PHOTOSYNTHESIS

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PHOTOSYNTHESIS

• Main biosynthetic pathway by which carbon and
  energy enter the web of life

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PHOTOSYNTHESIS

• Summary reaction similar to that of respiration
  in reverse
• Sugar and oxygen gas are produced from CO2,
  water, and light energy
• Converts light energy into chemical energy of
  glucose
• Photosynthesis summary equation

6CO 12 H O C H O 6 O 6 H O
2
2
2
2
6
6
12
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PHOTOSYNTHESIS

• Involves green plants, algae, and cyanobacteria,
  collectively known as autotrophs
• Essential to all life forms for making
• energy nutrients
• Essential to most life forms as a source of
  oxygen
• Reactions grouped in pathways

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Chloroplast site of photosynthesis

• Double membraned organelle
• Contains inner membranes of thylakoids
• Thylakoids have stacks of grana and contain
  chlorophyll to capture suns energy
• Space between thylakoids called stroma- matrix
  containing enzymes and raw materials for
  photosynthesis

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Light Requirements of Photosynthesis

• Visible spectrum
• ROY G BIV wavelengths
• Measured in nanometers, 400-700 nm range used in
  photosynthesis
• Contains energy packets called photons

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Pigments of Photosynthesis

• Main primary pigment chlorophyll a (dark green)
• Accessory pigments called antennae molecules
• chlorophyll b (yellow green)
• carotenoids (red-orange-yellow)
• xanthophylls (yellow)
• others in lower life forms

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Pigments arranged in PHOTOSYSTEMS

• Cluster of 200-300 pigments molecules per
  photosystem
• PS-I or P700 - absorbs an average of 700 nm light
• PS-II or P680 - absorbs an average of 680 nm
  light
• PS-II transfers energy to PS-I

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Two Photosystems Photosystem I and Photosystem
II

• Work together
• Excited electrons from PS-II used to generate ATP
  by chemiosmosis
• Resulting low energy electrons given to
  chlorophyll in PS-I
• Electrons in PS-II replaced by photolysis of
  water oxygen also produced

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Two Photosystems Photosystem I and Photosystem
II

• PS-I excited electrons used to reduce NADPH
• PS-I chlorophyll electrons replaced by depleted
  energy electrons from PS-II

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PAPER CHROMATOGRAPHY

• Lab technique for separating and identifying
  pigments of photosynthesis

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Reactions of Photosynthesis

• Light Dependent
• Also called
• -Light reaction
• -Photo phase
• -Light capturing
• phase

• Light Independent
• Also called
• -Dark reaction
• -Dark phase
• - Calvin cycle
• - Synthesis phase

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REACTION GROUPS

• Light Dependent Reaction - Light capturing
  reaction
• occurs in the grana of the chloroplast
• converts solar energy to ATP NADPH and stores
  it in chemical bonds of glucose
• Light Independent Reaction - Carbon dioxide
  conversion or fixation reaction
• occurs in the stroma of chloroplast - the space
  between the grana
• reduces CO2 to sugar molecule as C from carbon
  dioxide is incorporated into glucose

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Light Dependent Reactions

• Chlorophyll pigment systems absorb light energy
  (photons)
• Photons excite chlorophyll electrons boosting
  them to higher levels - called
• PHOTOEXCITATION
• Energy collected by many pigments passed to
  reactive chlorophyll
• Boosted electrons replaced by the PHOTOLYSIS of
  water
• Splitting of water releases oxygen

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Light Dependent Reactions

• Reactive chlorophyll electrons passed to electron
  acceptor molecule, NADP
• Generate chemical energy via photophosphorylation
• - ATP, NADPH
• - Used to power the light independent reaction

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Photophosphorylation

• CYCLIC
• - primitive pathway
• - produces ATP only
• - e- are recycled back
• to photosystems
• Example bacteria

• NONCYCLIC
• - most common
• - produces ATP and
• NADPH
• - relies on photolysis
• of water to replace e-
• Example plants and
• algae

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NADP
(chemiosmosis)
NADPH
ATP
PS-I
H2O 1/2 O2 2H
2 e
PS-II
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LIGHT INDEPENDENT REACTIONS

• These reactions occur in the stroma - the space
  between the grana
• While NOT directly dependent on light energy they
  are dependent on the ATP and NADPH2 produced by
  the light dependent reactions
• The light independent reactions are called the
  CALVIN CYCLE after their discoverer

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Light Independent Reaction

• Fixes C from inorganic CO2 into
• organic molecules such as glucose
• Requires 6 turns of the Calvin Cycle,
• 6 CO to make C H O (glucose)
• 2 6 12 6
• Calvin Cycle regenerates itself by
• producing RuBP to receive more CO2 to keep the
  cycle going

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Light Independent Reactions
6 Carbon sugar
CO2
2 3 carbon molecules
Ribulose biphosphate
PGAL
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What can the cell DO with the PGAL product of
photosynthesis??

• Regenerate the 5-C sugar, RuBP, for Calvin cycle
• Make more complex sugars
• 2 PGAL 1 glucose
• Make fats
• Make proteins
• Use for energy

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GENERAL SUMMARY

• Photosynthesis occurs in green plants and algae
  inside chloroplasts
• Photosynthesis transforms light energy into
  chemical energy
• Photosynthesis fixes CO2 into organic carbon
  Ex glucose sugar

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LIGHT REACTIONS SUMMARY

• Occurs in the chloroplasts GRANA
• Energy from sun transferred to electrons in
  chlorophyll
• Excited electrons transfer energy to cell energy
  molecules ATP and NADPH

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Light Reaction Summary

• Photosystem I depleted energy electrons from
  PS-II replace chlorophyll electrons
• Photosystem II electrons from photolysis of
  water replace chlorophyll electrons
• Oxygen released as a by-product

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CARBON FIXING REACTION SUMMARY

• Occurs in chloroplasts stroma
• Carbon dioxide is accepted by a starter sugar
  molecule, RuBP
• Uses energy from Light Reaction
• Regenerates the starter sugar molecule, RuBP
• Ultimately produces 3-C sugars PGAL, to form
  glucose, etc.
• Requires Calvin Cycle pathway

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PHOTOSYNTHESIS

• Pathway that captures the suns energy and
  transforms it into chemical energy for all life
  forms to use in life processes

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Historical Perspective of Photosynthesis

• Priestly - oxygen released
• Ingen-housz - requires light and involves green
  plants
• Senebier - uses CO2
• VanNeil plus Rubin and Kramer - splits water
• Engelmann - involves chlorophyll, needs ROY G BIV
  wavelengths

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Photosynthetic Pathways

• C3 Pathway - most common
• most plants in our temperate climate
• CO2 combines directly with RuBP to start Calvin
  Cycle
• Stomata open day, close night

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Photosynthetic Pathways

• C4 Pathway - dozen plant families such as corn,
  sugar cane, crab grass
• Adaptation for hot, dry seasons when stomata need
  to be closed
• Fixes CO2 twice
• CO2 fixed into oxaloacetic acid, stored in bundle
  sheath cells as malic acid when stomata open
• Malic acid serves as CO2 source when stomata
  close

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Photosynthetic Pathways

• CAM Pathway - desert plant such as cactus and
  other succulents
• Adaptation for extremely hot, dry, areas
• Plants conduct Crassulacean Acid Metabolism - CO2
  fixed as Crassulacean
• Acid and stored in vacuoles at night when
  stomata open
• Acid serves as CO2 source in day when stomata
  close

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Photosynthesis and Cell Respiration Comparison

• Compare the 2 processes with respect to
• purpose
• equations
• pathways
• cell location
• energonic or exergonic
• anabolic or catabolic