Chapter 8: Photosynthesis: Energy from the Sun CHAPTER 8 Photosynthesis: Energy from the Sun

Chapter 8: Photosynthesis: Energy from the Sun

CHAPTER 8Photosynthesis: Energy

from the Sun


Chapter 8: Photosynthesis: Energy from the SunPhotosynthesisPhotosynthesis

Identifying Photosynthetic Reactants andIdentifying Photosynthetic Reactants and Products Products

The Two Pathways of Photosynthesis: AnThe Two Pathways of Photosynthesis: An Overview Overview

Properties of Light and PigmentsProperties of Light and Pigments


Chapter 8: Photosynthesis: Energy from the SunLight Reactions: Light AbsorptionLight Reactions: Light Absorption

Making Sugar from COMaking Sugar from CO22: The Calvin–Benson Cycle: The Calvin–Benson Cycle

Photorespiration and Its Evolutionary CoPhotorespiration and Its Evolutionary Consequencesnsequences

Metabolic Pathways in PlantsMetabolic Pathways in Plants


Photosynthesis

• Life on Earth depends on the Life on Earth depends on the absorption of light energy from the absorption of light energy from the sun.sun.

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Photosynthesis

• In plants, photosynthesis takes place In plants, photosynthesis takes place in chloroplasts.in chloroplasts.

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Identifying Photosynthetic Reactants and Products• Photosynthesizing plants take in COPhotosynthesizing plants take in CO22, ,

water, and light energy, producing Owater, and light energy, producing O22 and carbohydrate. The overall reaction and carbohydrate. The overall reaction isis

6 CO6 CO22 + 12 H + 12 H22O + light O + light C C66HH1212OO66 + 6 O + 6 O22 + + 6 H6 H22OO

The oxygen atoms in OThe oxygen atoms in O22 come from water, come from water, not from COnot from CO22. Review Figures 8.1, 8.2. Review Figures 8.1, 8.2

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Figure 8.1

Figure 8.1Figure 8.1

figure 08-01.jpg


Figure 8.2


figure 08-02.jpg


The Two Pathways of Photosynthesis: An Overview• In the light reactions of In the light reactions of

photosynthesis, electron flow and photosynthesis, electron flow and photophosphorylation produce ATP photophosphorylation produce ATP and reduce NADPand reduce NADP++ to NADPH + H to NADPH + H++. .

Review Figure 8.3Review Figure 8.3

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Figure 8.3


figure 08-03.jpg


The Two Pathways of Photosynthesis: An Overview • ATP and NADPH + HATP and NADPH + H++ are needed for are needed for

the reactions that fix and reduce COthe reactions that fix and reduce CO22 in the Calvin–Benson cycle, forming in the Calvin–Benson cycle, forming sugars. sugars.


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Figure 8.3


figure 08-03.jpg


Properties of Light and Pigments

• Light energy comes in packets called Light energy comes in packets called photons, but it also has wavelike photons, but it also has wavelike properties. properties.


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Figure 8.4


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• Pigments absorb light in the visible Pigments absorb light in the visible spectrum. spectrum.


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Figure 8.5


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• Absorption of a photon puts a pigment Absorption of a photon puts a pigment molecule in an excited state with more molecule in an excited state with more energy than its ground state. energy than its ground state.


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Figure 8.6


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• Each compound has a characteristic Each compound has a characteristic absorption spectrum which reveals the absorption spectrum which reveals the biological effectiveness of different biological effectiveness of different wavelengths of light. wavelengths of light.

Review Figures 8.7, 8.8Review Figures 8.7, 8.8

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Figure 8.7


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Figure 8.8


figure 08-08.jpg



• Chlorophylls and accessory pigments Chlorophylls and accessory pigments form antenna systems for absorption form antenna systems for absorption of light energy. of light energy.

Review Figures 8.7, 8.9, 8.11Review Figures 8.7, 8.9, 8.11

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Figure 8.9


figure 08-09.jpg


Light Reactions: Light Absorption

• An excited pigment molecule may lose An excited pigment molecule may lose its energy by fluorescence, or by its energy by fluorescence, or by transferring it to another pigment transferring it to another pigment molecule. molecule.


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Figure 8.10


figure 08-10.jpg


Figure 8.11


figure 08-11.jpg


Electron Flow, Photophos-phorylation, and Reductions• Noncyclic electron flow uses two photosystems: Noncyclic electron flow uses two photosystems:

• Photosystem II uses PPhotosystem II uses P680680 chlorophyll, from chlorophyll, from which light-excited electrons pass to a redox which light-excited electrons pass to a redox chain that drives chemiosmotic ATP production. chain that drives chemiosmotic ATP production. Light-driven water oxidation releases OLight-driven water oxidation releases O2,2, passing electrons to Ppassing electrons to P680680 chlorophyll. chlorophyll.

• Photosystem I passes electrons from PPhotosystem I passes electrons from P700700 chlorophyll to another redox chain and then to chlorophyll to another redox chain and then to NADPNADP++, forming NADPH + H, forming NADPH + H++. Review Figure . Review Figure 8.128.12

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Figure 8.12 – Part 1

Figure 8.12 – Part 1Figure 8.12 – Part 1

figure 08-12a.jpg




figure 08-12b.jpg


Electron Flow, Photophos-phorylation, and Reductions • Cyclic electron flow uses PCyclic electron flow uses P700700

chlorophyll producing only ATP. chlorophyll producing only ATP.

• Its operation maintains the proper Its operation maintains the proper balance of ATP and NADPH + Hbalance of ATP and NADPH + H++ in the in the chloroplast. chloroplast.


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Figure 8.13


figure 08-13.jpg


Electron Flow, Photophos-phorylation, and Reductions • Chemiosmosis is the source of ATP in Chemiosmosis is the source of ATP in

photophosphorylation. photophosphorylation. • Electron transport pumps protons from Electron transport pumps protons from

stroma into thylakoids, establishing a proton-stroma into thylakoids, establishing a proton-motive force. motive force.

• Proton diffusion to stroma via ATP synthase Proton diffusion to stroma via ATP synthase channels drives ATP formation from ADP and channels drives ATP formation from ADP and PPii. .


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Figure 8.14


figure 08-14.jpg


Electron Flow, Photophos-phorylation, and Reductions • Photosynthesis probably originated in Photosynthesis probably originated in

anaerobic bacteria that used Hanaerobic bacteria that used H22S as a S as a source of electrons instead of Hsource of electrons instead of H22O. O.

• Oxygen production by bacteria was Oxygen production by bacteria was important in eukaryote evolution.important in eukaryote evolution.

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Light-Dependent Reactions


Photosystem II

Photosynthesis begins when Photosynthesis begins when pigments in photosystem II absorb pigments in photosystem II absorb light, increasing their energy level.light, increasing their energy level.


Photosystem II

These high-energy electrons are These high-energy electrons are passed on to the electron transport passed on to the electron transport chain.chain.

Electroncarriers

High-energy electron


Photosystem II

2H2O

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

Electroncarriers



Photosystem II

2H2O

hydrogen ionshydrogen ions

oxygen atomsoxygen atoms

energized electronsenergized electrons

+ O2

Electroncarriers



Photosystem II

2H2O

+ O2

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



Photosystem II

2H2O

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

+ O2



Photosystem II

2H2O

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

+ O2



Photosystem II

2H2O

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

+ O2


Photosystem II

2H2O

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

+ O2

Photosystem I


2H2O

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

+ O2

Photosystem I


2H2O

NADPNADP++ then picks up these high- then picks up these high-energy electrons, along with Henergy electrons, along with H++ ions, and becomes NADPH.ions, and becomes NADPH.

•

+ O2

2 NADP+

2 NADPH2


2H2O

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

+ O2

2 NADP+

2 NADPH2


2H2O

Soon, the inside of the membrane fills up with Soon, the inside of the membrane fills up with positively charged hydrogen ions, which makes positively charged hydrogen ions, which makes the outside of the membrane negatively the outside of the membrane negatively charged.charged.

+ O2

2 NADP+

2 NADPH2


2H2O

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

+ O2

2 NADP+

2 NADPH2


2H2O

HH++ ions cannot cross the membrane ions cannot cross the membrane directly.directly.

+ O2

ATP synthase

2 NADP+

2 NADPH2


2H2O

The cell membrane contains a protein The cell membrane contains a protein called called ATP synthaseATP synthase that allows H that allows H++ ions to pass through itions to pass through it

+ O2

ATP synthase

2 NADP+

2 NADPH2


2H2O

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

+ O2

ATP synthase

2 NADP+

2 NADPH2


2H2O

As it rotates, ATP synthase binds ADP As it rotates, ATP synthase binds ADP and a phosphate group together to and a phosphate group together to produce ATP.produce ATP.

+ O2

2 NADP+

2 NADPH2

ATP synthase

ADP


2H2O

Because of this system, light-dependent Because of this system, light-dependent electron transport produces not only high-electron transport produces not only high-energy electrons but ATP as well.energy electrons but ATP as well.

+ O2

ATP synthase

ADP2 NADP+

2 NADPH2


Making Sugar from CO2: The Calvin–Benson Cycle• The Calvin–Benson cycle makes sugar The Calvin–Benson cycle makes sugar

from COfrom CO22. .

• This pathway was elucidated through This pathway was elucidated through use of radioactive tracers. use of radioactive tracers. Review Figure 8.15 Review Figure 8.15

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Figure 8.15


figure 08-15.jpg


Making Sugar from CO2: The Calvin–Benson Cycle • The Calvin–Benson cycle has three The Calvin–Benson cycle has three

phases: fixation of COphases: fixation of CO22, reduction and , reduction and carbohydrate production, and carbohydrate production, and regeneration of RuBP. regeneration of RuBP.

• RuBP is the initial CORuBP is the initial CO22 acceptor, 3PG is acceptor, 3PG is the first stable product of COthe first stable product of CO22 fixation. fixation.

• Rubisco catalyzes the reaction of CORubisco catalyzes the reaction of CO22 and RuBP to form 3PG. and RuBP to form 3PG.


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Figure 8.16


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Figure 8.17


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Photorespiration and Its Evolutionary Consequences• Rubisco catalyzes a reaction between Rubisco catalyzes a reaction between

OO22 and RuBP in addition to that of CO and RuBP in addition to that of CO2 2

and RuBP. and RuBP. • Photorespiration significantly reduces Photorespiration significantly reduces

photosynthesis efficiency. photosynthesis efficiency. • Reactions that constitute Reactions that constitute

photorespiration are distributed over photorespiration are distributed over chloroplast, peroxisome, and chloroplast, peroxisome, and mitochondria organelles.mitochondria organelles.

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Photorespiration and Its Evolutionary Consequences • At high temperatures and low COAt high temperatures and low CO22

concentrations, the oxygenase concentrations, the oxygenase function of rubisco is favored.function of rubisco is favored.

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Photorespiration and Its Evolutionary Consequences • CC44 plants bypass photorespiration. plants bypass photorespiration.

• PEP carboxylase in mesophyll PEP carboxylase in mesophyll chloroplasts initially fixes COchloroplasts initially fixes CO22 in four- in four-carbon acids, which diffuse into bundle carbon acids, which diffuse into bundle sheath cells, where their sheath cells, where their decarboxylation produces locally high decarboxylation produces locally high concentrations of COconcentrations of CO22. .

Review Figures 8.19Review Figures 8.19

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Figure 8.19


figure 08-19.jpg


Photorespiration and Its Evolutionary Consequences • CAM plants operate much like CCAM plants operate much like C44

plants, but their initial COplants, but their initial CO22 fixation by fixation by PEP carboxylase is temporally PEP carboxylase is temporally separated from the Calvin–Benson separated from the Calvin–Benson cycle, rather than spatially separated. cycle, rather than spatially separated.


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Figure 8.21


figure 08-21.jpg


Metabolic Pathways in Plants• Plants respire in light and darkness, Plants respire in light and darkness,

but photosynthesize only in light. but photosynthesize only in light.

• A plant must photosynthesize more A plant must photosynthesize more than it respires, giving it a net gain of than it respires, giving it a net gain of reduced energy-rich compounds.reduced energy-rich compounds.

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Metabolic Pathways in Plants • Photosynthesis and respiration are Photosynthesis and respiration are

linked through the Calvin–Benson linked through the Calvin–Benson cycle, the citric acid cycle, and cycle, the citric acid cycle, and glycolysis. glycolysis.


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figure 08-22a.jpg




figure 08-22b.jpg

Documents

Chapter 8: Photosynthesis: Energy from the Sun CHAPTER 8 Photosynthesis: Energy from the Sun