Photosynthesis

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Photosynthesis

• Where it all starts!

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Light and Pigment

• Wavelength
• Distance between each wave
• Nanometers or billionths of a meter
• Shorter the wavelength, the greater the energy
• E in suns rays
• 380 750 nanometers
• Visible light

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Pigments

• Molecules that capture light energy
• Specific pigments can absorb only specific
  wavelengths of light
• If not absorbed, transmitted through or reflected
• Chlorophyll a
• Absorbs red and blue-violet light
• Reflects all other light
• mainly green

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Accessory pigments

• Capture other wavelengths of light
• Chlorophyll b
• Reflects green and yellow light

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Accessory pigments

• Carotenoids
• Reflect red, orange, and yellow
• Xanthophylls
• Reflect yellow, brown, purple, or blue
• Anthocyanins
• Red or purple flowers

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Accessory pigments

• Phycobilins
• Red or blue-green
• Signature pigments of red algae and cyanobacteria

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Chemical bonds

• Single covalent alternate with double covalent
  bond
• Magnesium atom
• Light catching

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2 stages of reactions

• Light-dependent
• Sunlight is converted to chemical bond energy of
  ATP
• Water molecules are split and coenzyme NADP
  picks up released H and electrons -gt NADPH
• Oxygen is released

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2 stages of reactions

• Light-independent
• Runs on energy donated by ATP
• Glucose and other carbohydrates are made with
  carbon and oxygen atoms, and with the H and
  electrons from NADPH

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Equation for Photosynthesis
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Inside

• Chloroplast
• The organelles of photosynthesis in all plants
  and certain protists
• Stroma
• Semifluid interior, enclosed by two outer
  membranes

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Chloroplast

• Thylakoid membrane
• Third membrane, forms a compartment inside the
  stroma
• Folds back onto itself connected by channels
• Stacks of pancakes
• Light dependent reactions occur

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Light-Dependent Reaction

• Light harvesting systems
• Found in thylakoid membrane
• Absorb photon energy
• Hold on to energy and pass it back and forth
• Photosystems
• Reaction centers, arrays of hundreds of pigments
  and other molecules

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Light-Dependent Reaction

• Freed electrons enter an electron transfer chain
• Transfer electrons or hydroden ions across the
  thylakoid membrane, from the stroma to the inner
  compartment
• H cannot diffuse across the lipid bilayer
• Can only pass through channels inside ATP
  synthase
• Type of active transport protein

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ATP Synthases

• Flow of hydrogen ions through ATP synthases
  powers the attachment of phosphate groups to ADP
  molecules in the stroma
• ATP forms

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Photosystem II

• Replaces lost electrons by pulling them from
  water, essentially unlimited
• Oxygen is released
• Hydrogen stays to contribute to the gradient that
  drives ATP formation

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Electrons?

• Photosystem I
• Light-harvesting complexes volley energy
• Chlorophylls release electrons
• Transferred to NADP
• Attracts hydrogen ions and forms NADPH

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Too much NADPH?

• Photosystem I may run independently of
  Photosystem II
• Cell can continue to make ATP
• Cyclic pathway
• Light energy frees electrons in photosystem I
• Released electrons enter transfer chain that
  moves hydroden ions into thylakoid
• The H gradient drives ATP fromation

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Too much NADPH?

• Electrons are recycled back to photosystem I
• Thus no NADPH is formed

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Light-Independent Reaction

• Light is not needed for reaction to occur
• Calvin-Benson Cycle
• Pathway that runs inside the stroma of
  chloroplast
• Uses ATP and NADPH formed in light-dependent
  reactions

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CO2

• Rubisco
• Enzyme that joins a carbon to five-carbon
  ribulose biphosphate (RuBP)
• Starts the Calvin-Benson Cycle
• Splits immediately into two stable molecules of
  phosphoglycerate (PGA)
• Carbon Fixation
• Process of securing carbon from environment by
  incorporating it into a stable organic compound

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PGA

• Each PGA gets
• a phosphate group from ATP
• Hydrogen and electrons from NADPH
• For every 6 molecules of CO2 fixed
• 12 phosphoglyceraldehydes (PGAL) are formed
• 10 PGAL are used to rearrange to generate RuBP
• The 2 other PGAL combine to make a six-carbon
  phosphate group

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Glucose

• Most of glucose is converted to sucrose
• Sucrose is a transportable form of carbohydrate
  in plants
• Use as energy source or building blocks for other
  complex molecules

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Calvin-Benson Cycle
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Putting it all together
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Different pathways

• Environments differ
• Different pathways
• Sunlight, air temp, rainfall, and soil
• Waxy cuticle
• Stomata (stoma)
• Water and gases move into and out of leaves

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C3 Plants

• Do not grow well in hot, dry climates
• Need irrigation
• C3 plants three carbon PGA
• Oxygen builds up
• Rubisco binds to oxygen, not CO2
• Alternate Rxn yeilds CO2
• Plant loses carbon instead of fixing it

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C3 Plants

• Less sensitive to cold
• Grow and compete best where temperatures drop
  below 25 C

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C4 Plants

• Evolved independently
• Four-carbon oxaloacetate forms first reaction in
  two cell types
• Mesophyll cells
• Will fix carbon no mater how much O2 present
• Deposits CO2 directly into bundle-sheath
• Enters Calvin-Benson Cycle
• CO2 level kept high enough to block completing
  reaction

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C4 Plants

• Use more ATP than C3
• Fix carbon twice
• Can make more sugar on dry days

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C3 vs. C4
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CAM Plants

• Crassulacean Acid Metabolism
• Open stomata at night fix carbon
• Calvin-Benson Cycle runs the next day
• Water storing
• Cacti and other succulents

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C4 vs. CAM
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Pastures of the Seas

• Photoautotrophs (Bacteria Protists)
• Marine Photoautotrophs
• 45 billion metric tons of Carbon from CO2
• Krill feed on photosynthesizers
• Fish, penguins, seabirds, and whales
• Whales feed on 4 tons of krill per day
• Krill fed on 1,200 tons of pasture

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Photosynthesis

• Construct a diagram that depicts the process of
  photosynthesis. Be sure to include the
  light-dependent reactions, light independent
  reactions, Photosystems I II, and the
  Calvin-Benson cycle.
• DO NOT JUST COPY THE DIAGRAMS FROM THE CHAPTER.