BIOL 205 :: Photosynthesis Lecture 1Introduction and the light reactions
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.
What is photosynthesis?
6CO2 + 12 H2O + hv C6H12O6 +6O2 +6H2O
•hv designates light•you should memorize this equation!! you will see it again
O2 = oxygenic photosynthesis
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)
Overview of Photosynthesis
Light ReactionsH2O
+CO2
NADPH
ATP
Dark ReactionsCO2
Sugars
O2
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
Chloroplast Morphology
Chloroplast Morphology-Terms
• Inner Membrane
• Outer Membrane
• Thylakoid
• Thylakoid lumen
• Stroma
• Granum
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.
The electromagnetic spectrum
PAR = photosynthetically available radiation
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
Chlorophyll
•Chl a has a methyl group •Chl b has a carbonyl group
Porphyrin ring delocalized e-
Phytol tail
Different pigments absorb light differently
Different pigments absorb light differently
2-minute quiz
1. What is the relationship between the structure of chlorophyll and its location in the chloroplast?
2. Why are plants green?
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
Light capture 1
LHCReaction Center
1. Most Chlorophyll is located in the Light Harvesting Complex
2. Sunlight is absorbed in the LHC and is passed from pigment to pigment
Remember the porphyrin ring?ß-carotene
Chl b
Chl a
Light capture - 2
LHCReaction Center
P680 chlorophylls*
3. Energy finally ends up in a pair of special chlorophyll a molecules: P680
4. e- in P680 Chl a goes to excited state and is shed = Charge separation
e-*
Optimal @ <680 nm
Light capture - 3
LHCReaction Center
+
e-*Q
5. High-energy e- accepted by quinone Q = primary e- acceptor
6. Q has been reduced; P680 Chl a has been oxidized
Oxidation by light = photo-oxidation
e- transport chain
7. The excited e- is shunted into the electron transport chain
Light capture - 4
H2O 1/2 O2
4H+
O2 evolving complex
8. The O2 evolving complex + Chl a+ strip e- from H2O and reduce Chl a+
Chl a+ = most powerful biological
oxidizing agent
e-
+
Light capture - 5
9. The reaction center is reset and ready to go again
Light Capture Movie
PSII electron transport chain
Qe- e-
H+ H+1. Q accepts 2 e- from P680 and removes 2 H+ from the stroma
2. Q passes the e- to cytochrome b/f complex & pumps the 2H+ into thylakoid lumen
b/f
cytochrome
complex
e- e-
3. As the e- moves through b/f more H+ are pumped into lumen
The PS II Electron transport chain
Qe- e-
H+ H+
2H+
b/f
cytochrome
H+
complex
plastocyanin
e-
PS II to PS I
plastocyanin
e- 4. e- end up on plastocyanin: a soluble electron carrier in the lumen
plastocyanin
e-
5. Plastocyanin serves as e- donor for PSI reaction center Chl a
Photosystem I
P700
e-*
+
plastocyanin
e-
1. Charge separation and photo-oxidation are similar to PSII
Optimal wavelength = 700nm
2. Plastocyanin acts as reducing agent on P700 Chl a
PSI electron transport chaine-Ferredoxin
Fe/S1. Ferredoxin recieves e- from P700*
FAD
NADP+
FAD-NAPD Reductase
H+
e-
2. e- moves throuh FAD (flavin adenine dinucleotide)
3. e- plus stromal H+ are used to reduce NADP+ to NADPH
NADPH = FINAL PRODUCT!
PSI electron transport chaine-
FerredoxinFe/S
FAD
NADP+
FAD-NAPD Reductase
H+
NADPH
e-e-
e-
ATP synthesis
stroma
lumen
H+
H+
H+
H+H+
H+ H+
H+H+
H+
H+
H+H+ 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!
ATP synthesis
stroma
lumen
H+
H+
H+
H+H+
H+ H+
H+H+
H+
H+
F0F1
complex
H+
ADP + Pi
ATP
The energy released as protons travel down their concentration gradient is used to fuel an ATP synthase
Light Reactions Products
In: CO2, H2O, sunlight
Out: O2, ATP, NADPH