Photosynthesis, now and at the beginning. Purple and green bacteria. Cyanobacteria (blue-green algae). Why do these tiny creatures use photosynthesis? How does it work? And how complex is it? Why has it arisen? What was needed, to think it out and to make it? What have scientists found out now about this? If you, dear reader, do not understand here all the technical details, do not worry about it. This will only help us the more, to find out, how much the tiny bacterium knows about physics, chemistry, electronics, and information processing.
Prof. T. D. Brock and M. T. Madigan report: "One of the most important biological processes on earth is photosynthesis, the conversion of light energy into chemical energy. Most phototrophic organisms are autotrophs, capable of growing on CO2 as sole carbon source. Energy from light is thus used in the reduction of CO2, to organic compounds. The ability to photosynthesize is dependent on the presence of special light-sensitive pigments, the chlorophylls, which are found in plants, algae, and some bacteria. Light reaches phototrophic organisms in distinct units called quanta. Absorption of light quanta by chlorophyll pigments begins the process of photosynthetic energy conversion.
"The growth of a phototrophic autotroph can be characterized by two distinct sets of reactions: the light reactions, in which light energy is converted into chemical energy, and the dark reactions, in which this chemical energy is used to reduce CO2 to organic compounds. For autotrophic growth, energy is supplied in the form of ATP, while electrons for the reduction of CO2 come from NADPH. The latter is produced by the reduction of NADP+ by electrons originating from various electron donors to be discussed below.
"The light reactions bring about the conversion of light energy in the form of ATP. Purple and green bacteria use light primarily to form ATP; they produce NADPH from reducing materials present in their environment, such as H2S or organic compounds. Green plants, algae, and cyanobacteria, however, do not generally use H2S or organic compounds to obtain reducing power. Instead, they obtain electrons for NADP+ reduction by splitting water molecules, producing O2 as a byproduct. The reduction of NADP+ to NADPH by these organisms is therefore a light-mediated event. Because molecular oxygen, O2, is produced, the process of photosynthesis in these organisms is called oxygenic photosynthesis. In contrast, the purple and green bacteria do not produce oxygen; their process is called anoxygenic photosynthesis." Brock and Madigan (1991:563, 564).
What does the chlorophyll in bacteria and other organisms do? How does it work?
Brock and Madigan: "Photosynthesis occurs only in organisms that possess some type of chlorophyll. Chlorophyll is a porphyrin, as are the cytochromes, but chlorophyll contains a magnesium atom instead of an iron atom at the center of the porphyrin ring as well as a long hydrophobic alcohol molecule. Because of this alcohol side chain, chlorophyll associates with lipid and hydrophobic proteins of photosynthetic membranes.
"The structure of chlorophyll a, the principal chlorophyll of higher plants, most algae and of the cyanobacteria, as shown in Figure 16.3 (in their book). Chlorophyll a is green in color because it absorbs red and blue light preferentially and transmits green light.
"The special properties of any pigment can best be expressed by its absorption spectrum. It indicates the degree to which the pigment absorbs light of different wavelengths. The absorption spectrum of cells containing chlorophyll a shows strong absorption of red light (maximum absorption at a wavelength of 680 nm) and blue light (maximum at 430 nm)." (1991:564).
"Why do organisms have several kinds of chlorophylls absorbing light at different wavelengths? One reason appears to be to make it possible to use more of the energy of the electromagnetic spectrum. Only light energy, which is absorbed will be used biologically. So that by having more than one chlorophyll, more of the incident light energy becomes available to the organism. By having different pigments, two unrelated organisms can coexist in a habitat, each using wavelengths of light that the other is not using. Thus, pigment diversity has ecological significance." (1991:565).
Why are there chlorophyll pigments within the cell? What are they doing there?
Brock and Madigan: "These pigments, and all the other components of the light-gathering apparatus, are associated with special membranes, the photosynthetic membranes. The location of the photosynthetic membranes within the cell differs between procaryotic and eucaryotic microorganisms. ... In procaryotes, chlorophasts are not present and the photosynthetic pigments are integrated into extensive internal membrane systems; a single reaction center probably contains 25-30 light-harvesting bacterio-chlorophyll molecules.
"Antenna chlorophyll molecules make possible a dramatic increase in the rate at which photosynthesis can be carried out. At the light intensities often prevailing in nature, reaction centers can only be excited about once per second. This would not be sufficient, to carry out a significant photosynthetic process. The additional antenna chlorophyll molecules permit collection of light energy at a much more rapid rate. Since reaction-center chlorophyll absorbs light energy only over a very narrow range of the spectrum, antenna pigments also perform the additional function of spreading out the spectral range available for use." (1991:565, 566).
Anoxygenic photosynthesis in purple and green bacteria. They use light, but do not make free oxygen. How do they do that? What must they all know and be able to do? How complex is anoxygenic photosynthesis?
Prof. T. D. Brock and M. T. Madigan: "The process of light-mediated ATP synthesis in all phototrophic organisms involves electron transport through a sequence of electron carriers. These electron carriers are arranged in the photosynthetic membrane in series from those with negative to those with more positive potentials. Conceptually, the process of photosynthetic electron flow resembles that of respiratory electron flow. As a matter of fact, in phototrophic bacteria capable of aerobic (dark) growth, many of the same electron transport components are present in the membranes of cells grown in either the light (anaerobic) or dark (aerobic). We now consider the structure of the photosynthetic apparatus in anoxygenic phototrophs and the details of photosynthetic electron flow in purple bacteria, where much is known concerning the molecular events of photosynthesis." (1991:566).
How do the electrons flow during photosynthesis, when they make chemical energy from the energy of light? Why are they able to do that?
Brock and Madigan: "It should be recalled that the photosynthetic reaction center is surrounded by light-harvesting antenna bacteriochlorophyll a molecules which function to funnel light energy to the reaction center. Light energy is transferred from the antenna to the reaction center. Light energy is transferred from the antenna to the reaction center in packets called excitons, mobile electronic singlet states, which migrate through the antenna to the reaction center at high efficiency. Photosynthesis begins when exciton energy strikes the special pair bacteriochlorophyll a molecules. The absorption of energy excites the special pair, converting it into a strong reductant, sufficiently strong to reduce an acceptor molecule of very low potential. This represents work done on the system by light energy." (1991:563-567).
"Before excitation, the bacterial reaction center has an energy of about +0.5 volts; after excitation it has a potential of about -0.7 volts, sufficient to reduce bacteriopheophytin a. The excited electron within the special pair proceeds to reduce a molecule of bacteriopheophytin within the reaction center. This transition occurs incredibly fast, taking about four trillionths of a second (4 x 10-12 seconds) to occur. Once reduced, bacteropheophytin a reduces a quinone molecule, which is part of the reaction center but is actually nearer the outer surface of the photosynthetic membrane. This transition is also very fast, taking less than 1 billionth of a second. The quinone is referred to as the primary electron acceptor. Relative to what has happened in the reaction center, further electron transport reactions occur rather slowly, on the order of microseconds to milliseconds.
"From the quinone, electrons are transported in the membrane through a series of iron-sulfur proteins and cytochromes, eventually returning to the reaction center. Key electron transport proteins include cytochrome bc1 and cytochrome c. Cytochrome c serves as an electron shuttle between the membrane-bound bc1 complex and the reaction center." Brock, T. D. and M. T. Madigan (1991:567, 568).
What does that mean? How does that work?
Brock and Madigan: "Synthesis of ATP during photosynthetic electron flow occurs as a result of the formation of a proton gradient generated by proton extrusion during electron transport and the activity of ATPases in coupling the dissipation of the proton gradient to ATP formation. The reaction series is completed when cytochrome c returns the electron to the special pair bacteriochlorophylls, which serves to return these molecules to their original ground state (Eo´ +0.5 volts). The reaction center is then capable of absorbing new energy and repeating the process. This method of making ATP is called cyclic photophosphorylation, since electrons are repeatedly moved around a closed circle; in cyclic photophosphorylation there is no net input or consumption of electrons as in respiration." (1991:568, 569).
"It is extremely important that the light-driven redox reaction just described takes place across the photosynthetic membrane. The spatial relationships of the electron transport components in the bacterial photosynthetic membrane are illustrated in Figure 16.10 (in his book). Note that protons are pumped to the center of the chromotophore membrane, thus setting up the proton gradient, which is used in ATP synthesis. The net result of the light reaction is the translocation of three protons across the membrane for each photochemically excited electron. It should be emphasized that the only thing light does in this process is create a strong reductant; the remaining reactions are not light dependent, but strictly thermodynamically favorable electron transfers.
"The importance of the chromatophore arrangement should be made clear. If the process of electron flow ... would occur in a flat membrane, the protons pumped across the membrane would become part of the general external environment and a proton gradient would not develop. In the chromatophore arrangement, however, the proton gradient is maintained and can be employed to do useful work, such as the synthesis of ATP shown." (1991:569).
How does that work? How complex is it?
Brock and Madigan: "As we have noted, purple and green bacteria do not produce O2 during photosynthesis. Their photosynthesis is thus said to be anoxygenic. The reactions described above have led to the conversion of light energy into high-energy phosphate bonds of ATP. However, if an anoxygenic phototroph is going to grow with CO2 as its sole or major carbon course, formation of ATP is not enough. Reducing power (NADPH) must also be made so that CO2 can be reduced to the level of cell material. The source of electrons for anoxygenic phototrophs is some reduced substance from the environment; unlike oxygenic phototrophs the source of reducing power in anoxygenic phototrophs is not water." (1991:569).
Oxygenic photosynthesis makes free oxygen with the help of light. How does that work? How complex is that? What must one know and be able to do, to think out this oxygenic photosynthesis? Why does it exist?
Prof. T. D. Brock and M. T. Madigan: "Electron flow in oxygenic phototrophs involves two distinct, but interconnected, photochemical reactions. Oxygenic phototrophs use light to generate both ATP and NADPH, the electrons of the latter arising from the splitting of water into oxygen and electrons. The two systems of light reactions are called photosystem I and photosystem II, each photosystem having a spectrally distinct form of reaction center chlorophyll a. Photosystem I chlorophyll, called P700, absorbs light best at long wavelengths (far red light), whereas photosystem II chlorophyll, called P680, absorbs best at shorter wavelengths (near red light).
"Like anoxigenic photosynthesis, oxygenic photosynthesis occurs in membranes. In eucaryotic cells, these membranes are found in the chloroplast, while in cyanobacteria, photosynthetic membranes are arranged in stacks within the cytoplasm. In both groups of phototrophs, the two forms of chlorophyll a are attached to specific proteins in the membrane...
"The path of electron flow in oxygenic phototrophs roughly resembles the letter Z turned on its side. And scientists studying oxygenic photosynthesis have come to refer to the electron flow of oxygenic phototrophs as the 'Z' scheme. We should first note that the reduction potential of P680 chlorophyll a molecule in photosystem II is very high, slightly higher than that of the O2/H2O couple. This is because the first step in oxygenic electron flow is the splitting of water into oxidizing and reducing equivalents, a thermodynamically unfavorable reaction.
"An electron from water is denoted to the P680 molecule following the absorption of a quantum of light near 680 nm. Light energy converts P680 into a moderately strong reductant, capable of reducing an intermediate molecule about -0.2 volts. The nature of this molecule is unknown, but it may be a pheophytin a molecule (chlorophyll a without the magnesium atom). From there the electron travels through several membrane carriers including quinones, cytochromes, and a copper-containing protein called plastocyanin, the latter donates electrons to photosystem I. the electron is accepted by the reaction center chlorophyll of photosystem I, P700, which has previously absorbed light quanta, and donated electrons to the primary acceptor of photosystem I; this acceptor has a very negative potential, about -0.75 volts.
"As in photosystem II, the primary acceptor of electrons from photosystem I has not been positively identified, but is thought to be a free radical form of chlorophyll a. At any rate, the acceptor in photosystem I, once reduced, is at a reduction potential sufficiently negative to reduce the iron-sulfur protein ferredoxin, which then reduces NADP+ to NADPH." Brock and Madigan (1991:570, 571).
Making ATP during oxygenic photosynthesis, how does that work? How complex is that?
Prof. T. D. Brock and M. T. Madigan: "Besides the net synthesis of reducing power (i.e., NADPH), other important events occur while electrons flow from one ecosystem to another. During transfer of an electron from the acceptor in photosystem II to the reaction center chlorophyll molecule in photosystem I, electron transport occurs in a thermodynamically favorable (negative to positive) direction. This generates a membrane potential (a proton gradient) from which ATP can be produced. This type of ATP generation has been called noncyclic photophosphorylation because the electron travels a direct route from water to NADP+.
"When sufficient reducing power is present, ATP can also be produced in oxygenic phototrophs by cyclic photophosphorylation involving only photosystem I. This occurs when the primary acceptor of photosystem I, instead of reducing ferredoxin (and hence NADP+), returns the electron to the P700 molecule via membrane-bound cytochromes b and f. This flow creates a membrane potential and synthesis of additional ATP.
"Photosystem I and II normally function together in the oxygenic process. However, under certain conditions many algae and some cyanobacteria are able to carry out cyclic photophosphorylation using only photosystem I, obtaining reducing power from sources other than water, in effect photosynthesizing anoxygenically as do purple and green bacteria. This alteration requires the presence of anaerobic conditions as well as a reducing substance, such as H2 or H2S. Under these conditions the electrons for CO2 reduction come not from water but from the reducing substance. In algae H2 is generally the reductant. And following a period of adaptation to anaerobic conditions, the enzyme hydrogenase is made and is used to assimilate H2 which reduces NADP+ to NADPH directly.
"A number of cyanobacteria can use H2S as an electron donor for anoxygenic photosynthesis. When H2S is used, it is oxidized to elemental sulfur (S°), and sulfur granules are deposited outside the cells similar to those produced by green sulfur bacteria. The filamentous cyanobacterium Oscillatora limnetica has been found in sulfide-rich saline ponds where it carries out anoxygenic photosynthesis along with photosynthetic green and purple bacteria and produces sulfur as an oxidation product of sulfide. In cultures of O. limnetica electron flow from photosystem II is strongly inhibited by H2S, thus necessitating anoxygenic photosynthesis if the organism is to survive in its sulfide-rich environment." Brock and Madigan (1991:571, 572).
Organisms, using the energy of light, do have still more pigments, besides those, which we have just studied. - Which ones?
Prof. T. D. Brock and M. T. Madigan: "Although a pigment with a ring structure like chlorophyll or bacteriochlorophyll is obligatory for photosynthesis, phototrophic organisms have other pigments that are involved, at least indirectly, in the capture of light energy. The most widespread accessory pigments are the carotenoids, which are almost always found in phototrophic organisms. Carotenoids are water insoluble pigments firmly embedded in the membrane... Carotenoids have long hydrocarbon chains with alternating C-C and C=C bonds, an arrangement called a conjugated double bond system. As a rule, carotenoids are yellow, red, or green in color and absorb light in the blue region of the spectrum. Carotenoids are usually closely associated with chlorophyll in the photosynthetic membrane. And there are approximately the same number of carotenoid as there are chlorophyll molecules. Carotenoids do not act directly in photophosphorylation reactions, but transfer, by way of fluorescence, some of the light energy they capture to chlorophyll. This transferred energy may thus be used in photophosphorylation in the same way as light energy captured directly by chlorophyll.
"Cyanobacteria and red algae contain phycobiliproteins, which are accessory pigments that are red or blue in color. The red pigment, called phycoerythrin, absorbs light most strongly at wavelengths around 550 nm, whereas the blue pigment, phycocyanin, absorbs most strongly at 620 to 640 nm. Phycobiliproteins contain open-chain tetrapyrroles called phycobilins, which are coupled to protein. Phycobiliproteins occur as high molecular weight aggregates called phycobilisomes, attached to the photosynthetic membranes. They are closely linked to the chlorophyll-containing system, which makes for very efficient energy transfer, approaching 100 percent, from biliprotein to chlorophyll." (1991:572, 573).
Why do organisms, using the energy of light, have extra pigments? For what do they need them?
Prof. T. D. Brock and M. T. Madigan: "The light-gathering function of accessory pigments seems to be of obvious advantage to the organism. Light from the sun is distributed over the whole visible range, yet chlorophyll absorbs well in only a part of this spectrum. By having accessory pigments, the organism is able to capture more of the available light. Another function of accessory pigments, especially of the carotenoids, is as photoprotective agents. Bright light can often be harmful to cells, in that it causes various photooxidation reactions that can actually lead to the destruction of chlorophyll and of the photosynthetic apparatus itself. The accessory pigments absorb much of this harmful light and thus provide a shield for the light-sensitive chlorophyll. Since phototrophic organisms must by their very nature live in the light, the photoprotective role of the accessory pigments is of obvious advantage." (1991:573).
The photosynthetic unit and its associated reaction center in oxygenic phototrophs. Light energy, absorbed by light-harvesting chlorophyll molecules, travels to the reaction center, where the actual ejection of an electron occurs, generating a charge separation. This light harvesting apparatus is located in the cell’s photosynthetic membrane. From M. T. Madigan et al. (1997:479) Fig. 13.6. Whoever has first thought out and made this photosynthetic unit in bacterial cells, also had to know about light-waves, their wave lengths, and the energy, which they carry. Chance cannot do that.
Some bacteria use the light of the sun as a source of energy. How have these cells, with their systems of photosynthesis, come into being? What was needed, to think them out and to make them, some 3.8-4 billion years ago?
Even the smallest bacterial and archaeal cell knows more about biochemistry, than any scientist does, now living on this earth? It knows that there are protons and electrons. And it knows how to use them. It knows that there is the sun with its light. And it knows the length of its waves. It knows and is able to use quantum physics and mathematics. It knows, how to make organic matter from inorganic matter. It is able to make itself, by doubling itself, within a few minutes or hours. No human scientist is able to make a living cell with its photosynthetic outfit. It is far too complicated. He is not even able, to make a single functional enzyme of this tiny creature. The assertion, that one only needs the laws of chemistry and physics, does not explain anything. That is only empty talk. This statement is not logical and contradicts itself. A law always comes from a lawgiver, from an intelligent person. And the laws of chemistry and physics come from the Creator, from the God of the Bible, whose name is Jehovah. King David was right, when he said in Psalms 14:1a, according to the King James Version: "The fool hath said in his heart, There is no God."