1. Chloroplast Organization
Impermeable to most ions & molecules
Highly permeable to small ions & molecules
Site of photosynthetic machinery
~50:50 lipid:protein ratio
Impermeable to most ions & molecules
Contains soluble enzymes that use NADPH & ATP to convert CO2 to sugar

2. Overview of Photosynthesis

3. Some Chromophores of Photosynthetic Systems
(Mg2+-free BChl-b)
Chlorophyll a
High absorption (extinction) coefficients:  ~ 105 M-1cm-1
Beer’s Law: Absorption = (path length)(concentration) = bC

4. Multiple Fates of Photonic Energy Absorbed by Chromophores
Electronic Excitation
decay by
giving off
light or heat 1
Possible Decay Pathways
Decay of Excitation via Electron Transfer

5. An Oxidation-Reduction Pair of Photosynthetic Systems

6. Bacterial Photoreaction Center
Direction of
electron flow
4 polypeptide components:
L, 31 kD (shown in red)
M, 36 kD (shown in blue)
H, 28 kD (shown in white)
C, c-type cytochrome(shown in yellow)
maximal absorption
@ 960 nm (a.k.a. P960)
~10 Å
Rate (electron transfer) ~ 1/r6
Sequence analysis & low-resolution structures of PSI & PSII show that the bacterial reaction center is analogous to these more complex plant photosystems

7. Photoinduced Charge Separation Process in the
Bacterial Photoreaction Center

8. Kinetics of Photoinduced Charge Separation in the
Bacterial Photoreaction Center
special pair
chlorophyll
pheophytin
tightly-bound
quinone (QA)
exchangeable
quinone (QB)
free
quinone
Factors contributing to efficient photoinduced charge separation in the forward direction:
The distance between BPh- and P960+ is large and energy of charge recombination is so thermodynamically-favorable it is in the slow regime of electron transfer rates
The small distance (< 10Å) between BPh- and the tightly-bound quinone helps to accelerate the forward electron transfer away from P960+, thereby maximizing the distance of charge separation across a medium of low dielectric
The small distance (< 10Å) between P960+ and the bound cytochrome helps to rapidly back-fill the electron “hole” before it has an opportunity to become reduced by BPh- or any other energy-wasting back electron transfer
Biological control of space and time to achieve thermodynamically unfavorable reaction