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Photosynthesis and Biosynthesis
As described earlier, chemoheterotrophs obtain the energy they need for growth from the catabolism of preformed organic compounds. These organisms make ATP using either substrate level or oxidative phosphorylation (or a combination of the two). Organisms categorized as phototrophs can make use of a third mechanism for synthesizing ATP called photophosphorylation. These organisms use light as their energy source. Photophosphorylation: Photophosphorylation is ATP synthesis involving light (usually sunlight) as the energy source. Organisms capable of photophosphorylation carry one or more types of light-trapping pigments, i.e., molecules that can respond to light energy in a specific manner. In the case of algae , cyanobacteria and green plants, the pigments involved are green chlorophyll molecules (chlorophyll a, b or c); in the case of anoxygenic phototrophic bacteria they are bacteriochlorophylls (bacteriochlorophyll a, b, c, d, or e), which occur in a variety of colors; in the case of Archaea in the genus Halobacterium the pigment is bacteriorhodopsin , a red or purple-colored integral protein. All of these pigment molecules have a feature in common, and that is their ability to respond to light energy. Chlorophylls and bacteriochlorophylls respond by passing energy and eventually by passing electrons to electron acceptor molecules, while bacteriorhodopsin responds by changing configuration and “pumping” hydrogen protons across the cell membrane. Light has both wave-form and particle-form properties, so although light occurs as different wavelengths, it also occurs as particles or packets of light energy called photons. When photons strike the bacteriorhodopsin molecules of Halobacterium salinarum , these function as light driven “proton pumps” that transport hydrogen protons across the cell membrane (out of the cell or into the periplasmic space). The accumulation of hydrogen protons creates a concentration and electrical gradient (the proton motive force ), and this causes a return flow of protons through ATP-synthase. The resulting formation of ATP from ADP and PO 4 is one example of photophosphorylation. When photons strike chlorophylls or bacteriochlorophylls, these molecules can pass energy from one to another, or certain atoms within them can give up electrons, i.e., pass electrons to other molecules. If the electrons are passed to proteins associated with a membrane-bound electron transport chain or system (integral proteins), their flow can trigger the same reactions we saw associated with oxidative phosphorylation , i.e., the transport of hydrogen protons across the membrane to establish a proton motive force , and the return flow of protons through ATP-synthase resulting in the formation of ATP. Again the ATP made would be the result of photophosphorylation. Not all of the light energy captured by phototrophs is used to make ATP. Some of it is used to reduce the coenzyme NADP to NADPH + H
. Recall that the reduced form of a coenzyme has a higher energy potential than does the oxidized form. Because this energy capture does not involve ATP, it is not photophosphorylation. Photophosphorylation can be described as cyclic or non-cyclic on the basis of whether or not the electrons leaving the pigment molecules return to them (or to other pigment molecules of the same type). During cyclic photophosphorylation , the electrons return to the pigment molecules (i.e., they are recycled), while during non-cyclic photophosphorylation , they do not. When pigment molecules give up electrons during non-cyclic photophosphorylation, these must be replaced, and this can result in the formation of a very important compound.
Photophosphorylation in Anoxygenic Phototrophs: Anoxygenic phototrophs include the green and purple sulfur bacteria, green and purple non-sulfur bacteria and other phototrophic prokaryotes not included in the phylum Cyanobacteria. These organisms are called anoxygenic (an = without, oxy = oxygen and genic = generation or production of), because they do not produce oxygen in association with photophosphorylation. Cyclic photophosphory- lation as it occurs in these organisms can involve different pigment molecules (alone or in combination), but in general they are all bacteriochlorophylls. These respond to light energy by passing electrons to an electron acceptor e.g., ubiquinone , bacteriopheophytin , or others. From the acceptor molecules the electrons travel along a series of cytochromes, and then back to the pigments. The diagram shown below is a simplified representation of the pathway taken by activated electrons during cyclic photophosphorylation. Bacteriopheophytin In this diagram light is striking pigment (or other electron acceptors) molecules and causing electrons associated with certain atoms to gain energy. Electrons can then either pass energy (via resonance) to electrons of other molecules (the antenna array), or "bounce" away to an atom in some other molecule (sometimes these travel in pairs as indicated here, and sometimes they 2e
- Cytochrome chain do not). The electrons leave their pigment molecules at reaction centers , and are passed to some type of electron acceptor (in ADP + Pi ATP this case bacteriopheophytin ). From there Light they can travel along a chain of cytochrome enzymes (via oxidation/reduction reactions), and supply the energy needed to “pump” hydrogen protons across the membrane. As electrons exit the cytochrome chain, they are picked up by atoms associated with pigment molecules (bacteriochlorophylls), and are ultimately returned to the atoms they came from (or to atoms of pigments of the same type). The flow of electrons is associated with ATP synthesis as explained below. Since the electrons involved in this process are recycled, the synthesis of ATP can continue as long as light energy is available. The pigment molecules, electron acceptors and cytochromes indicated above are all bound to the bacterial cell membrane, so this is where photophosphorylation occurs. The electron flow generated by light striking the pigment molecules is linked to the transport of hydrogen protons across the membrane and into the periplasmic space (as indicated above). The accumulation of protons outside the membrane creates a concentration and electrical gradient known as the proton motive force (again like water accumulated behind a dam). When the protons flow back across the membrane (down their concentration and electrical gradient), they pass through ATP-synthase (an enzyme complex) and supply the energy needed to convert ADP + Pi (inorganic phosphate) into ATP. As long as light is available, ATP can be generated. Green and purple sulfur bacteria can also capture light energy by means of a non-cyclic pathway. In this case, the electrons leaving the pigment molecules pass to an electron acceptor and then to the coenzyme NADP reducing it to NADPH + H+. The reduced coenzyme has a higher energy potential than it does in its oxidized state, so some of the energy available in the light (photons) is captured, but the pigment molecules are now missing electrons. Bacteriochlorophylls cannot pull electrons and hydrogen protons away from water molecules, so oxygen is not generated, but these pigments can pull
The diagram below shows a highly simplified representation of non-cyclic photophosphorylation as it occurs in algae, cyanobacteria and green plants. Plastoquinone Ferredoxin NADP ( Pheophytin as electron acceptor) NADPH + H+ 2e
- 2e - Cytochrome chain Light Light ADP + Pi ATP H 2 O Photosystem II Photosystem I In this diagram, light is striking pigment molecules associated with both photosystem I and photosystem II and is causing electrons to leave certain atoms. The electrons leaving photosystem II are passed to the electron acceptor pheophytin , and then to plastoquinone. From Plastoquinone the electrons are passed along a chain of cytochrome enzymes , and are then transferred to photosystem I. At the same time, the electrons leaving photosystem I are passed to an electron acceptor called ferredoxin (an iron sulfur protein). From ferredoxin, the electrons pass to an enzyme complex, and then to NADP , reducing it to NADPH + H+. The proteins and pigments of photosystem I and II are bound to membranes called thylakoids (thylakos = sac). The passage of electrons along the chain of cytochrome enzymes is linked to the transport of hydrogen protons across these membranes (into the region surrounded by them). This creates a concentration and electrical gradient, i.e., a proton motive force that drives the passage of hydrogen protons through ATP-synthase, resulting in the conversion of ADP + Pi into ATP. Since the electrons that leave photosystem II do not returned, the phosphorylation process is non-cyclic. Photosystem II can replace the lost electrons by pulling them away from water molecules, and thus generates molecular oxygen (2 H 2 O minus 4 electrons and 4 hydrogen protons = O 2 ). The photosystems described above are actually very complex, each containing multiple proteins, light- trapping pigments and other types of molecules. In green algae and higher plants, most of the pigments associated with the photosystems are chlorophylls , but carotenoid pigments can also be involved. In red algae and cyanobacteria, chlorophylls are often accompanied by phycobilins. Though electrons are depicted as moving in pairs within the diagrams presented here, this is not always the case. During cyclic photophosphorylation as it occurs in algae and cyanobacteria, electrons leave photosystem I, are passed to ferredoxin ; however, rather than being passed to NADP, the electrons are passed to a chain of cytochrome enzymes , and then return to photosystem I. ATP is generated as described above, but oxygen is not produced. A simplified version of cyclic photophosphorylation is provided in the diagram below.
Ferredoxin In this diagram, light is striking the pigment molecules of photosystem I and causing the electrons from some atoms to transfer energy to others. Eventually the electrons from some atoms gain enough energy to leave 2e
- Cytochrome chain or “bounce” away (sometimes in pairs). The electrons travel to an electron acceptor, in this case an ADP + Pi ATP iron-sulfur protein called ferredoxin. Light From ferredoxin, the electrons pass to a chain of cytochrome enzymes (supplying energy needed to pump protons across the membrane), and from there, back to photosystem I. Since the electrons leaving the pigments of photosystem I will Photosystem I eventually be returned to the same photosystem, the process is cyclic. Note that during cyclic photophos- phorylation energy is captured in the form of ATP, but not as NADPH + H
. In addition to this, there is no splitting of water molecules, and oxygen is not produced. Photosynthesis: Photosynthesis – Photosynthesis (photo = light, synthesis = building reactions) or light synthesis is often defined as a process allowing organisms to make sugar and oxygen using light energy, carbon dioxide and water. The chemical equation sometimes used to represent photosynthesis is shown below. 6 CO 2 + 6 H 2 O + light energy C 6 H 12 O 6 + 6 O 2 Although convenient, this equation is only partially correct, i.e., is only correct when applied to certain organisms. Algae, cyanobacteria and green plants produce oxygen in association with their photosynthetic processes, but anoxygenic phototrophs do not. In addition, although photoautotrophs use light energy and inorganic carbon as indicated by this equation, photoheterotrophs do not and neither do chemoautotrophs. Photosynthesis is actually a complex metabolic process involving two distinctly different phases, these are:
- Photophosphorylation (light dependent reactions) – As explained above, photophosphorylation involves making ATP using energy provided by light. When the process is non-cyclic and occurs within algae, cyanobacteria and green plants, it also results in the formation of NADPH + H+.
- The Calvin-Benson cycle (light independent reactions) – An anabolic pathway used to convert inorganic carbon (carbon dioxide) into sugar (fructose). This pathway requires energy and is driven by the ATP and NADPH + H
generated in association with photophosphorylation. As explained earlier, photophosphorylation may or may not result in the formation of oxygen. If it does, the oxygen is generated when water molecules donate electrons and hydrogen protons to photosystem II. The pigment systems of anoxygenic phototrophs do not split water and cannot generate oxygen.
The reactions of glycolysis and the Krebs cycle are of considerable importance because many of the metabolic intermediates formed, e.g., Pyruvic acid, oxaloacetic acid and a-ketoglutaric acid can have amino groups added to them to form amino acids , and these can be used to form proteins. The acetyl- CoA formed just prior to the Krebs cycle can donate two-carbon acetyl groups for use in the synthesis of fatty acids. The glyceraldehydes- 3 - phosphate formed during glycolysis is in equilibrium with dihydroxyacetone phosphate, and this can be converted into glycerol, the 3-carbon compound used in the production of triglycerides and phospholipids. Most of the nucleotides used to build nucleic acids (high-energy compounds, coenzymes and regulatory molecules) are obtained through the catabolism of nucleic acids ingested by cells (i.e., via salvage pathways ); however, all types of cells can also synthesize nucleotides from components provided by amino acids and 5-carbon sugars. Protein catabolism: Chemoheterotrophs cultured in the microbiology laboratory are often provided with proteins or protein breakdown products (peptone, peptides, proteoses) as nutrient sources (recall that nutrient agar contains beef extract and peptone). Like carbohydrates, these nutrients can be catabolized to provide cells with both energy and carbon. Proteins can be broken into individual amino acids, and these can be used to build other (different) proteins (i.e., via salvage pathways). Amino acids can also be deaminated , i.e., can have their amino groups removed, and can then be completely catabolized through chemical reactions associated with glycolysis and/or the Krebs cycle depending on the type of amino acid initially present. Lipid catabolism: Triglycerides and phospholipids are abundant within cells and can also be catabolized to yield both energy and carbon. The glycerol "backbone" associated with these molecules can be removed and phosphorylated to form dihydroxyacetone phosphate (DHAP). Since this molecule is in equilibrium with 3-phosphoglyceraldehyde (PGAL), it can readily enter the glycolysis pathway for additional catabolism. Fatty acids (hydrocarbon chains) can be broken into two-carbon units and oxidized through a process called b-oxidation. The resulting acetyl groups can then bind with coenzyme-A to form acetyl-CoA , and can then enter the Krebs cycle for additional processing.
Ultimate symbiosis: A more complete understanding of metabolic processes often leads to a greater appreciation for the interdependence of the organisms present on this planet. In a metabolic sense, all living organisms live symbiotically with other organisms, human beings included. The random devastation of the natural environment perpetuated by human activities is ultimately self-destructive. Without other life forms, we cannot survive. A brief summary of our obvious interdependence is illustrated in the diagram provided below. Photoautotrophs Chemoheterotrophs (Green plants, algae, (Fungi, protozoa, many cyanobacteria and types of bacteria and all other prokaryotes) animals including humans) Form sugar from CO 2 Catabolize sugar and other and metabolize sugar to organic compounds, releasing form other compounds. CO 2 and energy to make ATP. Calvin-Benson cycle Fermentation and respiration Oxygenic forms split Respiratory forms use O 2 as a water (H 2 O) to form O 2 final electron acceptor and in (molecular oxygen) the process generate H 2 O. Oxygenic photophosphorylation Cellular respiration. Remember that although photoautotrophs use light as an energy source, they can only do this when light is available. These organisms use a respiratory type metabolism and consume organic compounds and oxygen just as we do. When light is available, they produce more oxygen and sugar than they consume, but during hours of darkness the reverse is true. Recall that eutrophication can lead to fish kills when algae use excessive amounts of oxygen in water during periods of darkness (e.g., at night). It is important to remember that the cycling of carbon into and out of organic compounds (carbon cycle), and the cycling of oxygen with water, represents only a fraction of the chemical processes involved in metabolism. Bacteria also play crucial roles in the cycling of nitrogen, sulfur, phosphorous, iron, and other elements essential to life. Global Warming and Other Important Issues – A Biological Perspective: All life forms require a source of energy, and since energy cannot be created (produced) or destroyed, it flows through biological systems. Light energy (electromagnetic radiation from the sun) is the ultimate source of energy for organisms living on this planet. Only certain organisms can capture this energy and convert it to high-energy compounds available for metabolism (ATP and NADPH + H
); e.g., green plants, algae, cyanobacteria, certain other bacteria and a few types of archaea. These organisms have been capturing light energy for a very long time (millions of years at least). Some of the energy was used to drive metabolic processes within these organisms, and some was used to form organic compounds (cellular components for new cells). Eventually, geologic processes caused some cellular material to become buried under layers of sediments as deposits of coal and oil. Cellular respiration and fermentation allow organic compounds to be catabolized, often releasing CO 2 into the atmosphere, and these processes have also been occurring for a very long time (millions of years). Fire, the non-biological conversion of organic compounds and oxygen into carbon dioxide and water has also been around as a natural phenomenon for millions of years (volcanoes, lightning, etc. are natural
- Promoting education and new energy technology – Coal, oil and nuclear power are potentially dangerous, and alternative energy sources (solar, wind, hydrogen, etc.) are available. Promote education and encourage the development/use of alternative, ecologically "friendly" energy sources.
- Spreading the word – Tell other people; the microbiology web site is available to everyone, encourage other people to read it. We are biological entities and live symbiotically with other biological entities in a complex ecosystem maintained by interactions we do not fully understand. Human activity is negatively impacting this ecosystem in ways that will ultimately be damaging to us; our future is at stake. Fortunately, we don't have to worry about the fate of microorganisms, they will be just fine.
