy.
Formation of ATP and NADPH via ETC.

32. Dark Rxn Overview Occurs in the stroma of the chloroplast.
Obtains CO2 (Use Carbon)
From surrounding Air
Use ATP (Need Energy)
Use NADPH (Need High Energy Electrons)
Both Obtained from Light Reactions

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The Calvin cycle uses ATP and NADPH from the light-dependent reactions to produce high-energy sugars.
Because the Calvin cycle does not require light, these reactions are also called the light-independent reactions.
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The Calvin Cycle
Six carbon dioxide molecules enter the cycle from the atmosphere and combine with six 5-carbon molecules.
CO2 Enters the Cycle
The Calvin cycle uses ATP and NADPH to produce high-energy sugars.
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The Calvin Cycle
The result is twelve 3-carbon molecules, which are then converted into higher-energy forms.
The Calvin cycle uses ATP and NADPH to produce high-energy sugars.
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The Calvin Cycle
The energy for this conversion comes from ATP and high-energy electrons from NADPH.
Energy Input 12
12 ADP 12
NADPH
The Calvin cycle uses ATP and NADPH to produce high-energy sugars.
12 NADP+
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The Calvin Cycle
Two of twelve 3-carbon molecules are removed from the cycle.
Energy Input 12
12 ADP 12
NADPH
The Calvin cycle uses ATP and NADPH to produce high-energy sugars.
12 NADP+
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38. 6-Carbon sugar produced Sugars and other compounds
The Calvin Cycle
The molecules are used to produce sugars, lipids, amino acids and other compounds. 12
12 ADP 12
NADPH
The Calvin cycle uses ATP and NADPH to produce high-energy sugars.
12 NADP+
6-Carbon sugar produced
Sugars and other compounds
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39. 5-Carbon Molecules Regenerated Sugars and other compounds
The Calvin Cycle
The 10 remaining 3-carbon molecules are converted back into six 5-carbon molecules, which are used to begin the next cycle. 12
12 ADP
6 ADP 12
NADPH 6
The Calvin cycle uses ATP and NADPH to produce high-energy sugars.
12 NADP+
5-Carbon Molecules Regenerated
Sugars and other compounds
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40. The two sets of photosynthetic reactions work together
The light-dependent reactions trap sunlight energy in chemical form.
The light-independent reactions use that chemical energy to produce stable, high-energy sugars from carbon dioxide and water.
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