Plants and Photosynthesis

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Plants and Photosynthesis
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Photosynthesis

• Organisms
• Autotrophs Self Feeders
• Photo- Light
• Chemo- Oxidize inorganics (Ex Sulfur, Ammonia),
  unique to bacteria
• Heterotrophs Other Feeders

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History

• Jean-Baptiste van Helmont (1600s)
• grew willow tree
• Weighed soil before and after
• Added only water
• Tree gained 75 kg
• No change in mass of soil
• Concluded mass in plants comes from water

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Site of Photosynthesis
Upper Epidermis
MesophyllCells
LowerEpidermis
Vein
Stoma
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Site of Photosynthesis
Thylakoids
Stroma
Granum
Inner OuterMembranes
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Photosynthesis

• Conversion of Light E into Chem E
• Light E
• Travels in waves (photons)
• Wavelength (?) crest to crest (measured in nm)
• ? inversely related to frequency
• Higher frequency more E
• Different ? different properties

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Nature of Light

• Visible spectrum is 380750 nm

Wavelength (nanometers)
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Nature of Light

• Pigments absorb certain ? and reflect or transmit
  others

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Nature of Light

• Spectrophotometers measure amount of Light
  pigments absorb or reflect

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Nature of Light

• Pigments
• Absorb and reflect light
• Specific pigment specific light
• Chlorophylls
• a and b both absorb blues and reds
• a is 1? pigment for photosynthesis focuses
  solar E onto a pair of e-s

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Nature of Light

• Accessory pigments funnel the E they collect to
  a central Chlorophyll A
• Carotenoids
• Carotenes reflect oranges
• Xanthophylls reflect yellows
• Phycocyanins reflect blues
• Some accessory pigments provide photoprotection
  against excess light
• Carotenoids in human eyes serve same function

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Absorption/Action Spectra
Visible Light
Collectively
Chlorophyll
Carotenoids
Phycocyanin
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Engelmanns Experiment

• Simple experiment in 1883
• Compare to action spectrum

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Photosynthesis

• Can be divided into
• Light-dependent rxn
• Makes E storing compounds NADPH and ATP to fuel
  L-i rxn
• Occurs in thylakoids
• Light-independent rxn
• Uses NADPH and ATP to produce glucose, a more
  stable form of E
• Occurs in stroma

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Photosynthesis
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Light-dependent rxn
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Light-dependent rxn

• Light is absorbed in photosystem II, an antenna
  complex of hundreds of pigments that funnel E to
  a reaction center
• Rxn Center central chlorophyll a
  molecule next to a protein,
  the 1 e- acceptor

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Light-dependent rxn

• Chemi- osmosis

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Photosynthesis
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Light-dependent rxn
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Light-dependent rxn

• The e-s from the broken bonds slide down the ETC,
  slowly losing E
• The e-s are recharged by sunlight in photosystem
  I and are passed along more carrier proteins to
  NADP, reducing it to NADPH

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sun
O2
Light-dependent
H
H20
H
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Light-dependent
sun
sun
O2
ADP
H
H
H20
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Light-dependent rxn summary

• H2O is broken up by sunlight
• O2 is released as waste
• e-s flow down ETC, pump H ions, and finally make
  NADPH
• H ions diffuse across thylakoid membrane and
  help form ATP
• ATP and NADPH move on to the light-independent rxn

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Photosynthesis
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L-i rxn C fixation
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L-i rxn Reduction

• 12 ATPs phosphorylate the 12 3PGs to form 12 1,3
  bisphosphoglycerates
• A pair of e-s from NADPH reduces each 1,3
  bisphosphoglycerate to glyceraldehyde-3-phosphate
  (G3P)
• The electrons reduce a carboxyl group to a
  carbonyl group

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L-i rxn Reduction
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L-i rxn Reduction

• Two G3Ps can now be removed from the cycle to
  make glucose or be used for as any other carb the
  plant cell needs

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Light-independent rxn summary

• Carbon Fixation
• CO2 needed to begin the process
• Synthesis of G3P (Glyceraldehyde 3 phosphate)
• ATP and NADPH are used
• Regeneration of 5C compound
• Need more ATP to reset the cylce

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Photorespiration

• Stomata not only allow gas exchange, but
  transpiration also
• Hot, dry day stomata close
• Problem CO2 ?, O2 ?
• Rubisco can bind either CO2 OR O2 to RuBP
• When O2 binds, no useful cellular E is produced

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Photorespiration

• When rubisco adds O2 to RuBP, RuBP splits into a
  3-C piece and a 2-C piece
• The 2-C fragment is exported from the chloroplast
  and degraded to CO2 by mitochondria and
  peroxisomes
• Photorespiration decreases photosynthetic output
  by siphoning organic material from the Calvin
  cycle
• Up to 50 of the C fixed by Calvin cycle can be
  drained away on a hot, dry day

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C4 Plants

• Mesophyll cells use PEP carboxylase to fix CO2 to
  phosphoenolpyruvate, forming oxaloacetate (4C)
• PEP carboxylase has a very high affinity for CO2
  and can fix CO2 efficiently when rubisco cannot -
  on hot, dry days with the stomata closed

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C4 Plants

• Oxaloacetate then dumps the extra CO2 into the
  Calvin cycle in bundle-sheath cells
• Rubisco can then work with a high concentration
  of CO2, thus minimizing photorespiration
• C4 plants thrive in hot regions with intense
  sunlight
• Examples sugar, corn

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C4 Plants
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CAM Plants

• Crassulacean Acid Metabolism
• CO2 is fixed at night, but NO photosynthesis
  takes place at night
• During the day, the light reactions supply ATP
  and NADPH to the Calvin cycle and CO2 is released
  from the organic acids

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CAM Plants

• Allows plants to keep their stomata closed during
  the hot, dry hours of day and open in the cooler
  hours of night
• Less water is lost in the process
• Less photorespiration occurs
• Ex succulent plants, cacti, pineapples, and
  several other plant families

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CAM Plants
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• Both C4 and CAM plants add CO2 into organic
  intermediates before it enters the Calvin cycle
• In C4 plants, carbon fixation and the Calvin
  cycle are spatially separated
• In CAM plants, carbon fixation and the Calvin
  cycle are temporally separated
• Both eventually use the Calvin cycle to
  incorporate light energy into the production of
  sugar