OBJECTIVES

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OBJECTIVES  

•  The Process That Feeds the Biosphere
• Distinguish between autotrophic and heterotrophic
  nutrition.
• Distinguish between photoautotrophs and
  chemoautotrophs.
• Describe the structure of a chloroplast, listing
  all membranes and compartments.    

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• The Pathways of Photosynthesis
• Write a summary equation for photosynthesis.
• In general terms, explain the role of redox
  reactions in photosynthesis.
• Describe the two main stages of photosynthesis in
  general terms.
• Explain how carotenoids protect the cell from
  damage by light.
• List the wavelengths of light that are most
  effective for photosynthesis.
• Explain what happens when a solution of
  chlorophyll a absorbs photons. Explain what
  happens when chlorophyll a in an intact
  chloroplast absorbs photons.

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• List the components of a photosystem and explain
  the function of each component.
• Trace the movement of electrons in noncyclic
  electron flow. Trace the movement of electrons in
  cyclic electron flow. Explain the functions of
  cyclic and noncyclic electron flow.
• Describe the similarities and differences in
  chemiosmosis between oxidative phosphorylation in
  mitochondria and photophosphorylation in
  chloroplasts.
• State the function of each of the three phases of
  the Calvin cycle.
• Describe the role of ATP and NADPH in the Calvin
  cycle.
• Describe what happens to rubisco when O2
  concentration is much higher than CO2
  concentration.
• Describe the major consequences of
  photorespiration. Explain why it is thought to be
  an evolutionary relict.
• Describe two important photosynthetic adaptations
  that minimize photorespiration.
• List the possible fates of photosynthetic
  products.

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Chapter 10Photosynthesis
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• Simplified Equation
• 6CO2 6H2O ? C6H12O6 6O2

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Definitions

• Autotrophic Nutrition Nutritional mode of
  synthesizing organic molecules from inorganic raw
  materials

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• Photoautotroph Autotrophic organisms that use
  light as an energy source to synthesize organic
  molecules plants, algae, some prokaryotes
• Carbon source is CO2
• Examples Cyanobacteria, plants, bacteria, algae

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• Chemoautotroph Autotrophic organisms that use
  the oxidation of inorganic substances (sulfur,
  ammonia) as an energy source to synthesize
  organic molecules - bacteria

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• Heterotrophic Nutrition Nutritional mode of
  acquiring organic molecules from compounds
  produced by other organisms consumers
  (eat/decompose)

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Chloroplast

• Leaf Cross Section
• Chlorophyll is the green pigment in chloroplasts
  that absorb light used to drive photosynthesis
• Chloroplasts found primarily in mesophyll cells
• CO2 enters and O2 exits the leaf through stomata
• Water absorbed by the roots is transported to
  leaves through veins, which also export sugar
  from leaves to nonphotosynthetic part of plant

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B. Structure

• Intermembrane Space Chloroplast is bound by a
  double membrane which partitions its contents
  from the cytosol. A narrow intermembrane space
  separates the two membranes

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• Thylakoids Flattened membranous sacs inside the
  chloroplast
• Chlorophyll is found in the thylakoid membranes
• Function in the steps of photosynthesis that
  initially convert light to chemical energy
• Thylakoid Space Space inside the thylakoid
• Grana Stacks of thylakoids in a chloroplast
• Stroma Viscous fluid outside the thylakoids in
  which reactions occur which use chemical energy
  to convert CO2 to sugar

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Photosynthesis

• The Nature of Sunlight
• Wavelike properties of light
• Electromagnetic Energy waves that are
  disturbances of electric and magnetic fields
• Wavelength is the distance between 2 crests
• Visible light 380-750nm
• Particle like properties
• Discrete particles photons
• Photo has fixed energy inversely proportional to
  wavelength

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A. Pigments Substances that absorb visible light

• Different pigments absorb different wavelengths
• Color of a substance is due to reflected or
  transmitted wavelengths

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• Chlorophyll a A light absorbing pigment that
  participates in light reactions absorbs red and
  blue wavelengths
• Accessory pigments can absorb light and transfer
  energy to chlorophyll a broaden absorption
  spectrum
• Chlorophyll b
• Carotenoids

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C. Photoexcitation of Chlorophyll

• Absorbed photons boosts one of the molecules
  electrons in its ground state to its excited
  state
• Only photons absorbed by a molecule are those
  with an energy state equal to the difference in
  energy between ground and excited states.

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D. Photosystems

• Antennae Complex several hundred chlorophyll a,
  b and carotenoid molecules absorb photons and
  pass energy to specialized chlorophyll a molecule
• Reaction Center Chlorophyll specialized
  chlorophyll a molecule located in the reaction
  center that can transfer an excited electron to
  initiate the light reaction
• Primary Electron Acceptor Traps electrons
  released from reaction center chlorophyll

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• Photosystem I Reaction center chlorophyll P700
• Photosystem II Reaction center chlorophyll P680
  same chlorophyll molecules, associated with
  different proteins

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Two Phases of Photosynthesis
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• Light Reactions Convert light energy to
  chemical bond energy ATP and NADPH. Occur in
  thylakoid membranes

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a. Noncyclic Electron Flow
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b. Cyclic Electron Flow- involves only PSI and
generates ATP

• Excited electrons leave from chlorophyll a and
  return to reaction center
• Excited electrons ? primary electron acceptor ?
  ferredoxin (Fd) ? ETP to P700
• Flow of electrons ? production of ATP by cyclic
  phosphorylation
• Necessary to make additional ATP demands for
  Calvin cycle

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2. Dark Reactions

• Calvin cycle uses ATP and NADPH to convert CO2 to
  sugar

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1. Phase 1 Carbon fixation

• Each molecule of CO2 is attached to ribulose
  bisphosphate (RuBP)
• Catalyzed by rubisco
• Unstable 6-carbiin intermediate is split into
  molecules of 3-phosphoglycerate
• Uses 6 ATP

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2. Phase 2 Reduction

• ATP phosphorylates 3-phosphoglycerate ? 1,3
  bisphosphoglycerate
• HADPH reduces 3-phosphoglycerate ?
  glyceraldehydes 3-phosphate (G3P)
• For every 3 CO2 molecules that enters the Calvin
  cycle, 6 G3P molecules produced (1 counted as a
  net gain, must regenerate 3RuBP)

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3. Regeneration of starting material (RuBP)

• Complex series of reactions rearranges 5 G3P to 3
  RuBP molecules
• Require 3 ATP
• For net synthesis of 1 G3P, Calvin cycle uses 9
  ATP and 6 NADPH

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IV. Photorespiration

• Define A metabolic pathway that consumes
  oxygen, evolves carbon dioxide, produces no ATP
  and decreases photosynthetic output.

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• See Flow Chart When O2 concentration in the
  leafs air spaces is higher than CO2
  concentration, rubisco accepts O2 and transfers
  to RuBP

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V. C4 Plants

• Adaptive process that enhances carbon fixation
  under conditions that favor photorespiration (hot
  and arid)

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• Bundle-Sheath Cells
• Arranged into tightly packed sheaths around the
  veins of the leaf
• Calvin cycle confined to bundle sheath
• Mesophyll Cells
• More loosely arranged in the area between the
  bundle sheath and the leaf surface

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• Step 1 CO2 is added to phosphophenyl pyruvate
  (PEP) to produce a 4-carbon compound enzyme has
  a high affinity for CO2, none for O2
• Step 2 4-carbon compound (oxaloacetae) converted
  to 2nd 4-C compound (malate)
• Step 3 Malate exported through plasmodermata
  from mesophyll to bundle-sheath cells
• 4-C compound releases CO2, which is then fixed by
  rubisco in Calvin cycle

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VI. CAM Plants

• Adaptation in succulent plants adapted to arid
  conditions. Open stomata during the night and
  close them during the day
• When stomata are open at night, CO2 is taken up
  and incorporated into a variety of organic acids
• Organic acids made at night are stored in
  vacuoles of mesophyll cells until morning, when
  stomata close
• During the daytime, light reactions supply ATP
  and NADPH for the Calvin cycle. CO2 is released
  from organic acids and is used for making sugar

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B. C4 vs. CAM

• Similarities CO2 is first incorporated into
  organic intermediates before it enters the Calvin
  cycle
• Differences In the initial steps of carbon
  fixation in C4 plants are structurally separate
  from Calvin cycle in CAM plants the two steps
  occur at different times

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