downhill, progressing from pigments that absorb at shorter wavelengths (higher energy) to ones with longer wavelength absorbance bands (lower energy) (34). Photo-synthetic electron transfer is initiated when the excitation energy reaches P680 or P700. The energy transfer from the light-harvesting complexes to the reaction centers is so efficient that more than 90% of the captured energy is passed on to the reaction center within a few hundred picoseconds.
The anoxygenic purple bacteria capture light with the help of bacteriochlorophyll (bChl) (22), which absorbs in the infrared (>700 nm), green (570 nm), and UVA (320400 nm) spectral regions (17). These bacteria have different types of light-harvesting complexes and other mechanisms of regulating resonance transfer. However, their pigments still pass excitation energy down a gradient to the reaction center bChls. Like in plants, most anoxygenic bacteria contain two types of light-harvesting complexes:
Figure 3. (a) Molecular model of chlorophyll a and h. (b) absorbance spectra of chlorophyll a and b.
light-harvesting complex I (LH I) and light-harvesting complex II (LH II) (35).
The ATP synthase is the only major thylakoid membrane complex not directly involved in electron transport and does not contain chlorophyll (Fig. 1). It is a multisubunit protein complex consisting of two domains. CF0 is integral to the membrane and contains a proton channel. CF1 is extrinsic to the membrane and catalyzes ATP synthesis. ATP synthase utilizes the proton gradient across the thylakoid membrane during the light reaction to synthesize ATP (36). This chemiosmotic process of ATP synthesis in chloroplasts is very similar to the system mitochondria employ to generate ATP. This reaction in chloroplasts is called photophosphorylation and can be summarized as follows:
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