Light Reaction
The Light Reactions depend upon the presence of chlorophyll, the primary light-harvesting pigment in the membrane of photosynthetic organisms. The functional components of the photochemical system are light harvesting pigments, a membrane electron transport system, and an ATPase enzyme. The photosynthetic electron transport system of is fundamentally similar to a respiratory ETS, except that there is a low redox electron acceptor (e.g. ferredoxin ) at the top (low redox end) of the electron transport chain, that is first reduced by the electron displaced from chlorophyll.
There are several types of pigments distributed among various phototrophic organisms. Chlorophyll is the primary light-harvesting pigment in all photosynthetic organisms. Chlorophyll is a tetrapyrrole which contains magnesium at the center of the porphyrin ring. It contains a long hydrophobic side chain that associates with the photosynthetic membrane. Cyanobacteria have chlorophyll a, the same as plants and algae. The chlorophylls of the purple and green bacteria, called bacteriochlorophylls are chemically different than chlorophyll a in their substituent side chains. This is reflected in their light absorption spectra. Chlorophyll a absorbs light in two regions of the spectrum, one around 450nm and the other between 650 -750nm; bacterial chlorophylls absorb from 800-1000nm in the far red region of the spectrum.
Carotenoids are always associated with the photosynthetic apparatus. They function as secondary light-harvesting pigments , absorbing light in the blue-green spectral region between 400-550 nm. Carotenoids transfer energy to chlorophyll, at near 100 percent efficiency, from wave lengths of light that are missed by chlorophyll. In addition, carotenoids have an indispensable function to protect the photosynthetic apparatus from photooxidative damage. Carotenoids have long hydrocarbon side chains in a conjugated double bond system. Carotenoids "quench" the powerful oxygen radical, singlet oxygen, which is invariably produced in reactions between chlorophyll and O2 (molecular oxygen). Some non-photosynthetic bacterial pathogens, i.e., Staphylococcus aureus, produce carotenoids that protect the cells from lethal oxidations by singlet oxygen in phagocytes.
Phycobiliproteins are the major light harvesting pigments of the cyanobacteria. They also occur in some groups of algae. They may be red or blue, absorbing light in the middle of the spectrum between 550 and 650nm. Phycobiliproteins consist of proteins that contain covalently-bound linear tetrapyrroles (phycobilins). They are contained in granules called phycobilisomes that are closely associated with the photosynthetic apparatus. Being closely linked to chlorophyll they can efficiently transfer light energy to chlorophyll at the reaction center.
All phototrophic bacteria are capable of performing cyclic photophosphorylation as described above and in Figure 16 and below in Figure 18. This universal mechanism of cyclic photophosphorylation is referred to as Photosystem I. Bacterial photosynthesis uses only Photosystem I (PSI), but the more evolved cyanobacteria, as well as algae and plants, have an additional light-harvesting system called Photosystem II (PSII). Photosystem II is used to reduce Photosystem I when electrons are withdrawn from PSI for CO2 fixation. PSII transfers electrons from H2O and produces O2, as shown in Figure 20.
Fig. 15. The cyclical flow of electrons during anoxygenic photosynthesis.