Photosynthesis:%20Energy%20from%20Sunlight

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Photosynthesis Energy from Sunlight
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10 Photosynthesis Energy from Sunlight

• 10.1 What Is Photosynthesis?
• 10.2 How Does Photosynthesis Convert Light Energy
  into Chemical Energy?
• 10.3 How Is Chemical Energy Used to Synthesize
  Carbohydrates?
• 10.4 How Have Plants Adapted Photosynthesis to
  Environmental Conditions?
• 10.5 How Does Photosynthesis Interact with Other
  Pathways?

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10 Photosynthesis Energy from Sunlight
To predict how plants will respond to rising CO2
levels, biologists have performed large-scale
experiments. Photosynthesis rates increase as
atmospheric CO2 concentration increases.
Opening Question What possible effects will
increased atmospheric CO2 have on global food
production?
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10.1 What Is Photosynthesis?

• Photosynthesis synthesis from light
• Energy from sunlight is captured and used to
  convert CO2 to more complex carbon compounds.

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Figure 10.1 The Ingredients for Photosynthesis
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10.1 What Is Photosynthesis?

• Using stable 18O isotopes, Ruben and Kamen
  determined that water is the source of O2
  released during photosynthesis

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Figure 10.2 The Source of the Oxygen Produced by
Photosynthesis
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Working with Data 10.1 Water Is the Source of
the Oxygen Produced by Photosynthesis

• The stable isotope 18O was used to confirm the
  hypothesis that O2 generated during
  photosynthesis came from water.
• Algal cells were exposed to water, and CO2
  generated from K2CO3 and KHCO3.

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Working with Data 10.1 Water Is the Source of
the Oxygen Produced by Photosynthesis

• Experiment 1 water contained more 18O than 16O.
• Experiment 2 the CO2 contained more 18O than
  16O.
• A mass spectrometer measured the isotopic content
  of reactants and the O2 produced.

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Working with Data 10.1 Water Is the Source of
the Oxygen Produced by Photosynthesis

• Question 1
• In Experiment 1, was the isotopic ratio of O2
  similar to that of H2O or to that of CO2?
• What about in Experiment 2?

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Working with Data 10.1, Table 1
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Working with Data 10.1 Water Is the Source of
the Oxygen Produced by Photosynthesis

• Question 2
• What can you conclude from these data?

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10.1 What Is Photosynthesis?

• Photosynthesis is an oxidationreduction process.
• Oxygen atoms in H2O are in a reduced state they
  are oxidized to O2.
• Carbon atoms are in the oxidized state in CO2
  they are reduced to a carbohydrate.

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10.1 What Is Photosynthesis?

• Water is the donor of protons and electrons in
  oxygenic photosynthesis.
• In anoxygenic photosynthesis, other molecules
  donate the protons and electrons.
• Example purple sulfur bacteria use H2S.

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10.1 What Is Photosynthesis?

• Two pathways occur in different parts of the
  chloroplast
• Light reactions Convert light energy to chemical
  energy as ATP and NADPH.
• Light-independent reactions Use ATP and NADPH
  (from the light reactions) plus CO2 to produce
  carbohydrates.

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Figure 10.3 An Overview of Photosynthesis
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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• Light is a form of energyelectromagnetic
  radiation.
• It is propagated as wavesthe amount of energy is
  inversely proportional to its wavelength.
• Light also behaves as particles, called photons.

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Figure 10.4 The Electromagnetic Spectrum
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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• Certain molecules absorb photons of specific
  wavelengths.
• When a photon hits a molecule, it can
• Bounce offscattered or reflected
• Pass throughtransmitted
• Be absorbed, adding energy to the molecule
  (excited state)

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In-Text Art, Ch. 10, p. 189
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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• The absorbed energy boosts an electron in the
  molecule into a shell farther from the nucleus.
• This electron is held less firmly making the
  molecule more unstable and reactive.

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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• Molecules that absorb specific wavelengths in the
  visible range are called pigments.
• Other wavelengths are scattered or transmitted,
  which imparts the colors that we see.
• Chlorophyll absorbs blue and red light and
  scatters green.

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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• Absorption spectrum plot of wavelengths absorbed
  by a pigment.
• Action spectrum plot of photosynthesis against
  wavelengths of light to which it is exposed.
• The rate of photosynthesis can be measured by the
  amount of O2 released.

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Figure 10.5 Absorption and Action Spectra
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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• The major pigment in photosynthesis is
  chlorophyll a.
• It has a hydrocarbon tail that anchors it in a
  protein complex in the thylakoid membrane called
  a photosystem.

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Figure 10.6 The Molecular Structure of
Chlorophyll a
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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• Chlorophyll a and accessory pigments
  (chlorophylls b and c, carotenoids, phycobilins)
  are arranged in light-harvesting complexes, or
  antenna systems.
• Several complexes surround a reaction center in
  the photosystem.

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Figure 10.7 Energy Transfer and Electron
Transport (Part 2)
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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• Light energy is captured in the light harvesting
  complexes and transferred to the reaction
  centers.
• Accessory pigments absorb light in other
  wavelengths, increasing the range of light that
  can be used.
• Types of accessory pigments characterize
  different groups.

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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• When a pigment molecule absorbs a photon, the
  excited state is unstable and the energy is
  quickly released.
• The energy is absorbed by other pigment molecules
  and passed to chlorophyll a in a reaction center.

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Figure 10.8 Noncyclic Electron Transport Uses
Two Photosystems
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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• The excited chlorophyll a molecule (Chl) gives
  up an electron to an acceptor.
• Chl acceptor ? Chl acceptor
• A redox reaction The chlorophyll gets oxidized
  the acceptor molecule is reduced.

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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• The electron acceptor is the first in a chain of
  carriers in the thylakoid membrane.
• The final electron acceptor is NADP, which gets
  reduced
• NADP H 2e ? NADPH

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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• Noncyclic electron transport uses two
  photosystems
• Photosystem I has P700 chlorophyll absorbs best
  at 700 nm.
• Photosystem II has P680 chlorophyll absorbs best
  at 680 nm.

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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• Photosystem II
• When excited chlorophyll (Chl) gives up its
  electron, it is unstable, and grabs another
  electron (it is a strong oxidizer).
• The electron comes from water
• 2Chl H2O ? 2Chl 2H ½O2

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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• The energetic electrons are passed through a
  series of membrane-bound carriers to a final
  acceptor at a lower energy level.
• A proton gradient is generated and is used by ATP
  synthase to make ATP.

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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• Photosystem I
• An excited electron from the Chl reduces an
  acceptor.
• The oxidized Chl takes an electron from the last
  carrier in photosystem II.
• The energetic electron is passed through several
  carriers and reduces NADP to NADPH.

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Figure 10.8 Noncyclic Electron Transport Uses
Two Photosystems
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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• Cyclic electron transport
• Uses photosystem I and electron transport to
  produce ATP instead of NADPH.
• Cyclic the electron from the excited chlorophyll
  passes back to the same chlorophyll.

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Figure 10.9 Cyclic Electron Transport Traps
Light Energy as ATP
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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• ATP is formed by photophosphorylation, a
  chemiosmotic mechanism.
• H is transported across the thylakoid membrane
  into the lumen, creating an electrochemical
  gradient.

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Figure 10.10 Photophosphorylation
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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• Water oxidation creates more H in the thylakoid
  lumen and NADP reduction removes H in the
  stroma.
• Both reactions contribute to the H gradient.

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10.2 How Does Photosynthesis Convert Light Energy
into Chemical Energy?

• High concentration of H in the lumen drives
  movement of H back into the stroma through
  protein channels.
• The channels are ATP synthases that couple
  movement of protons with formation of ATP.

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10.3 How Is Chemical Energy Used to Synthesize
Carbohydrates?

• CO2 fixation CO2 is reduced to carbohydrates.
• Occurs in the stroma.
• Energy in ATP and NADPH is used to reduce CO2.

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10.3 How Is Chemical Energy Used to Synthesize
Carbohydrates?

• Calvin and Benson used the 14C radioisotope to
  determine the sequence of reactions in CO2
  fixation.
• They exposed Chlorella to 14CO2, then extracted
  the organic compounds and separated them by paper
  chromatography.

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Figure 10.11 Tracing the Pathway of CO2 (Part 1)
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Figure 10.11 Tracing the Pathway of CO2 (Part 2)
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10.3 How Is Chemical Energy Used to Synthesize
Carbohydrates?

• The first compound to be formed is
  3-phosphoglycerate, 3PG, a 3-carbon sugar
  phosphate.

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10.3 How Is Chemical Energy Used to Synthesize
Carbohydrates?

• The pathway of CO2 fixation is cyclic the Calvin
  cycle.
• CO2 first binds to 5-C RuBP the 6-C compound
  immediately breaks down into two molecules of
  3PG.
• The enzyme rubisco (ribulose bisphoshate
  carboxylase/oxygenase) is the most abundant
  protein in the world.

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Figure 10.12 RuBP Is the Carbon Dioxide Acceptor
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Working with Data 10.2 Tracing the Pathway of CO2

• In experiments to determine the reactions of
  photosynthesis, the green alga Chlorella was
  exposed to CO2 made with the radioactive 14C
  isotope.
• Cells were exposed for various periods of time.

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Working with Data 10.2 Tracing the Pathway of CO2

• The first reaction in CO2 fixation can occur in
  the dark.
• Cells were exposed to 20 minutes of light
  followed by various lengths of darkness.

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Working with Data 10.2, Table 1
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Working with Data 10.2 Tracing the Pathway of CO2

• Question 1
• Using the data in the table, plot radioactivity
  in 3PG versus time.
• What do the data show?

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Working with Data 10.2 Tracing the Pathway of CO2

• Question 2
• Why did the amount of radioactively labeled RuBP
  go down after 30 seconds in the dark?

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10.3 How Is Chemical Energy Used to Synthesize
Carbohydrates?

• The Calvin cycle
• Fixation of CO2 to 3PG
• Reduction of 3PG to G3P
• Regeneration of RuBP, the CO2 acceptor.
• For every turn of the cycle, one CO2 is fixed and
  one RuBP is regenerated.

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Figure 10.13 The Calvin Cycle
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10.3 How Is Chemical Energy Used to Synthesize
Carbohydrates?

• Glyceraldehyde 3-phosphate (G3P) is the product
  of the Calvin cycle.

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10.3 How Is Chemical Energy Used to Synthesize
Carbohydrates?

• Some G3P is exported to the cytosol and converted
  to hexoses (glucose and fructose) used in
  respiration.
• Hexoses may be converted to sucrose and
  transported to other parts of the plant and used
  for energy or to build other molecules.

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10.3 How Is Chemical Energy Used to Synthesize
Carbohydrates?

• Some G3P is used to synthesize glucose and starch
  within the chloroplast.
• The stored starch is used at night so that
  photosynthetic tissues can continue to export
  sucrose to the rest of the plant.

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10.3 How Is Chemical Energy Used to Synthesize
Carbohydrates?

• Products of the Calvin cycle are crucial to the
  entire biosphere.
• The covalent bonds generated by the cycle provide
  almost all of the energy for life.

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10.3 How Is Chemical Energy Used to Synthesize
Carbohydrates?

• Photosynthetic organisms (autotrophs) use this
  energy for growth, reproduction, and development.
• Heterotrophs cannot photosynthesize and depend on
  autotrophs for both energy and raw materials.

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10.3 How Is Chemical Energy Used to Synthesize
Carbohydrates?

• The Calvin cycle is stimulated by light
• Protons pumped from stroma into thylakoids
  increase the pH, which favors activation of
  rubisco.
• Electron transport reduces disulfide bonds in
  Calvin cycle enzymes to activate them.

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Figure 10.14 The Photochemical Reactions
Stimulate the Calvin Cycle
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10.4 How Have Plants Adapted Photosynthesis to
Environmental Conditions?

• Rubisco is an oxygenase as well as a carboxylase.
• It can add O2 to RuBP instead of CO2, reducing
  the amount of CO2 fixed.
• Its affinity for CO2 is about 10 times higher,
  thus carboxylation is usually favored.

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10.4 How Have Plants Adapted Photosynthesis to
Environmental Conditions?

• But if O2 concentration in the leaf is high, O2
  combines with RuBP, resulting in
  photorespiration
• RuBP O2 ? phosphoglycolate 3PG
• Phosphoglycolate (2 carbons) does not enter the
  Calvin cycle, but another metabolic pathway
  converts it to 3PG.

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10.4 How Have Plants Adapted Photosynthesis to
Environmental Conditions?

• 2 phosphoglycolate O2 ? 3PG CO2
• 75 of the carbon from phosphoglycolate are
  recovered for the Calvin cycle photorespiration
  reduces CO2 fixation by 25.
• Photorespiration consumes O2 and releases CO2.
  Occurs only in the light.

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Figure 10.15 Organelles of Photorespiration
(Part 1)
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Figure 10.15 Organelles of Photorespiration
(Part 2)
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10.4 How Have Plants Adapted Photosynthesis to
Environmental Conditions?

• Photorespiration is more likely at high
  temperatures.
• On hot, dry days stomata (leaf pores) are closed
  to prevent water loss. CO2 concentration falls as
  it is used in photosynthesis, and thus O2
  concentration increases photorespiration occurs.

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10.4 How Have Plants Adapted Photosynthesis to
Environmental Conditions?

• Plants differ in how they fix CO2.
• C3 plants first product of CO2 fixation is 3PG.
  Cells in the leaf mesophyll have abundant
  rubisco.
• On hot days, plants close stomata to conserve
  water, which limits entry of CO2 and
  photorespiration occurs.

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10.4 How Have Plants Adapted Photosynthesis to
Environmental Conditions?

• In C4 plants, oxaloacetate (4 carbons) is the
  first product of CO2 fixation.
• They have a mechanism to increase CO2 near
  rubisco and isolate it from O2.
• CO2 is fixed in mesophyll cells by PEP
  carboxylase into a 3-carbon compound,
  phosphoenolpyruvate (PEP), then to oxaloacetate.
• PEP carboxylase has no oxygenase activity and
  fixes CO2 even when levels are low.

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Figure 10.16 Leaf Anatomy of C3 and C4 Plants
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10.4 How Have Plants Adapted Photosynthesis to
Environmental Conditions?

• Oxaloacetate is converted to malate it diffuses
  to bundle sheath cells, which have modified
  chloroplasts that concentrate CO2 around rubisco.
• Malate is decarboxylated to pyruvate and CO2.
  Pyruvate moves back to mesophyll cells to
  regenerate PEP, which requires ATP.
• The CO2 enters the Calvin cycle.

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Figure 10.17 The Anatomy and Biochemistry of C4
Carbon Fixation (Part 1)
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Figure 10.17 The Anatomy and Biochemistry of C4
Carbon Fixation (Part 2)
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10.4 How Have Plants Adapted Photosynthesis to
Environmental Conditions?

• C4 plants must use some energy to pump up CO2
  concentration in bundle sheath cells.
• In cool, cloudy conditions, C3 plants have an
  advantage, but in warmer, dryer climates, C4
  plants have the advantage, since photorespiration
  does not occur.

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10.4 How Have Plants Adapted Photosynthesis to
Environmental Conditions?

• C3 plants are more efficient but C4 plants may
  have evolved in response to declining CO2 levels
  12 mya.
• Atmospheric CO2 levels have been increasing over
  the last 200 years. Further increases may give C3
  plants an advantage if CO2 becomes high enough to
  prevent photorespiration.

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10.4 How Have Plants Adapted Photosynthesis to
Environmental Conditions?

• Some plants have crassulacean acid metabolism
  (CAM).
• CO2 is initially fixed into a 4-C molecule by PEP
  carboxylase, but fixation and the Calvin cycle
  are separated in time, not space.
• Night CO2 fixed by PEP carboxylase stomata are
  open, but less water loss occurs. Malate is
  stored.

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10.4 How Have Plants Adapted Photosynthesis to
Environmental Conditions?

• Day Stomata close to conserve water malate
  moves to chloroplasts and is decarboxylated.
• This supplies CO2 for the Calvin cycle, and
  light reactions provide ATP and NADPH.
• CAM plants include water-storing plants
  (succulents) of the family Crassulaceae, many
  cacti, pineapples, and others.

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Table 10.1
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10.5 How Does Photosynthesis Interact with Other
Pathways?

• Green plants can synthesize all the molecules
  they need from simple starting materials CO2,
  H2O, phosphate, sulfate, ammonium ions, and other
  minerals.
• They use the carbohydrates produced in
  photosynthesis to produce energy by respiration
  and (rarely) fermentation. Respiration occurs in
  both the light and dark.

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10.5 How Does Photosynthesis Interact with Other
Pathways?

• Photosynthesis and respiration are closely linked
  through the Calvin cycle.
• Partitioning of G3P is important
• Some goes to the cytosol and enters glycolysis
  and cellular respiration or is used to make other
  compounds.
• Some enters gluconeogenesis sugars are formed
  and transported to other parts of the plant.

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Figure 10.18 Metabolic Interactions in a Plant
Cell (Part 1)
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Figure 10.18 Metabolic Interactions in a Plant
Cell (Part 2)
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10.5 How Does Photosynthesis Interact with Other
Pathways?

• Only 5 of total sunlight energy is transformed
  to the energy of chemical bonds.
• Understanding the inefficiencies of
  photosynthesis may be important as climate change
  drives changes in photosynthetic activity of
  plants.

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Figure 10.19 Energy Losses in Photosynthesis
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10 Answer to Opening Question

• Higher CO2 concentration generally leads to
  increased photosynthesis, especially in C3
  plants.
• C3 crops such as wheat and rice may grow more,
  but the parts that we eat (seeds) may not grow
  more.

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10 Answer to Opening Question

• Increased plant growth may be countered by the
  effects of CO2 on climateincreased temperatures
  and changing rainfall patterns.
• Biologists estimate that increased CO2 will
  result in moderately increased food production.