Life, 6th Edition

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

• Photosynthesis
• Identifying Photosynthetic Reactants and Products
• The Two Pathways of Photosynthesis An Overview
• Properties of Light and Pigments

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Chapter 8 Photosynthesis Energy from the Sun

• Light Reactions Light Absorption
• Making Sugar from CO2 The CalvinBenson Cycle
• Photorespiration and Its Evolutionary
  Consequences
• Metabolic Pathways in Plants

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Photosynthesis

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

• In plants, photosynthesis takes place in
  chloroplasts.
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Identifying Photosynthetic Reactants and Products

• Photosynthesizing plants take in CO2, water, and
  light energy, producing O2 and carbohydrate. The
  overall reaction is
• 6 CO2 12 H2O light ? C6H12O6 6 O2 6 H2O
• The oxygen atoms in O2 come from water, not from
  CO2. Review Figures 8.1, 8.2
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Figure 8.1
figure 08-01.jpg

• Figure 8.1

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Figure 8.2
figure 08-02.jpg

• Figure 8.2

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The Two Pathways of Photosynthesis An Overview

• In the light reactions of photosynthesis,
  electron flow and photophosphorylation produce
  ATP and reduce NADP to NADPH H.
• Review Figure 8.3
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Figure 8.3
figure 08-03.jpg

• Figure 8.3

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The Two Pathways of Photosynthesis An Overview

• ATP and NADPH H are needed for the reactions
  that fix and reduce CO2 in the CalvinBenson
  cycle, forming sugars.
• Review Figure 8.3
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Figure 8.3
figure 08-03.jpg

• Figure 8.3

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Properties of Light and Pigments

• Light energy comes in packets called photons, but
  it also has wavelike properties.
• Review Figure 8.4
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Figure 8.4
figure 08-04.jpg

• Figure 8.4

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Properties of Light and Pigments

• Pigments absorb light in the visible spectrum.
• Review Figure 8.5
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Figure 8.5
figure 08-05.jpg

• Figure 8.5

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Properties of Light and Pigments

• Absorption of a photon puts a pigment molecule in
  an excited state with more energy than its ground
  state.
• Review Figure 8.6
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Figure 8.6
figure 08-06.jpg

• Figure 8.6

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Properties of Light and Pigments

• Each compound has a characteristic absorption
  spectrum which reveals the biological
  effectiveness of different wavelengths of light.
• Review Figures 8.7, 8.8
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Figure 8.7
figure 08-07.jpg

• Figure 8.7

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Figure 8.8
figure 08-08.jpg

• Figure 8.8

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Properties of Light and Pigments

• Chlorophylls and accessory pigments form antenna
  systems for absorption of light energy.
• Review Figures 8.7, 8.9, 8.11
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Figure 8.9
figure 08-09.jpg

• Figure 8.9

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Light Reactions Light Absorption

• An excited pigment molecule may lose its energy
  by fluorescence, or by transferring it to another
  pigment molecule.
• Review Figures 8.10, 8.11
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Figure 8.10
figure 08-10.jpg

• Figure 8.10

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Figure 8.11
figure 08-11.jpg

• Figure 8.11

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Electron Flow, Photophos-phorylation, and
Reductions

• Noncyclic electron flow uses two photosystems
• Photosystem II uses P680 chlorophyll, from which
  light-excited electrons pass to a redox chain
  that drives chemiosmotic ATP production.
  Light-driven water oxidation releases O2, passing
  electrons to P680 chlorophyll.
• Photosystem I passes electrons from P700
  chlorophyll to another redox chain and then to
  NADP, forming NADPH H. Review Figure 8.12
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Figure 8.12 Part 1
figure 08-12a.jpg

• Figure 8.12 Part 1

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Figure 8.12 Part 2
figure 08-12b.jpg

• Figure 8.12 Part 2

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Electron Flow, Photophos-phorylation, and
Reductions

• Cyclic electron flow uses P700 chlorophyll
  producing only ATP.
• Its operation maintains the proper balance of ATP
  and NADPH H in the chloroplast.
• Review Figure 8.13
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Figure 8.13
figure 08-13.jpg

• Figure 8.13

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Electron Flow, Photophos-phorylation, and
Reductions

• Chemiosmosis is the source of ATP in
  photophosphorylation.
• Electron transport pumps protons from stroma into
  thylakoids, establishing a proton-motive force.
• Proton diffusion to stroma via ATP synthase
  channels drives ATP formation from ADP and Pi.
• Review Figure 8.14
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Figure 8.14
figure 08-14.jpg

• Figure 8.14

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Electron Flow, Photophos-phorylation, and
Reductions

• Photosynthesis probably originated in anaerobic
  bacteria that used H2S as a source of electrons
  instead of H2O.
• Oxygen production by bacteria was important in
  eukaryote evolution.
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Light-Dependent Reactions
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• Photosynthesis begins when pigments in
  photosystem II absorb light, increasing their
  energy level.

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

Photosystem II
Electroncarriers
High-energy electron
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• Enzymes on the thylakoid membrane break water
  molecules into

Photosystem II
2H2O
Electroncarriers
High-energy electron
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• hydrogen ions
• oxygen atoms
• energized electrons

Photosystem II
O2
2H2O
Electroncarriers
High-energy electron
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• The energized electrons from water replace the
  high-energy electrons that chlorophyll lost to
  the electron transport chain.

Photosystem II
O2
2H2O
High-energy electron
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• As plants remove electrons from water, oxygen is
  left behind and is released into the air.

Photosystem II
O2
2H2O
High-energy electron
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• The hydrogen ions left behind when water is
  broken apart are released inside the thylakoid
  membrane.

Photosystem II
O2
2H2O
High-energy electron
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• Energy from the electrons is used to transport H
  ions from the stroma into the inner thylakoid
  space.

Photosystem II
O2
2H2O
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• High-energy electrons move through the electron
  transport chain from photosystem II to
  photosystem I.

Photosystem II
O2
2H2O
Photosystem I
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• Pigments in photosystem I use energy from light
  to re-energize the electrons.

O2
2H2O
Photosystem I
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• NADP then picks up these high-energy electrons,
  along with H ions, and becomes NADPH.

O2
2H2O
2 NADP
2
NADPH
2
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• As electrons are passed from chlorophyll to
  NADP, more H ions are pumped across the
  membrane.

O2
2H2O
2 NADP
2
NADPH
2
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• Soon, the inside of the membrane fills up with
  positively charged hydrogen ions, which makes the
  outside of the membrane negatively charged.

O2
2H2O
2 NADP
2
NADPH
2
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• The difference in charges across the membrane
  provides the energy to make ATP

O2
2H2O
2 NADP
2
NADPH
2
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• H ions cannot cross the membrane directly.

ATP synthase
O2
2H2O
2 NADP
2
NADPH
2
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• The cell membrane contains a protein called ATP
  synthase that allows H ions to pass through it

ATP synthase
O2
2H2O
2 NADP
2
NADPH
2
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• As H ions pass through ATP synthase, the protein
  rotates.

ATP synthase
O2
2H2O
2 NADP
2
NADPH
2
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• As it rotates, ATP synthase binds ADP and a
  phosphate group together to produce ATP.

ATP synthase
O2
2H2O
ADP
2 NADP
2
NADPH
2
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• Because of this system, light-dependent electron
  transport produces not only high-energy electrons
  but ATP as well.

ATP synthase
O2
2H2O
ADP
2 NADP
2
NADPH
2
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Making Sugar from CO2 The CalvinBenson Cycle

• The CalvinBenson cycle makes sugar from CO2.
• This pathway was elucidated through use of
  radioactive tracers.
  
  Review Figure 8.15
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Figure 8.15
figure 08-15.jpg

• Figure 8.15

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Making Sugar from CO2 The CalvinBenson Cycle

• The CalvinBenson cycle has three phases
  fixation of CO2, reduction and carbohydrate
  production, and regeneration of RuBP.
• RuBP is the initial CO2 acceptor, 3PG is the
  first stable product of CO2 fixation.
• Rubisco catalyzes the reaction of CO2 and RuBP to
  form 3PG.
• Review Figures 8.16, 8.17
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Figure 8.16
figure 08-16.jpg

• Figure 8.16

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Figure 8.17
figure 08-17.jpg

• Figure 8.17

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Photorespiration and Its Evolutionary Consequences

• Rubisco catalyzes a reaction between O2 and RuBP
  in addition to that of CO2 and RuBP.
• Photorespiration significantly reduces
  photosynthesis efficiency.
• Reactions that constitute photorespiration are
  distributed over chloroplast, peroxisome, and
  mitochondria organelles.
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Photorespiration and Its Evolutionary
Consequences

• At high temperatures and low CO2 concentrations,
  the oxygenase function of rubisco is favored.
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Photorespiration and Its Evolutionary
Consequences

• C4 plants bypass photorespiration.
• PEP carboxylase in mesophyll chloroplasts
  initially fixes CO2 in four-carbon acids, which
  diffuse into bundle sheath cells, where their
  decarboxylation produces locally high
  concentrations of CO2.
• Review Figures 8.19
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Figure 8.19
figure 08-19.jpg

• Figure 8.19

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Photorespiration and Its Evolutionary
Consequences

• CAM plants operate much like C4 plants, but their
  initial CO2 fixation by PEP carboxylase is
  temporally separated from the CalvinBenson
  cycle, rather than spatially separated.
• Review Figure 8.21
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Figure 8.21
figure 08-21.jpg

• Figure 8.21

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Metabolic Pathways in Plants

• Plants respire in light and darkness, but
  photosynthesize only in light.
• A plant must photosynthesize more than it
  respires, giving it a net gain of reduced
  energy-rich compounds.
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Metabolic Pathways in Plants

• Photosynthesis and respiration are linked through
  the CalvinBenson cycle, the citric acid cycle,
  and glycolysis.
• Review Figure 8.22
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Figure 8.22 Part 1
figure 08-22a.jpg

• Figure 8.22 Part 1

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Figure 8.22 Part 2
figure 08-22b.jpg

• Figure 8.22 Part 2