Lecture 6 - Photosynthesis: The Light Reactions
Chem 454: Regulatory Mechanisms in Biochemistry University of Wisconsin-Eau Claire
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Introduction
Photosynthesis represents for the biosphere Text the major source of Free energy (1017 kcal/year stored) Carbon (1010 tons/year assimilated) Oxygen
CO2 + H2O Light (CH2O) + O2 carbohydrate
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Overview
Analogous to the combination of oxidative phosphorylation and the citric acid cycle.
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1. Chloroplasts
Like oxidative phosphorylation, Text photosynthesis is compartmentalized
4 4 1 Mitochondria
Mitochondria are bound by a double membrane
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2. Electron Transfer
Light absorption by Text chlorophyll induces electron transfer
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2. Electron Transfer
Absorption of light leads to photoinduced Text charge separation
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2. Electron Transfer
Absorption of light leads to photoinduced Text charge separation
8 8 2.1 Bacteria vs. Green Plants
Plants have two photosystems Photosystem I 13 polypeptides chains 60 Chlorophylls 3 4Fe-4S centers 1 Quinone Photosystem II 10 polypeptide chains 30 Chlorophylls 1 Non-heme Fe 4 Mn+2 (Mn+2, Mn+3, Mn+4 , Mn+5) 9 9
2.1 Bacteria vs. Green Plants
Bacteria have a single system 4 Bacteriochlorophylls b 2 Bacteriopheophytin b 2 Quinones 1 Fe2+
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2.1 Bacteria vs. Green Plants
Bacteria have a single system
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2.2 The Bacterial Photosynthetic Reaction Center
12 12 2.3 Electron Transfer
The rate of electron transfer is dependent up two factors: Distance Driving Force
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3.3 Q-Cytochrome c Oxidoreductase
The Q cycle:
QH + 2 Cyt c + + + 2 ox 2 H matrix Q + 2 Cyt cred + 4 H cytosol
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3. Two Photosystems of Plants
In green plants there are two photosynthetic Text reaction centers.
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3.1 Photosystem II
The core of PSII is similar to the bacterial system.
16 16 3.1 Photosystem II
2 Q + 2 H2O O2 + 2 QH2
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3.1 Photosystem II
Four photons are required to generate one oxygen molecule
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3.1 Photosystem II
The 4 Manganese center is where the electrons are extracted from the water.
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3.1 Photosystem II
An equivalent of 4 H+ are moved across the thylakoid membrane by PSII
20 20 3.2 Cytochrome bf
The cytochrome bf complex transfers the electrons from plastoquinone to platocyanin:
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3.2 Cytochrome bf
2+ + + 2 QH2 + 2 Pc(Cu ) Q + 2 Pc(Cu ) + 2 H thylakoid lumen
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3.3 Q-Cytochrome c Oxidoreductase
The Q cycle:
QH + 2 Cyt c + + + 2 ox 2 H matrix Q + 2 Cyt cred + 4 H cytosol
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3.3 Photosystem I
Though larger, the core of PSI is also similar to the bacterial system.
24 24 3.3 Photosystem I
The receptor of the the electrons from PSI is the small one-electron redox protein, Ferredoxin.
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3.3 Photosystem I
+ 2+ Pc(Cu ) + Fdox Pc(Cu ) + Fdred
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3.3 Photosystem I
The Z-scheme of photosynthesis
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3.4 Ferredoxin-NADP+ Reductase
Ferredoxin-NADP+ reductase
28 28 3.4 Ferredoxin-NADP+ Reductase
Ferredoxin-NADP+ reductase contains the coenzyme FAD.
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3.4 Ferredoxin-NADP+ Reductase
Ferredoxin-NADP+ reductase contains the coenzyme FAD.
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4. The Proton Gradient
PSII, Cytochrome bf and ferredoxin-NADP+ Text reductase each contribute to the proton gradient.
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4. The Proton Gradient
André Jagendorf's Text demostration 1966, was one of the earliest pieces of evidence to support Peter Mitchell's chemiosmotic hypothesis.
32 32 4.1 ATP Synthase
The ATP synthase closely resembles the ATP synthase of mitochondria
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4.1 ATP Synthase
Comparing photosynthesis to oxidative phosphorylation:
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4.2 Flow of Electrons Through Photosystem I Normally photosynthesis produces both ATP and NADPH.
When there is not NADP+ available, cyclic flow of electrons through PSI can be used to produce ATP without reducing NADP+
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4.2 Flow of Electrons Through Photosystem I Cyclic flow of electrons through PSI:
36 36 4.3 Stoichiometry of Photosynthesis
+ + 2 H2O + 2 NADP + 10 H stroma
+ O2 + 2 NADPH + 12 H lumen
12 protons are expected to be used for one turn of the ATP synthase, therefore, producing 3 AtP's from 3ADP's and 3Pi's
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