BIO 330 Cell Biology
Lecture Outline
Spring 2011
Chapter 11: Photosynthesis
I. Photosynthesis: Overview
A. Two linked processes
Energy transduction reactions (light-dependent reactions)
Electron transport system
Photophosphorylation (ATP production from light energy)
Photoreduction (NADPH production)
Carbon assimilation reactions (light-independent reactions; carbon fixation)
Calvin cycle
B. Chloroplast structure
Inner membrane
Outer membrane
Thylakoid membrane / thylakoids / grana
II. Light Harvesting
A. Photoexcitation
Photon of light is absorbed by pigment, raising it from ground state to an excited state
Resonance energy transfer: energy transfer from one pigment to an adjacent one
Photochemical reduction
Chlorophyll a and b
Porphyrin ring
B. Photosystems and light-harvesting complexes
Light harvesting complexes are associated with photosystems
Photosystem I absorption maximum is 700nm
Photosystem II absorption maximum is 680 nm
Both are required for photosynthesis to function effectively; each electron must be
photoexcited twice, once by each PS
III. NADPH Synthesis
A. Overview
Electron transfer system forms NADPH via photoreduction
Oxygen is produced as a byproduct via photolysis of water
Membrane-bound (or associated) complexes are analogous to those in respiration
B. Photosystem II is activated by 4 photons
P680 Chlorophyll passes an electron to Pheophytin, creating charge separation
Electron is passed to QA, then to Plastoquinone (QB), which picks up H+ from stroma to
be reduced to plastoquinol (QBH2) (plastoquinones are analogous to CoQ)
QBH2) passes an electron to the next complex (Cyt b6/f complex)
To replace the electron on P680 chlorophyll, water is hydrolyzed, producing one
molecule of oxygen
C. Cytochrome b6/f complex is analogous to Complex III in respiration
BIO 330 Cell Biology
Lecture Outline
Spring 2011
Cyt b accepts electrons from QBH2), then passes them to a Fe-S protein, then Cyt f, then
to Plastocyanin (analogous to Cyt c in respiration)
Moves 4 H+ to lumen
D. Photosystem I absorbs 4 photons like PSII, sends electrons to A0, then to A1
The missing electron is replaced by an electron from plastocyanin
A1 passes an electron to ferredoxin
Ferredoxin reduces NADP+ to NADH, which also uses a H+ from the stroma
E. Summary of process
8 photons + 2 H2O + 6H+stroma + 2NADP+  8 H+lumen + O2 + 2 NADPH
NADPH is produced (reducing potential)
Proton gradient is established (electrochemical energy storage)
IV. ATP Synthesis
A. CF0CF1 complex is analogous to the mitochondrial F0F1 complex
CF0 subunits I and II form stationary stalk like b of mt complex
CF0 subunit IV forms proton translocator like a of mt complex
CF0 subunits III in ring turn as in c ring of mt complex
CF1 components are similar to mt complex F1
B. Noncyclic electron flow
If ATP is needed in higher quantities than NADPH, electron flow can be diverted from PSI
into ATP synthesis instead of NADPH synthesis.
Cyclic phosphorylation
V. Carbon Fixation: The Calvin Cycle
A. Ribulose-1,5-bisphosphate + CO2  2 3-phosphoglycerate
Carboxylation
B. 3-phosphoglycerate  glyceraldehyde-3-phosphate
Reduction
ATP and NADPH utilization
C. Regeneration of Ribulose-1,5-bisphosphate
ATP utilization
VI. Carbon Fixation: Carbohydrate Synthesis
A. Triose phosphates G-3-P and DHAP are endproducts of the Calvin cycle
These are incorporated into carbohydrates for storage or used immediately for energy
B. Trioses are exported from the stroma to the cytosol
Phosphate translocator – an antiporter for Pi and trioses
C. Glucose-1-Phosphate is formed in either cytosol or stroma
Sucrose synthesis occurs in cytosol from G-1-P
Starch synthesis form in stroma from G-1-P