Introduction to Photosynthesis

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Introduction to Photosynthesis

• Chapter 10
• p. 181-188

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Autotrophs Producers of the Biosphere

• Autotroph self-feeding produce own organic
  molecules from CO2 inorg. molec. in environment
• i.e. plants, algae, some bacteria
• Heterotroph feeds on others must consume
  other orgs to obtain nutrients, O2, energy
• i.e. animals, fungi, most bacteria

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Chloroplasts Sites of Photosynthesis

• Plants appear green due to pigment chlorophyll
  inside thylakoid space of chloroplasts
• Found in mesophyll tissue of leaves
• 30-40 per cell
• Stomata pores through which CO2 enters O2
  leaves

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Stomata
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Chloroplast Structure
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Tracking Atoms through Photosynthesis

• O2 given off by plants comes from H2O NOT CO2
• Chloroplast splits H2O ? 2H O
• C.B. van Niel proved 2H from split H2O goes to
  glucose O released as atmospheric O2

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Photosynthesis is a Redox Reaction

• Unlike cell respir., photosynthesis is endergonic
• Energy comes from sun
• Reverses direction of e- flow from H2O ? CO2
  (oxidized)
• CO2 is reduced to glucose
• CO2 H2O energy ? C6H12O6 O2

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The 2 Stages of Photosynthesis

• 1) Light Reactions (photo)
• e- H transferred to NADP (cousin of NAD)
• O2 given off as byproduct
• Produces 1 ATP (photophosphorylation)
• Occurs in the thylakoid

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The 2 Stages of Photosynthesis

• 2) Calvin Cycle (synthesis)
• CO2 incorporated into organic molecules already
  present (carbon fixation)
• Fixed C is reduced to glucose (add e-)
• Powered by NADPH ATP from light rxns
• Occurs during day in most plants relies on light
  rxns
• Occurs in stroma

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Photosynthesis Overview
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Nature of Sunlight

• Light electromagnetic energy (radiation)
• Travels in waves distance between called
  wavelength
• Also acts as photons particles of light energy
• Electromagnetic Spectrum entire radiation
  spectrum
• Visible light 380-750nm
• Amt energy inversely proportional to wavelength
• Purple photon gt red photon

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Photosynthetic Pigments

• Pigment substance that absorbs visible light at
  different wavelengths
• Reflected/transmitted wavelength is color we see
• Spectrophotometer measures absorbed wavelengths

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Photosynthetic Pigments

• Chlorophyll a main photosynthetic pigment
• Absorbs red blue photons reflects green
• Only pigment directly involved in light rxns
• Other pigments (Chlorophyll B Carotenoids)
  transfer photons to chlorophyll a provide
  photoprotection

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Excitation of Chlorophyll by Light

• When molecules absorb light energy (photons), e-
  jump to next orbital
• Ground state ? excited state
• Specific to wavelength
• Unstable e- will fall back quickly, releasing
  energy (heat)
• Fluorescence energy released as light

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Reactions of Photosynthesis

• Chapter 10
• p. 189-198

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Photosystems

• Consists of 3 sections
• 1) Light-Harvesting Complex
• contain all 3 types pigments
• ? surface area to absorb more light
• 2) Reaction-Center at center receives energy
  from light-harvesting complex becomes excited
• Contains special chlorophyll a molecules whose
  e-s move to higher energy level
• 3) Primary Electron Acceptor receives e-s from
  excited chlorophyll a molecules catches them
• e-s then enter into Noncyclic Electron Flow

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Types of Photosystems

• Photosystem II absorbs 680nm best (P680)
• P700 P680 identical, but surrounded by diff.
  proteins
• Work together to make ATP NADPH for Calvin
  Cycle
• Photosystem I Reaction center chlorophyll a
  absorbs 700nm best (P700)

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Noncyclic Electron Flow

• Predominant route for e-s
• Steps
• 1) Photots. II absorbs light, P680 excited
• 2) e-s captured by Primary e- Acceptor
• P680 now has e- hole
• 3) e-s replaced in P680 by split H2O molecule O2
  released inside thylakoid
• 4) ETC takes e-s from Primary e- Acceptor to
  Photosystem I
• Composed of plastoquinone (Pq), 2 cytochromes,
  plastocyanin (Pc)

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Noncyclic Electron Flow

• 5) As e-s fall down chain, energy is harvested
  to make ATP by chemiosmosis outside thylakoid
• Noncyclic Photophosphorylation
• 6) Final e- acceptor is P700
• P700 e-s excited by light energy are captured by
  Primary e- Acceptor
• Fills hole created by Primary e- Acceptor of
  Photo II
• 7) Primary e- Acceptor passes e-s to 2nd ETC ?
  ferredoxin (Fd)
• 8) NADP reductase transfers e-s from Fd to
  NADP? makes NADPH in stroma

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Summary of Noncyclic Electron Flow

• P680 ? Primary e- Acceptor ? 1st ETC ? P700 ?
  Primary e- Acceptor ? Fd ? NADP reductase ?
  NADPH

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Cyclic Electron Flow

• Calvin cycle uses more ATP than NADPH
• If ATP runs low, chloroplast switches to cyclic
• Involves Photosystem I (P700) only
• Fd takes e-s to cytochrome complex of 1st ETC
  returns them to P700
• No NADPH produced no O2 released

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Calvin Cycle

• C enters as CO2, leaves as sugar (G3P)
• Cycle must turn 3 xs to make glucose
• Must fix 3 Cs into org. molecules
• Occurs in 3 phases
• 1) C Fixation 3Cs from 3CO2 are incorporated
  into RuBP, catalyzed by rubisco
• 2) Reduction e-s from NADPH reduce 6 1,3
  biphosphate ? 6 G3P (? energy)
• Spends 6 ATP
• 3) Regeneration of RuBP G3P rearranged ? RuBP
  (can pick up CO2 again)
• Spends 3 ATP

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Calvin Cycle - Summarized

• For each turn of Calvin
• In Out
• 9 ATP 9 ADP
• 6 NADPH 6 NADP
• 3 CO2 1 G3P (will
• become
  glucose)

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Alternate Methods of C Fixation

• In hot, dry climates, stomata remain closed to
  prevent H2O loss
• Also prevents CO2 in O2 out
• Result is Photorespiration

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Photorespiration

• Most plants are called C3 because C fixation
  creates a 3-C compound
• Closed stomata ? CO2 inside leaf, and ? O2
• O2 will be picked up by rubisco (instead of CO2)
• Photorespiration uses light (photo) to consume
  O2 (respiration)
• No ATP produced no sugar made
• May be ancient evolutionary adaptation

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C4 Plants

• C fixed by PEP carboxylase to form 4-C compound
  (oxaloacetate ? malate)
• PEP carbox. has ?? affinity for CO2 can fix
  CO2 when rubisco cant
• 4-C cmpnd (malate) enters Bundle Sheath cells
  where CO2 breaks off enters Calvin
• Keeps CO2 levels ? for rubisco
• Minimizes photorespiration ? sugar production

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CAM Plants

• Water-storing plants (cacti, pineapple, etc.)
  close stomata during day, open at night
• Store org. molec. until day when light rxns can
  provide ATP NADPH