Light Reaction of Photosynthesis

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Light Reaction of Photosynthesis
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• occurs in thylakoid (stacks of paper
  plates)
• thylakoids found in chloroplast

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Light enters and is absorbed by photosystem II
H2O
Light
LIGHT REACTIONS
ATP
NADPH
Chloroplast
O2
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• Photosystem II

• consist of pigments, proteins, and organic
  molecules.

act as antennas that absorb light energy
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• Photosystem II

• Photon energy is transferred from pigment to
  pigment until it reaches
  chlorophyll a

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in chlorophyll a
CH3
P I G M E N T
in chlorophyll b
CHO
Porphyrin ring light-absorbing head of
molecule note magnesium atom at center
Hydrocarbon tail interacts with
hydrophobic regions of proteins inside thylakoid
membranes of chloroplasts H atoms not shown
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Fills in lost e- from H20 being split
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to the
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Photosystem II

• Photo II aka. P680- absorbs wavelengths of light
  at 680nm

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• Light Reaction overview
• 1)pigments absorb light photon the photon is
  relayed around photo II
• 2) an e-, from chlorophyll a bounces to the next
  energy level caught by the primary e- acceptor

Solar E. --gt Chemical E.
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H2O
CO2
Light
NADP
CALVIN CYCLE
LIGHT REACTIONS
NADPH
CH2O (sugar)
O2
Primary acceptor
e
Light
P680
Photosystem II (PS II)
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• Light Reaction overview
• 3)meanwhile an enzyme splits H2O -gt H, e-, O
• an e- fills in the e- lost by the P680
  molecule
• O2 goes up your nose

Solar E. --gt Chemical E.
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H2O
CO2
Light
NADP
CALVIN CYCLE
LIGHT REACTIONS
NADPH
O2
CH2O (sugar)
Primary acceptor
e
H2O
2 H
O2
1/2
Energy of electrons
Light
P680
Photosystem II (PS II)
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• Light Reaction overview
• 4)Each photexcited e- is now passed down an e-
  transport chain
• 5) The e- fall synthesizes ATP (H
  gradient from H20 splitting)

Solar E. --gt Chemical E.
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H2O
CO2
Light
NADP
CALVIN CYCLE
LIGHT REACTIONS
ATP
H
H
H
H
O2
CH2O (sugar)
H
H
Primary acceptor
H
Electron transport chain
Pq
H
H2O
e
Cytochrome complex
2 H
O2
Pc
Energy of electrons
Light
P680
ATP
Photosystem II (PS II)
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• Light Reaction overview
• 6) chlorophyll in PS I absorbs light photon the
  photon is relayed
• an e- is bounced to the next energy level
  caught by the Primary e- acceptor

Solar E. --gt Chemical E.
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Photosystem I and II what makes them different?

• Photo II aka. P680- absorbs wavelengths of light
  at 680nm

Photo I aka. P700- absorbs wavelengths of light
at 700nm
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H2O
CO2
Light
NADP
CALVIN CYCLE
LIGHT REACTIONS
NADPH
CH2O (sugar)
Primary acceptor
Primary acceptor
Electron transport chain
Pq
e
e
H2O
Cytochrome complex
2 H
O2
1/2
Pc
P700
Light
P680
Light
ATP
Photosystem I (PS I)
Photosystem II (PS II)
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• FYI
• P680 and P700 are identical chlorophyll a
  molecules.
• difference in light absorption comes from
  association with different proteins in the
  thylakoid membrane.

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• Light Reaction overview
• 7) e-s passed down the second chain
• 8) 2 e-s hop on an NADP bus and form NADPH

Solar E. --gt Chemical E.
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NADPH
off to the dark side
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• Photophosphorylation - Light energy
  generates ATP

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2 H
2 H
2 H
2 H
2 H
2 H
2 H
2 H
2 H
2 H
2 H
2 H
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• Purpose of Light Reactions (revisited)
• Generate ATP
• Generate NADPH
• these will provide E. reducing power to the
  Calvin Cycle.

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Light Reaction
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e- carrier
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e- carrier
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via. ATP synthase
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e
ATP
e
e
NADPH
e
e
e
Mill makes ATP
Photon
e
Photon
Photosystem II
Photosystem I
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Go to the dark side
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• Cyclic Electron Flow-
• occurs under condition where there is not enough
  ATP for Calvin Cycle.

• Uses photosystem I but not II.

• No production of NADPH

• No production of O2

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• Cyclic Electron Flow-
• Rise in NADPH (b/c Calvin Cycle consumes more
  ATP)
• triggers this process, goal to make ATP catch
  up with NADPH.
• Side steps Photo II (P680).

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Primary acceptor
Fd
Cytochrome complex
Pc
Photosystem I
ATP
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• Cyclic Electron Flow-
• Excited electrons from Photo I are passed by
  electron acceptor to Fd (ferrodoxin).
• Then passed to ETC to make ATP .
• No NADPH is made and there is no release of O2.