Photosynthesis is the conversion of light energy into chemical energy that can be used in the formation of cellular material from CO₂. Photosynthesis is a type of metabolism separable into a catabolic and anabolic component. The catabolic component of photosynthesis is the light reaction, wherein light energy is transformed into electrical energy, then chemical energy. The anabolic component involves the fixation of CO₂ and its use as a carbon source for growth, usually called the dark reaction. In photosynthetic procaryotes there are two types of photosynthesis and two types of CO₂ fixation.

The Light Reactions depend upon the presence of chlorophyll, the primary light-harvesting pigment in the membrane of photosynthetic organisms. Absorption of a quantum of light by a chlorophyll molecule causes the displacement of an electron at the reaction center. The displaced electron is an energy source that is moved through a membrane photosynthetic electron transport system, being successively passed from an iron-sulfur protein (X ) to a quinone to a cytochrome and back to chlorophyll (Figure 16 below). As the electron is transported, a proton motive force is established on the membrane, and ATP is synthesized by an ATPase enzyme. This manner of converting light energy into chemical energy is called cyclic photophosphorylation.

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.

The chlorophylls are partially responsible for light harvesting at the photochemical reaction center. The energy of a photon of light is absorbed by a special chlorophyll molecule at the reaction center, which becomes instantaneously oxidized by a nearby electron acceptor of low redox potential. The energy present in a photon of light is conserved as a separation of electrical charge which can be used to generate a proton gradient for ATP synthesis.

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 nonphotosynthetic 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.

Figure 17. The distribution of photosynthetic pigments among photosynthetic microorganisms.

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.

Most of the phototrophic procaryotes are obligate or facultative autotrophs, which means that they are able to fix CO₂ as a sole source of carbon for growth. Just as the oxidation of organic material yields energy, electrons and CO₂, in order to build up CO₂ to the level of cell material (CH₂O), energy (ATP) and electrons (reducing power) are required. The overall reaction for the fixation of CO₂ in the Calvin cycle is CO₂ + 3ATP + 2NADPH₂ ----------> CH₂O + 2ADP + 2Pi + 2NADP. The light reactions operate to produce ATP to provide energy for the dark reactions of CO₂ fixation. The dark reactions also need reductant (electrons). Usually the provision of electrons is in some way connected to the light reactions. A model for coupling the light and dark reactions of photosynthesis is illustrated in Figure 21 below.

The general scheme for finding electrons for CO₂ fixation is to open up Photosystem I and remove the electrons, eventually getting them to NADP which can donate them to the dark reaction. In bacterial photosynthesis the process may be quite complex. The electrons are removed from Photosystem I at the level of a cytochrome, then moved through an energy-consuming reverse electron transport system to an iron-sulfur protein, ferredoxin, which reduces NADP to NADPH₂. The electrons that replenish Photosystem I come from the oxidation of an external photosynthetic electron donor, which may be H₂S, other sulfur compounds, H₂, or certain organic compounds.

In plant photosynthesis, the photosynthetic electron donor is H₂O, which is lysed by photosystem II, resulting in the production of O₂. Electrons removed from H₂O travel through Photosystem II to Photosystem I as described in Figure 20 above. Electrons removed from Photosystem I reduce ferredoxin directly. Ferredoxin, in turn, passes the electrons to NADP.

The differences between plant and bacterial photosynthesis are summarized in Table 6 below. Bacterial photosynthesis is an anoxygenic process. The external electron donor for bacterial photosynthesis is never H₂O, and therefore, purple and green bacteria never produce O₂ during photosynthesis. Furthermore, bacterial photosynthesis is usually inhibited by O₂ and takes place in microaerophilic and anaerobic environments. Bacterial chlorophylls use light at longer wave lengths not utilized in plant photosynthesis, and therefore they do not have to compete with oxygenic phototrophs for light. Bacteria use only cyclic photophosphorylation (Photosystem I) for ATP synthesis and lack a second photosystem.

Table 6. Differences between plant and bacterial photosynthesis

	plant photosynthesis	bacterial photosynthesis
organisms	plants, algae, cyanobacteria	purple and green bacteria
type of chlorophyll	chlorophyll a  absorbs 650-750nm	bacteriochlorophyll  absorbs 800-1000nm
Photosystem I (cyclic photophosphorylation)	present	present
Photosystem I (noncyclic photophosphorylation)	present	absent
Produces O2	yes	no
Photosynthetic electron donor	H2O	H2S, other sulfur compounds or  certain organic compounds

While photosynthesis is highly-evolved in the procaryotes, it apparently originated in the Bacteria and did not spread or evolve in Archaea. But the Archaea, in keeping with their unique ways, are not without representatives which can conduct a type of light-driven photophosphorylation. The extreme halophiles, archaea that live in natural environments such as the Dead Sea and the Great Salt Lake at very high salt concentration (as high as 25 percent NaCl) adapt to the high-salt environment by the development of "purple membrane", actually patches of light-harvesting pigment in the plasma membrane. The pigment is a type of rhodopsin called bacteriorhodopsin which reacts with light in a way that forms a proton gradient on the membrane allowing the synthesis of ATP. This is the only example in nature of non photosynthetic photophosphorylation. These organisms are heterotrophs that normally respire by aerobic means. The high concentration of NaCl in their environment limits the availability of O₂ for respiration so they are able to supplement their ATP-producing capacity by converting light energy into ATP using bacteriorhodopsin.