BIOL 205 :: Photosynthesis Lecture 1

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BIOL 205 Photosynthesis Lecture 1 Introduction
and the light reactions
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What is photosynthesis?

• The process of converting solar energy into
  chemical energy.
• Can use only water and carbon dioxide to create
  sugars chemical energy.
• Responsible for removal of 200 billion tons of
  C from the atmosphere yearly.

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What is photosynthesis?

• hv designates light
• you should memorize this equation!! you will see
  it again

O2 oxygenic photosynthesis
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What is photosynthesis?

• 2 stages of photosynthesis
• Light reactions and Dark reactions
• Light reactions convert sunlight into chemical
  energy (ATP NADPH)
• Dark reactions use those products to form sugars
  (stored chemical energy)

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Overview of Photosynthesis
Dark Reactions
CO2
ATP
NADPH
Light Reactions
Sugars
H2O CO2
O2
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Where does photosynthesis take place?

• Prokaryotes

• Prokaryotes have both anoxygenic and oxygenic
• Cyanobacteria have oxygenic
• -Photosynthesis on thylakoids (from plasma
  membrane)
• -Cyanobacteria are source of eukaryotic
  photosynthesis

• Eukaryotes

• Oxygenic only
• Takes place in chloroplasts

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Chloroplast Morphology
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Chloroplast Morphology-Terms

• Inner Membrane
• Outer Membrane
• Thylakoid
• Thylakoid lumen
• Stroma
• Granum

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How can light provide energy for plants?

• Light is composed of particles photons
• Light behaves like a wave
• Can e described w/ wavelength frequency
• Only a small portion of the electromagnetic
  spectrum.

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The electromagnetic spectrum
PAR photosynthetically available radiation
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Pigments

• Pigment a light absorbing molecule
• Associated with the thylakoid membranes
• Chlorophyll
• Chl a and Chl b (Chl c in some algae)
• Xanthophylls
• Carotenoids
• ß-carotene

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Chlorophyll

• Chl a has a methyl group
• Chl b has a carbonyl group

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Different pigments absorb light differently
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Different pigments absorb light differently
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2-minute quiz
1. What is the relationship between the structure
of chlorophyll and its location in the
chloroplast?
2. Why are plants green?
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Light Reactions 1 Light capture and redox

• 2 spatially functionally distinct units
  Photosystems
• Photosystem II 1st stage
• Photosystem I 2nd stage
• Named after order of discovery

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Light capture 1
1. Most Chlorophyll is located in the Light
Harvesting Complex
Chl a
Chl b
Remember the porphyrin ring?
LHC
ß-carotene
Reaction Center
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Light capture - 2
3. Energy finally ends up in a pair of special
chlorophyll a molecules P680
P680 chlorophylls

Optimal _at_ lt680 nm
LHC
Reaction Center
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Light capture - 3
e-
Q

LHC
Reaction Center
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Light capture - 4
8. The O2 evolving complex Chl a strip e- from
H2O and reduce Chl a
Chl a most powerful biological oxidizing agent

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Light capture - 5
9. The reaction center is reset and ready to go
again
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Light Capture Movie
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PSII electron transport chain
3. As the e- moves through b/f more H are pumped
into lumen
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The PS II Electron transport chain
H
H
Q
e-
e-
cytochrome
b/f
complex
e-
2H
plastocyanin
H
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PS II to PS I
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Photosystem I
e-
1. Charge separation and photo-oxidation are
similar to PSII Optimal wavelength 700nm

P700
2. Plastocyanin acts as reducing agent on P700
Chl a
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PSI electron transport chain
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PSI electron transport chain
e-
Ferredoxin Fe/S
e-
e-
FAD
H
e-
FAD-NAPD Reductase
NADP
NADPH
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ATP synthesis
1. O2 evolving complex liberates H into lumen
from water 2. Q and Cyt b/f pump H from stroma
into lumen. 3. NADP scavenges protons from the
stroma pH separation across membrane Proton
Motive Force!
H
H
stroma
lumen
H
H
H
H
H
H
H
H
H
H
H
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ATP synthesis
ADP Pi
H
F0F1 complex
ATP
stroma
H
The energy released as protons travel down their
concentration gradient is used to fuel an ATP
synthase
lumen
H
H
H
H
H
H
H
H
H
H
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Light Reactions Products
In CO2, H2O, sunlight Out O2, ATP, NADPH