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

Overview of Photosynthesis

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

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

An Oxidation-Reduction Pair of Photosynthetic Systems

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

Photoinduced Charge Separation Process in the Bacterial Photoreaction Center

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