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Gerald Karp
Cell and Molecular Biology
Fourth Edition
Chapter 6:
Photosynthesis and the Chloroplast
Copyright © 2005 by John Wiley & Sons, Inc.
Heterotrophs (CO2, H2S),
Autotrophs,
• Chemoautotrophs: NH3, H2SO3- (only for
bacteria)
• Photoautotrophs: P.S. plant, algae, flagellated
protists, purple, green bacteria, cyanobacteria.
• 2 billions years, H2S is the e- source
• CO2 + 2 H2S → (CH2O) + H2O + 2S (sulfur
spring and deep sea)
• CO2 + H2O → (CH2O) + O2 need light
(cyanobacteria)
1881 German biologist T.
Engelmann
found the bacteria surround the
algae in order to absorp the O2
released by bacteria
6.2 An overview of
photosynthetic metabolism
• Light dependent reaction
• 2H20 → O2 + 4H+ + 4e- → NADPH, ATP
• Light independent reaction
• CO2 → CH2O (600x1012/year Kg),
• O2 (400x1012/year Kg)
6.3 The absorption of light
• 1 mole of 680 nm photon contains 42
Kcal/mole (1.8V of redox potential)
• In vitro:
• In vivo:
1. Photosynthetic pigments
• Chlorophyll:
• A. porphyrin ring : absorption of light
• B. hydrophobic phytol tail: in thylakoid
membrane
• Accessory pigments: carotenoids (capture
energy and removal excited energy)
6.4 Photosynthetic units and
reaction centers
• 1932: Emerson and Arnold of the CIT.
About three hundred chlorophyll
molecules absorbed to produce one
molecule of oxygen ----- Photosynthetic
unit
• Only one member of the group ”the
reaction-center chlorophyll” actually
transfer electrons to an electron acceptor.
1. Oxygen formation: coordinating
the action of two different
photosynthetic systems
• PSII operations: obtain electrons from
water
•
•
a. The flow of electrons from PSII to plastoquinone
b. The flow of electrons from water to PSII
• From PSII to PSI
• PSI operations: The production of NADPH
Z-scheme
ferredoxin
PC
*Electron carriers,
*proton gradient,
*ATPsynthase
From PSII to PSI
pH:8
pH:5
PS1 operations: The production
of NADPH
2001, 3D structure of PS1 from a
cyanobacterium was discovered at
2.5 A resolution.
12 polypeptides, 96 chlorophylls,
22 carotenoids and other elements
An overview of photosynthetic
electron transport
• Summary of the light-dependent reactions
Killing weeds by inhibiting electron
transport
• 1. Inhibitors of electron carries (herbicides)
• 2. Binding to a core protein of PSII ( QB).
6.5 Photophosphorylation
• Noncyclic photophosphorylation
• H2O → e- → NADP+ → NADPH, ATP
• CO2 → CH2O need 3 ATP and 2 NADPH
• Cyclic photophosphorylation
• PS1 → A0 → A1→ ferredoxin →NADPH
→ cytb6f → PC → PSI
Z-scheme
ferredoxin
PC
*Electron carriers,
*proton gradient,
*ATPsynthase
6.6 carbon dioxide fixation and
the synthesis of carbohydrate
• 1. Carbohydrate synthesis in C3 plants
•
Photorespiration and peroxisomes
• 2. Carbohydrate synthesis in C4 plants
• 3. Carbohydrate synthesis in CAM plants
Chromatogram showing the results of an experiment in
which algal cells were incubated for 5 secs in (14C)O2 prior
to immersion in alcohol.
3phosphoglycerate
Rubisco, only fix 3 molecules
CO2/sec, 10 kg for every human
Redox control of the Calvin cycle
Several key enzymes of the Calvin cycle are only active in the light when
ATP and NADPH are being produced by PS
In the light, a fraction of electrons are
transferred to the small protein
thioredoxin, which reduces the disulfide
groups of certain Calvin cycle enzymes
which become ative.