Photosynthesis is the reduction of carbon dioxide into biomass using energy derived from the sun, solar energy. The way in which organisms ultimately convert this light energy into chemical energy for growth is referred to as phototrophy. All photosynthetic organisms are phototrophic, however not all phototrophic organisms are photosynthetic, since many of them are also able to harvest and process other forms of food, in ways which are independent of light. When photosynthesis is carried out in the presence of oxygen, it is referred to as oxidative photosynthesis; when carried out without oxygen it is referred to as anoxygenic photosynthesis. Eukaryotes, which are the higher plants and algae, and some prokaryotes such as cyanobacteria, all perform oxidative photosynthesis; some other bacteria, such as purple bacteria produce energy via anoxygenic photosynthesis. The main principles of energy transduction are however the same in both methods. In all organisms, the basic principles of photosynthesis are that photons are absorbed by light-harvesting complexes (LHCs), which results in transfer of excitation energy from LHCs to reaction centres (RCs), and then a primary charge separation across the photosynthetic membrane (Hu et al., 2002).
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All evidence which has been collected to date suggests that (bacterio) chlorophyll-based photosynthesis first arose in prokaryotic bacteria, and was then transferred across to eukaryotes via endosymbiotic transfer. It is currently believed that chloroplasts, along with mitochondria, were once bacteria in their own right, since they carry out many functions independent to the rest of the cell. It is believed that they first entered the cells of eukaryotic organisms forming a symbiotic relationship, and have since evolved to become the organelle that is present in cells of eukaryotes today (Raymond et al., 2002). If this is the case, then it would suggest that there would be many similarities in the equipment and methods used for light-harvesting by both eukaryotes and prokaryotes, and also similarities in the way that the light is used. However it is inevitable that there will also be differences, since prokaryotes are single celled organisms, whereas eukaryotes are usually many-celled, and so the method of light-harvesting and use would be expected to be far more complex.
Overview of light-dependent photosynthesis in eukaryotes
The light dependent part of photosynthesis in plants takes place in the grana region of the chloroplast. The thylakoid membranes found in the grana contain a cytochrome transport system through which electrons flow, generating ATP. When light hits a chlorophyll molecule, the energy is absorbed by an electron; this energy boosts the electron up to a higher energy level. Some parts of the chlorophyll membranes form light-trapping antennae, which then channel these excited electrons to an area known as the action centre, where they are passed onto protein embedded in the thylakoid membrane, and are channeled into the electron carrier system. During the light dependent reactions of photosynthesis, NADP picks up two hydrogen atoms when water is split, forming NADPH2; this compound is used to convert carbon dioxide to glucose during the Calvin Cycle, or the light-independent reactions of photosynthesis. Since the NADP takes hold of the two hydrogen atoms from water, oxygen is given off as a waste product, and the NADPH2 and ATP which are generated in the grana then pass forward to the stroma for the light-independent reactions to take place (Armstrong, 2001). All eukaryotes are currently believed to function in an identical manner, and they are believed to have identical structures inside their chloroplasts. There are therefore no problems with eukaryotes in the fact that there re only certain species which have been studied in great depth.
Overview of light-dependent photosynthesis in prokaryotes
There are two different mechanisms for collecting light energy that are currently known of. The first of these methods uses rhodopsins, which are retinal-binding proteins that respond to light stimuli. There are various homologous types of rhodopsins which are known in microbes, including energy-conserving transmembrane proton pumps€ (bacteriorhodopsin, proteorhodopsin, xanthorhodopsin), transmembrane chloride pumps (halorhodopsins), and light sensors (sensory rhodopsins). This is the simplest method of phototrophy, in which light energy directly drives proton expulsion from cells through bacteriorhodopsins and proteorhodopsins. This creates a force that can be used in conjunction with ATP synthase to produce ATP, or can alternatively be used to push forward various other transport processes. Since these reactions do not mediate electron transfer reactions, organisms that use these rhodopsins are phototrophic, but have not yet been shown to be photosynthetic.
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The second method is dependent upon photochemical reaction centres that contain bacteriochlorophyll. In this method of phototrophy, redox reactions are used to power the synthesis of ATP; light initiates electron transfer through oxidation of a chlorophyll molecule and reduction of an electron acceptor. Secondary electron transfer reactions that do not require light create the proton-motive force that can be combined with the redox reaction to result in ATP synthesis. This mechanism is dependent upon photochemical reaction centres (RCs), which are (B) Chl-containing proteins. There are two variations of these RCs, although they have similar structures, and they have been shown to have evolved from a common ancestor
There are five prokaryotic phyla which are currently known to be photosynthetic: Cyanobacteria, proteobacteria, green sulfur bacteria, green filamentous bacteria and grampositive heliobacteria (Raymond et al., 2002). Little is currently known about the detailed workings of photosynthesis in the majority of these phyla, with the majority of prokaryotic studies having so far been carried out on cyanobacteria and purple bacteria. While it is recognized that the unstudied bacteria may be somewhat different to the bacteria that we have studied, from the current knowledge of their structures and function it can be seen that there are many similarities, and it is therefore assumed that photosynthesis will be very similar, if not identical between the prokaryotic phyla.
Excited electrons from chlorophyll are captured by Photosystem II, which is a series of electron carrier proteins embedded in the thylakoid membrane. The electron carriers then use the energy that this generates to pump hydrogen ions into the thylakoid space, where the diffuse back through ATPase; the flow of these protons is used to generate ATP. The partially spent electron is then channeled to a second reaction centre, Photosystem I, where another photon of energy boosts the electron to an even higher energy level. The electrons then have enough energy to be used in the production of NADPH. Photosystem II contains all of the proteins, pigments and cofactors necessary for light-driven movement of electrons from water to reduced plastoquinone. There are three Chl-protein complexes in the core - the reaction centre where the initial charge separation occurs, and two Chl a light harvesting antennae. Aside from their role in light capture, these antennae are also believed to contribute to the protein environment of the water-splitting apparatus (Green and Durnford, 1996).
Similarities and differences between eukaryotes and prokaryotes
Structure of the photosynthetic unit (PSU)
In all photosynthetic organisms there are two regions for photosynthesis reactions, Photosystem I (SI) and Photosystem II. The way in which these are arranged is believed to be similar in both eukaryotes and prokaryotes. In chloroplasts found in eukaryotes, the thylakoid membrane is continuous, but is divided into two distinct regions, the grana stacks and the stroma lamellae. Most of the PSII regions are concentrated on the grana region, while the PSI regions are concentrated on the stroma (Green and Durnford, 1996). In the purple bacteria, the most closely studied of the prokaryotic cells; the PSU is composed of two pigment protein complexes, called LH2 and the RC-LH1 core complex. These are essentially the same as PSI and PSII. These are both integral membrane proteins, and their structures are quite similar. They both consist of a pair of low molecular weight apoproteins which combine to a small number of Bchl a and carotenoid molecules.
Photosynthetic organisms produce a variety of light-harvesting antenna structures to increase the efficiency of light-dependent electron transport. Examples of these structures include phycobilisomes, chlorosomes and a variety of light-harvesting (B)Chl and carotene-(B)Cl proteins (Bryant and Frigaard, 2006). The major light harvesting antenna complex of higher plants (LCHII) is found in both regions of the thylakoid membrane, and so can transfer energy to both PSII and PSI. It has been found that a subpopulation of LHCII can move from grana to stroma, depending on light quality, redox potential and phosphorylation (Green and Durnford, 1996). In prokaryotes, the LH2 cannot transfer energy directly to the RC, but must transfer it through LHI (Codgell et al., 2004); it is unclear if this is also the case for eukaryotes. In the majority of cyanobacteria, there is an antenna complex associated with Photosystem II (PSII) reactions. This complex consists of phycobiliproteins, which are a family of water-soluble polypeptides, and these covalently bind to chromophores, which are pigments. The phycobiliproteins associate with linker polypeptides and form a supramolecular antenna on the outer surface of the intracytoplasmic membranes, known as a phycobilisome (Ting et al., 2002).
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There are known to be many other structures for antennae, all of which are designed for different specific frequency absorptions. Another example is that found in Prochlorococcus, in which the antenna proteins bind to different pigments that are present in the organism and so are better at absorbing blue wavelengths of light than green. Since the suite of pigments found in this bacteria are unique, it is likely that the exact structure of the antennae present are also likely to be unique, although this is difficult to ascertain since we still know very little about the composition of many prokaryotes (Ting et al., 2002). In some other species of bacteria there have been shown to be a third type of light-harvesting antenna that is structurally different to LHI and LHII, although little else is currently known about it (Hu et al., 2002). This means that it is perfectly viable that there may be other variations present in species of bacteria which we have not yet studied in detail.
In prokaryotes it is known that a variety of environmental factors control the size and composition of the PSU, obviously with light intensity being the prime determinant. There is currently evidence to suggest that the antennae may even be organized differently in different parts of the photosynthetic membrane (Codgell et al., 2004). Again, it is unclear whether this is the same fro eukaryotes.
A bacteriochlorophyll molecule a (Bchl a) is excited by the absorption of a photon, to its first exited state. It can remain like this for several nanoseconds, so the process is more efficient if two photons can be absorbed within a very short space of time, since the Bchl a will be pushed up to a second level of excitement. The physics of light harvesting is not particularly stringent, resulting in a wide variety of light harvesting antennae being present in different organisms. The versatility of light harvesting systems is essential to allowing a species of organism to proliferate in widely different environments; for example cyanobacteria have a very versatile light-harvesting system which has been shown to undergo several stages of evolution in some varieties of cyanobacteria. This is what leads that phylum of bacteria to be so successful to adapting to the many different environments in which it can be found. The physics of electron transport is very strict however, so the structure of reaction centres in which this is initiated is not particularly variable; the structures if RCs in most organisms is very similar, with few differences.
PSI and PSII
Photosystem I and Photosystem II are two multi-subunit membrane protein complexes which drive photosynthesis in plants, and indeed in all oxygenic photosynthesis. Both of these complexes are composed of a reaction centre (RC) and a peripheral antenna (Nugent and Evans, 2004). Light-induced charge translocation occurs in the RC and the antenna is responsible for absorbing light and channeling it to the RC. The peripheral antenna of PSI is known as LHCI and that of PSII is LHCII (Bem-Shem et al., 2004).
PSI in plants is much larger than PSI in cyanobacteria, and coordinates many more cofactors. LHCI gathers solar energy and channels it to the RC. The core complex in plant PSI contains more than 100 chlorophyll molecules. The vast majority of them are positioned almost exactly the same as those in cyanobacterial PSI (Nugent and Evans, 2004). The PSI in green-sulfur bacteria and heliobacteria is different, as it is composed of a single peptide, with only around 20 chlorophyll molecules; this makes it far less efficient, and will result in a lower yield of photons to the electron translocation process. Weak interaction between the molecules in LHCI in plants allows for the composition to be adjusted according to environmental conditions, such as levels of light (Bem-Shem et al., 2004). In Photosystem I, the reaction centre and internal antenna are combined in a single Chl-protein complex that accepts electrons from a plastocyanin or a cytochrome and delivers them on to a different molecule for continued electron transfer (Green and Durnford, 1996).
LHCI and LHCII
LHC I proteins are unique in the family of LHC proteins in their ability to capture photons at long wavelengths, allowing them to absorb light from the red end of the spectrum. This is due to a set of chlorophylls found at their periphery, termed 'red chlorophylls', which are not present on the other members of the family, such as LHCII. This is particularly important in areas of dense vegetation, since around 40% of absorbed light in that situation would come from the low wavelength end of the spectrum. These units are highly efficient, as almost every single photon which is absorbed will eventually be trapped and will result in electron translocation. Recent research has also suggested that up to 80% of photons are only captured by the antennae after being transiently captured by the 'red chlorophylls' which suggests that they play a vital role in overall light harvesting. It is currently believed that these 'red traps' are fundamental in maintaining the highly efficient yields which are currently found in LHCI (Bem- Shem et al., 2004).
It has been found in cyanobacteria also that these 'red traps' occur, however in cyanobacteria it has been shown that the red chlorophylls not only act to trap light of longer wavelengths, but also act in a protective manner under high light levels (Bem-Shem et al., 2004). However it has also been shown that the number of 'red traps' in cyanobacteria varies from species to species, and since knowledge is currently limited to the few species which have been cultivated for study, there is no guarantee that this will be the same for all species, since there have already been shown to be other subtle differences in light-harvesting in some species of these prokaryotes. Bahatyrova et al. (2004) highlighted that although we currently know much about the structure of photosynthetic membranes, we still have much to learn about the size and organizational them; as can be seen in many other areas of biochemistry, simply because the composition between the photosynthetic equipment in eukaryotes and prokaryotes is similar, it does not mean that the organization will be the same. Therefore, it is important to avoid too much assumption before the organization as been studied further.
LHCII has a more rigid arrangement than LHCI; in place of weak interactions are extensive hydrophobic interactions, which allow for little variation. The entire structure of LHII has still not been elucidated however, so there is far less knowledge about the structure and functioning of LHCII than LHCI.
The role of pigments
The pigment proteins of photosynthesis are responsible for the absorption of light, and the first steps toward its conversion to chemical energy. All of the pigment proteins are capable of bonding both chlorophylls and carotenoids, with chlorophylls performing the majority of the light harvesting, and carotenoids protecting against excess light energy, preventing the excitation of too many electrons. In some marine organisms, where the levels of light are significantly less, carotenoids have also been found to contribute toward the absorption of light. There are two main categories of pigment proteins. The first is Chl a, which is the type of pigment protein found in cyanobacteria and eukaryotic chloroplasts. The second type of pigment protein is the type found in light-harvesting antennae in eukaryotes; the type actually consists of a variety of different Chls, all of which could be bound in the antennae, along with Chl a (Green and Durnford, 1996).
The main light absorbing pigments in prokaryotes are bacteriochlorophyll a and b, and carotenoids. These pigments are noncovalently bound to integral proteins to form either reaction centres or antenna complexes. The complete structure and organization of how the reaction centres and antenna complexes are arranged are still unknown.
Photosynthetic eukaryotes are divided into three groups based on their light-harvesting pigments. The first group is the chlorophytes, which is composed of green algae and higher plants. These eukaryotes have antennae containing Chl a/b. Some marine green algae have been found to contain different carotenoids which enhance light-harvesting capacity. The second group are the chromophytes, which have antennae containing Chl a/c. They have a higher carotenoid ratio, which is believed to enhance absorption significantly from the green region of the spectrum. The last group is the rhodophytes which is composed solely of the red algaes. This group uses phycobilisomes as the major PSII antenna; for this reason it is believed to be more recently evolved from a cyanobacterial ancestor than the other eukaryotes (Green and Durnford, 1996)
Types of light energy absorbed
The wavelength of light absorbed by prokaryotic and eukaryotic organisms is of a different wavelength. Photosynthetic bacteria absorb different wavelengths depending on their energy needs. Purple bacteria absorb light at a wavelength of approximately 500nm through carotenoids, and 800nms through BChls. Pigments of outer LHCs absorb at higher frequencies than inner ones. This is possibly due to the fact that the outer LHCs must pass the energy further, since they must transfer to the RCs through the inner LHCs; if they absorbed lower energy light it would not have enough energy to survive the distance. Green plants absorb in the majority of the visible spectrum of light, with the exception of greens, and also the longest wavelengths of light in the far-infrared end of the spectrum. In particular, chlorophyll itself, which is where photosynthesis occurs in plants, absorbs the red and blue, and then the energy which is contained in these specific wavelengths is fixed during photosynthesis (Howell, 1994).
Based on current knowledge it can be seen that although there are some differences between the structure and function of light-harvesting equipment in prokaryotes and eukaryotes, there are not nearly as many differences as could be expected, considering the vast difference in size and complexities of the organisms.
Much of the current knowledge as to the structure and function of prokaryotic light-harvesting equipment comes from the study of purple bacteria (Cogdell, 1999). However it must be acknowledged that while these make an excellent model to study, there is no guarantee that other types of photosynthetic bacteria are composed in the same way, or that the way in which they function is the same. This is especially pertinent since the very method of photosynthesis employed may be different to purple bacteria; some bacteria use anoxygenic photosynthesis, as is the case with purple bacteria, while others photosynthesise in the presence of oxygen. Photosynthetic bacteria are found in a much wider variety of habitats than photosynthetic eukaryotes, and it is known that they have been in existence for far longer. Therefore it is likely that at least some species will have evolved independently of others, due to the environmental conditions in which they are present. This makes it very likely that certain species will have equipment that is varied from what we currently recognize. Although it has been shown that certain elements of the light-harvesting equipment in prokaryotes is structured rigidly due to the process in which it is involved, there are other parts that have demonstrated variation even between the few bacteria that have so far been studied in depth. Therefore it is likely that some parts of the light-harvesting equipment may be the same, while other parts have changed. In order to fully understand the differences between prokaryotic and eukaryotic organisms, the differences within prokaryotes must first be understood, so it is important to examine the way in which light is harvested and used in bacteria from a wide range of conditions. While the differences between prokaryotes and eukaryotes are important, since they offer explanations as to other processes which may be different within the organism, the similarities between the two are also extremely important. This is since if we can understand the similarities between the two, and how eukaryotes systems have evolved from prokaryotes, it may go further to proving the hypothesis that eukaryotic systems originally arise from a symbiotic relationship, and may go some way to helping us to understand evolution on a broader level.