Photosynthesis

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Photosynthesis

• The Dark and Light Reactions

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Photosynthesisusing light energy to convert CO2
and water into sugars

• 6 CO2 6 H2O light energy ? C6H12O6 6 O2

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Photosynthesis Takes Place in the Chloroplast
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Light Reactions occur on the thylakoid
membranes

• convert light energy and water into chemical
  energy (ATP and NADPH)
• produce O2 as a waste product

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How Light Works

• Visible radiation drives the light reactions
• Light is a type of energy called electromagnetic
  energy.
• Different pigments embedded in thylakoid
  membranes

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• Different pigments embedded in thylakoid
  membranes absorb light of different wavelengths.
• The wavelengths absorbed by the light harvesting
  pigments are mainly in the blue and red range.
• The light harvesting pigments of chloroplasts are
• 1. chlorophyll a absorbs in blue and red
• 2. chlorophyll b absorbs in blue and orange
• 3. carotenoids absorbs mainly blue-green range

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• Having all these different pigments broadens the
  range of visible light that is usable for
  photosynthesis.
• Only chlorophyll a passes electrons to the
  electron transport chain and thus is directly
  involved in the light reactions.
• The other pigments convey the light energy to
  chlorophyll a.

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• Photosystem II absorbs light energy
• 2 electrons are ejected
• Electrons move down transport chain generating
  ATP

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• Photosystem I absorbs light energy
• 2 electrons move to an electron acceptor
• Then on to NADP

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• Water splits into H ions electrons and Oxygen

PHOTOLYSIS the splitting of water
Electrons
H ions
WATER
Oxygen
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• 8. The electrons from splitting water replace
  those lost in PSII
• 9. The H joins NADP to form NADPH
• 10. Oxygen in released
• 11. ATP and NADPH move to dark reactions

O2
H Ions
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Photosynthesis The Dark Reactions

• ATP NADPH2 CO2?C6H12O6

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Dark Reactions occur in the stroma

• Carbon dioxide is split, providing carbon to make
  sugars.
• The ultimate product is glucose.
• While this system depends on the products from
  the light reactions, it does not directly require
  light energy.

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Carbon Fixation

• 3 CO2 molecules attach to 3, 5 carbon sugars
  (RuBP)
• A 6 carbon sugar is formed and O2 is released
• This reaction ins catalyzed by the enzyme
  Rubisco.
• This 6 carbon sugar rearranges and splits into 2,
  3 carbon sugars (PGA)

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Reduction

• 6 ATP contribute phosphate groups
• 8 NADPH drop off electrons
• These are used to make the high energy compound
  G3P

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Regeneration

• 1 G3P is used to form glucose
• The others are used to regenerate the original 5
  carbon sugar
• 9 ATP and 6 NADPH are used per turn of the calvin
  cycle
• These are regenerated by Light Reactions
• 2 G3P Glucose
• http//video.google.com/videosearch?gbv2hlenq
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• CO2 combines with the phosphorylated 5-carbon
  sugar ribulose bisphosphate.
• This reaction is catalyzed by the enzyme ribulose
  bisphosphate carboxylase oxygenase (RUBISCO)(an
  enzyme which can fairly claim to be the most
  abundant protein on earth).
• The resulting 6-carbon compound breaks down into
  two molecules of 3-phosphoglyceric acid (PGA).
• The PGA molecules are further phosphorylated (by
  ATP) and are reduced (by NADPH) to form
  phosphoglyceraldehyde (PGAL).
• Phosphoglyceraldehyde serves as the starting
  material for the synthesis of glucose and
  fructose.
• Glucose and fructose make the disaccharide
  sucrose, which travels in solution to other parts
  of the plant (e.g., fruit, roots).
• Glucose is also the monomer used in the synthesis
  of the polysaccharides starch and cellulose.

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