Photosynthesis

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Photosynthesis

• Chapter 10

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Photosynthesis

• Fundamentals
• Autotrophs vs Phototrophs
• Anatomy
• Development of Formula
• Nature of sunlight
• Photosystems
• Overview
• Light Reactions
• Calvin Cycle

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IA.Autotrophs vs Phototrophs

• Autotrophs? make organic molecules from raw
  material
• Photoautotroph? use energy of sun to produce
  food, explants algae, cyanobacteria euglena
• Chemoautotrophs? reduces inorganic compounds for
  energy,ex sulfur bacteria
• Heterotrophs? live and use organic materials made
  by other organisms, ex us,fungi

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IB. Anatomy

• Carried out by green parts of a plant
• Stomata regulates flow of CO2 H2O
• Mesophyll layer of leaf site of photosynthesis,
  contains chloroplasts(endosybiotic bacteria)

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The Leaf Chloroplasts

• Chloroplast? 3 functional compartments
• Intermembrane space
• Thylakoid space
• stroma

• The leaf

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Pigments and Chromatography

• Paper chromatography is a technique used to
  separate a mixture into its component molecules.
  The molecules migrate, or move up the paper, at
  different rates because of differences in
  solubility, molecular mass, and hydrogen bonding
  with the paper
• In paper chromatography the pigments are
  dissolved in a solvent that carries them up the
  paper. To separate the pigments of the
  chloroplasts, you must use an organic solvent

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Chromatography
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Retention Factor

• Rf distance traveled by spot/ distance
  traveled by solvent
• If the for an unknown is close to or same as that
  for a known compound, the two are most likely
  similar or identical

Solvent Front
Solute front
Sample origin
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IC. Development of Formula

• Using glucose as our target product, the equation
  describing the net process of photosynthesis is
• 6CO2 6H2O light energy ?C6H12O6 6O2
• In reality, photosynthesis adds one CO2 at a
  time
• CO2 H2O light energy ?CH2O O2
• CH2O represents the general formula for a sugar.

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• Clues to the mechanism of photosynthesis came
  from the discovery that the O2 given off by
  plants comes from H2O, not CO2.
• Before the 1930s, the prevailing hypothesis was
  that photosynthesis occurred in two steps
• Step 1 CO2 ? C O2
• Step 2 C H2O ? CH2O
• C.B. van Niel challenged this hypothesis.
• Studied sulfur bacteria that use H2S, not water,
  in photosynthesis.
• They produce yellow globules of sulfur as a
  waste.
• Van Niel proposed this reaction
• CO2 2H2S ?CH2O H2O 2S
• Applied this to plants proposed
• CO2 2H2O ?CH2O H2O O2

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Tracking the atoms

• Other scientists confirmed van Niels hypothesis.
• Used 18O, a heavy isotope, as a tracer, labeled
  either CO2 or H2O.
• Found that the 18O label only appeared if water
  was the source of the tracer.
• Essentially, hydrogen extracted from water is
  incorporated into sugar and the oxygen released
  to the atmosphere (where it will be used in
  respiration).

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ID. Nature of Sunlight

• Electromagnetic energy travels in waves but also
  behaves as discrete particles
• Visible ?spectrum of colors of diff wave lengths

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Nature of Sunlight

• Absorption Spectrum
• Action Spectrum

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Absorption Spectrum
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Absorption Spectrum
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Action Absorption Spectra? photosynthesis is
greatest in the blue and red end
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Engelmanns Experiment
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Pigments

• Absorb and reflect light resulting in the color
  of the leaves
• Green wave length of light is not absorbed, it is
  reflected back hence leaf appears green

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Why leaves are green interaction of light with
chloroplasts
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IE. Photosystems

• Light harvesting unit embedded in the thylakoid
  membrane
• Consists of several hundred chlorophyll
  molecules? energy funnel
• Array of pigment molecules that act as a light
  antenna

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Light Harvesting Complex

• Mostly chlorophyll a, some carotenoids and other
  pigments
• Various molecules absorb at varying wavelenghts
  all direct energy to a single pigment
  molecule(P700 0r P680)
• Two photosystems PS II(P680) PS I(P700)? named
  in order of discovery, but PSII functions first
• See fig 10.12

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IF. Overview

• Light Reaction
• Takes place in the thylakoid membrane
• Converts light energy to chemical energy
• Water is split, ATP NADPH is made, O2 evolved
• Calvin Cycle (Dark Reaction)
• Takes place in the stroma
• Carbon fixation
• Energy ? ATP NADPH of the light Rxn

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Overview
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II. Light Reactions

• NonCyclic Phosphorylation
• Oxygen is evolved
• Water is split
• NADPH produced
• Cyclic Phosphorylation
• No oxygen is released
• NADP not reduced

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Non Cyclic Phosphorylation

• Light energy is used to generate ATP and NADPH
• Predominant route of electron flow

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Cyclic Phosphorylation

• Takes place under certain conditions, uses PSI
• WHY??
• Calvin Cycle uses more ATP than NADPH
• Cyclic electron flow makes up the difference?
  produces ATP, no NADPH
• NADPH regulates the shift from noncyclic to cyclic

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Cyclic Phosphorylation
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How is ATP Sythesized
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Chemiosmosis in Mitochondria Chloroplasts
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III. Calvin Cycle

• Takes place in phases
• Phase 1? Carbon fixation
• Phase 2?Reduction
• Phase 3? regeneration of CO2 acceptor
• CO2 enters the cycle and leaves as sugar.
• The actual sugar product of the Calvin cycle is
  not glucose, but a three-carbon sugar,
  glyceraldehyde-3-phosphate (G3P).

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Phase 1? Carbon Fixation

• Each CO2 molecule is attached to a five-carbon
  sugar, ribulose bisphosphate (RuBP).
• This is catalyzed by RuBP carboxylase or rubisco
  (most abundant enzyme on earth)
• The six-carbon intermediate splits in half to
  form two molecules of 3-phosphoglycerate per CO2.

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Phase 1? Carbon Fixation
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Phase 2?Reduction

• Each 3-phosphoglycerate receives another
  phosphate group from ATP to form 1,3
  bisphosphoglycerate.
• A pair of electrons from NADPH reduces each 1,3
  bisphosphoglycerate to G3P(6 molecules)
• One of these six G3P (3C) is a net gain of
  carbohydrate.
• One exits the cycle to be used by the plant cell
• The other five (15C) must remain in the cycle to
  regenerate three RuBP

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Phase 2
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Phase 3? regeneration of CO2 acceptor

• Five G3P molecules are rearranged to form 3 RuBP
  molecules
• It costs three ATP and two NADPH per CO2
• The G3P from the Calvin cycle is the starting
  material for metabolic pathways that synthesize
  other organic compounds, including glucose and
  other carbohydrates

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Review of Photosynthesis
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Alternate Mechanisms of Carbon Fixation

• Photorespiration
• C4 Plants
• CAM Plants

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Photorespiration

• A metabolic pathway that consumes O2, releases
  CO2, generates no ATP
• Rubisco(RuBP carboxylase)? catalyzes both
  carboxylation and oxidation of RuBP
• Rubisco has alternate sites for O2 and CO2( both
  are substrates for the enzyme)
• This reverses carbon fixation undoes results of
  Calvin cycle

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Photorespiration

• A metabolic process that is counter productive
• Under hot dry, bright condition
• An evolutionary quirk ?early atmosphere had very
  little oxygen
• Produces no ATP
• Decreases the photosynthetic output
• Certain species of plants have evolved
  alternate modes of carbon fixation to minimize
  photorespiration

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C4 Plants

• Some plants preface the Calvin cycle with a Rxn
  that incorporate CO2 into 4C compounds
• Occurs in corn, sugar cane, crabgrass, at least
  19 plant families use this

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C4 Leaf Anatomy

• Mesophyll cells ? light Rxns CO2 fixation
• Bundle Sheath? Calvin cycle

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C4 Plants

• CO2 combines w/ PEP
• PEP Carboxylase has a greater affinity for CO2
  that rubisco
• Oxaloacetate (4C) or Malate(4C) ? bundle sheath
• Decarboxylate? pyruvate moves back to mesophyll
• CO2 picked up by RuBP(CO2 released w/in the
  bundle sheath, not lost)

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CAM Plants

• Crassulaceae Acid Metabolism
• Aloe , Jade , Cacti
• Keep in as much water as possible by opening
  stomata only at night
• Store CO2 ? organic acids(in vacuoles)
• During the day these organic acids release CO2?
  Calvin cycle takes place

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C4 and CAM Plants
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Figure 10.20 A review of photosynthesis