Copyright Pearson Prentice Hall 8-3 The Reactions of Photosynthesis

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?


Inside a ChloroplastInside a Chloroplast

Chloroplasts contain thylakoids—saclike Chloroplasts contain thylakoids—saclike photosynthetic membranes.photosynthetic membranes.

Chloroplast

Singlethylakoid


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


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


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


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..




Photosystem II


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.



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



Photosystem II

2H2O

Enzymes on the thylakoid membrane break water Enzymes on the thylakoid membrane break water molecules into:molecules into:

Electroncarriers




Photosystem II

2H2O

hydrogen ionshydrogen ions oxygen atomsoxygen atoms energized electronsenergized electrons

+ O2

Electroncarriers




Photosystem II

2H2O

+ O2

3) The energized electrons from water replace the high-energy electrons that chlorophyll lost to the electron transport chain.




Photosystem II

2H2O

4) As plants remove electrons from water, oxygen is left behind and is released into the air.

+ O2




Photosystem II

2H2O

The hydrogen ions left behind when water is broken apart are released inside the thylakoid membrane.

+ O2




Photosystem II

2H2O

Energy from the electrons is used to transport H+ ions from the stroma into the inner thylakoid space.

+ O2



Photosystem II

2H2O

5) High-energy electrons move through the electron transport chain from photosystem II to photosystem I.

+ O2

Photosystem I



2H2O

Pigments in photosystem I use energy from light to re-energize the electrons.

+ O2

Photosystem I



2H2O

6) NADP+ then picks up these high-energy electrons, along with H+ ions, and becomes NADPH.

+ O2

2 NADP+

2 NADPH2



2H2O

As electrons are passed from chlorophyll to NADP+, more H+ ions are pumped across the membrane.

+ O2

2 NADP+

2 NADPH2



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



2H2O

The difference in charges across the membrane provides the energy to make ATP

+ O2

2 NADP+

2 NADPH2



2H2O

H+ ions cannot cross the membrane directly.

+ O2

ATP synthase

2 NADP+

2 NADPH2



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



2H2O

As H+ ions pass through ATP synthase, the protein rotates.

+ O2

ATP synthase

2 NADP+

2 NADPH2



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



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


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.


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



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.



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


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+



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



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


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.

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Copyright Pearson Prentice Hall 8-3 The Reactions of Photosynthesis