Photosynthesis

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Photosynthesis
Chapter 10
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Objectives

• Describe the structure and function of a
  chloroplast.
• Describe methods of solar capture and ATP
  production.
• Describe how carbohydrates are synthesized.
• Compare and contrast the 3 modes of
  photosynthesis.

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Key Terms

• ATP Synthase
• Chloroplast
• Chlorophyll
• Stroma
• Thylakoid
• light reactions
• Calvin cycle
• Antenna complex

• Ribulose biphosphate
• PGAL
• Photosystem
• Photorespiration
• CAM photosynthesis
• C3 Photosynthesis
• C4 Photosynthesis
• Chemiosmosis

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Do you know this reaction?
6 H2O
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Photosynthesis occurs in 2 stages.
Stage 1
Stage 2
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Solar Energy Capture
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Solar energy
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Most available energy is not used

• 42 of solar energy reaches Earths surface
• 2 of the 42 utilized by plants, remainder
  becomes heat
• Only 0.1 - 1.6 is incorporated into plant
  material.
• Photosynthesis uses the portion of the
  electromagnetic spectrum known as visible light.

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Photosynthetic Pigments
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Electromagnetic Spectrum
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Absorption Spectra
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Photosynthetic Pigments

• Chlorophyll a and b
• Absorbs violet, blue and red better than other
  colors.
• Green NOT absorbed!

Animation Light and Pigments
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Photosynthetic Pigments

• Chlorophyll a - main photosynthetic pigment
• chlorophyll b - accessory pigments broadens the
  spectrum used for photosynthesis
• carotenoids - accessory pigments that absorb
  excessive light that would damage chlorophyll

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Leaf Structure
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Chlorophyll (Terpene)
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Structure Function of Chloroplasts
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Chloroplasts

• Internal membranes, thylakoids, are organized
  into grana.
• Thylakoid membranes house pigments for capturing
  light and the machinery to produce ATP.
• clustered together to form a photosystem
• acts as an antenna, gathering light energy
  harvested by multiple pigment molecules

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Chloroplasts
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Interaction of Light with Chloroplasts
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Stage 1Light-dependent Reactions
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The Light Reactions (Solar Energy Capture)
In membranes of thylakoids Involves two
light-gathering units Photosystem I Photosystem
II Converts sunlight to chemical energy
(ATP). O2 is a "waste product"
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Photosynthesis Output

• Increases linearly at low light intensities but
  lessens at higher intensities.
• Reaches a saturation point

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Electron Pathways

• Cyclic electrons originate return to PSI
  reaction center
• ATP is produced
• Noncyclic electrons leave PSII and go to PSI
• ATP is produced
• H2O is oxidized yielding H, e- and O2
• NADP becomes NADPH

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Photosystem

• Network of pigments that channels excitation
  energy gathered by any of the molecules to the
  reaction center.
• reaction center allows photon excitation to move
  away from chlorophylls and is the key conversion
  of light to chemical energy

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Photosystem II complex (PSII) (P680 - Oxygen evolving apparatus) Photosystem II complex (PSII) (P680 - Oxygen evolving apparatus)
                                                                                                                                                                                                                               
Membrane protein complex found in photosynthetic organisms (higher plants, green algae and cyanobacteria) Harnesses light energy to split H2O into O2, protons and electrons. Responsible for the production of atmospheric oxygen Also involved in the production of a substantial proportion of the global biomass.
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Reaction Center

• Allows photon excitation to move away from
  chlorophylls and is the key conversion of light
  to chemical energy

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Photosystem Function

• Bacteria use a single photosystem, Photosystem I
  (P700)
• electron is joined with a proton to make hydrogen
• electron is recycled to chlorophyll

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Photosystem Function

• Plants use two photosystems
• photosystem I (P700) and II (P680)
• generate power to reduce NADP to NADPH with
  enough left over to make ATP
• two stage process photosystem II I.
• noncyclic photophosphorylation
• ejected electrons end up in NADPH

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Cyclic Electron Pathway
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Linear Electron Pathway
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(Chlorophyll a)
Chlorophylls a b and accessory pigments
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ATP Production
Chemiosmosis ATP production tied to an electro-
chemical gradient.
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Stage 2Light-independent Reactions
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Carbohydrate Synthesis
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Calvin Cycle
Sometimes called light-independent reactions.
Essential Question Does the Calvin cycle
continue running in a plant kept in the dark?
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Calvin Cycle
Takes place in the stroma Makes sugar from CO2
(CO2 becomes CH2O) Energy supplied by ATP made
by light reaction Electrons supplied by
NADPH G3P (glyceraldehyde-3-phosphate PGAL)
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Light-Independent Reactions
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P3G(glyceraldehyde-3-phosphate PGAL)

• Product of Calvin cycle
• Important biochemical reactant

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Modes of Photosynthesis

• C3
• C4
• CAM

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C3 Photosynthesis

• Most plants are C3. (Kentucky Bluegrass)
• Stomata open during the day
• Photosynthesis throughout the leaf. CO2 enters
  Calvin Cycle directly - fixed using RuBP
  carbolylase
• More efficient than C4 and CAM under cool and
  moist conditions and under normal light (fewer
  enzymes and no specialized anatomy).

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C4 Pathway

• Plants adapted to warmer environments deal with
  the loss of CO2 in two ways
• C4 conducted in mesophyll cells, Calvin cycle in
  bundle sheath cells
• creates high local levels of CO2 to favor
  carboxylation reaction of rubisco
• isolates CO2 production spatially

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C4 Photosynthesis

• CO2 undergoes preliminary fixation into malate
  (a C4 molecule) before entering Calvin cycle.
• Malate stored in large vacuoles in mesophyll
  cells (CO2 fixation is partitioned by space)
• Hot, dry climates
• Photosynthesis takes place in inner cells
• Include several thousand species in at least 19
  families. (crabgrass, corn, and many summer
  annuals)

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C4 Photosynthesis(Contd)

• Adaptive Value
• Photosynthesizes faster than C3 plants under high
  light intensity and high temperatures
• Better water use efficiency because PEP
  Carboxylase brings in CO2 faster and does not
  need to keep stomata open as much (less water
  lost by transpiration)

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C4 Photosynthesis
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Leaf Structure
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Comparison of C3 and C4 Leaf Anatomy
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CAM Photosynthesis(Crassulacean Acid Metabolism )

• The CO2 converted to an acid and stored during
  night as an acid.
• Stomata open at night, usually closed during the
  day. (Reduces loss of water vapor)
• Daytime - acid broken down and the CO2 is
  released to RUBISCO for photosynthesis
• Include many succulents, also some orchids and
  bromeliads.

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CAM PhotosynthesisAdaptive Value

• Better Water Use Efficiency than C3 under arid
  conditions (transpiration rates are lower, no
  sunlight, lower temperatures, lower wind speeds,
  etc.)

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CAM PhotosynthesisAdaptive Value

• CAM-idle under extreme conditions.
• Stomata closed night and day.
• O2 from photosynthesis is used for respiration,
  CO2 from respiration used for photosynthesis.
• Cannot CAM-idle forever.
• Survival of dry spells and rapid recovery when
  water is available again. (No dormancy)

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Carbon Fixation
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Photorespiration

• O2 is incorporated into RuBP, which undergoes
  additional reactions that release CO2.
• decreased yields of photosynthesis

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THATS ALL FOLKS