PHOTOSYNTHESIS

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PHOTOSYNTHESIS

• Converts light energy into chemical energy
  through a complex series of biochemical reactions
• 6CO2 6H2O -gt C6H12O6 6O2
• Photosynthesis occurs inside the chloroplasts
• White light from the sun is composed of different
  colors of different wavelengths visible spectrum

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CHLOROPLAST STRUCTURE

• Outer, double membrane
• Inner Membrane system
• THYLAKOIDS flattened sacs
• GRANA stacks of thylakoids
• Light harvesting pigments embedded in thylakoid
  membrane
• Surrounding thylakoids, liquid STROMA

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• Photosynthesis depends upon green pigment
  CHLOROPHYLL (absorbs light in the blue-violet and
  orange-red and reflects light in green region
• Accessory pigments help harvest light energy

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LIGHT DEPENDENT RXNS

• Visible light (traveling in photons, packets of
  energy) is changed into chemical energy
• H2O is split into O2 and H
• PS I and II absorb light energy
• This light energy is transferred to reaction
  center, a Chlorophyll a that donates e- to
  electron carrier

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LIGHT DEP. RXNS contd

• Lost e- from PSII is replaced by e- from H2O
• At end of electron flow, electrons combine with
  NADP to form NADPH
• As electrons flow along electron transport chain,
  protons build up inside thylakoids

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H3C H C1 "/, H C C H CHJ H CH3 H H H H H C
CH1 J,,/HJI I I I I I I I II! I /c, /C"", /c""'
/C, /C, /C""' /c, /C"", /c /c, /c" /CH1 H1C C C
C C C C C C C C /C" I I I I I I I I I I H3C
CHJ H1C, /C H H H H H CHJ H CH3 H C " H1
CH3 f,'-carotene
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LIGHT DEP. RXNS contd

• These built up protons will diffuse down
  concentration gradient through ATP synthase
• TAH-DAH!! ATP is made!!!
• PRODUCTS
• O2, ATP and NADPH!!!!

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CALVIN CYCLE

• Pathway that produces organic compounds, using
  energy stored in ATP and NADPH from the light
  reactions
• Occurs in the stroma
• CO2 is fixed into organic compounds
• RuBP (ribulose bisphosphate) is the 5 carbon
  sugar that CO2 is bound to by the enzyme rubisco

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More CALVIN

• The new 6 carbon molecule is immediately split
  into 2 3-carbon molecules (PGA)
• PGA converted to PGAL by addition of the
  phosphate from ATP and the hydrogen from NADPH
• ADP, NADP and phosphate are used again in the
  light reactions to form more ATP and NADPH

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• To make one molecule of G3P, three turns of the
  cycle 3 CO2 molecules
• CO2 is fixed to RuBP by rubisco produces
  unstable intermediate -gt 3-phosphoglycerate
• RuBP is regenerated one per each trip through
  the cycle
• Uses 9 ATP and 6 NADPH

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ALTERNATIVE PATHWAYS

• Calvin cycle plants C3 because of PGAL that is
  formed (3 carbon)
• Water loss through stomates is big problem
• When air is hot and dry, stomates close to
  prevent water loss
• BAD THING CO2 levels fall and O2 levels rise,
  resulting in carbon fixation inhibition

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C4 PATHWAY

• During hottest part of day, C4 plants partially
  close stomates
• A special enzyme fixes CO2 into 4-carbon
  compounds that is stored in bundle sheath cells
  and can then enter the Calvin cycle
• Corn, sugar cane and crabgrass

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CAM Photosynthesis

• Adaptation to hot, dry climates
• Open stomates at night and close them during the
  day (minimizes water loss)
• CO2 that enters at night is fixed into a variety
  of organic compounds and stored in vacuoles in
  morning, stomates close and CO2 is then released
  during the day and enters Calvin cycle
• These plants grow very slowly cactuses,
  pineapples

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Rates of Photosynthesis

• 4 limiting factors
• Light intensity, temperature, CO2, O2
• Active site of Rubisco can bind to O2 or CO2
  Photorespiration results in release of
  previously fixed CO2 that would otherwise remain
  in organic form

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RATES OF PHOTOSYNTHESIS

• As light intensity increases, so does rate of
  photosynthesis
• Levels off at a max rate, when all electrons are
  excited
• Same thing for CO2 levels
• Temperature increase, rate increases to a point
  then, enzymes denature and stomates close to
  prevent water loss, thus decreasing rate at high
  temperatures