B.Sc Micro II Microbial physiology Unit 1 Bacterial Photosynthesis

Several bacteria like cynobacteria and other purple-sulfer bacteria have capacity to generate energy by photosynthesis without chlorophyll.

1. Microbial physiology Bacterial Photosynthesis Unit 1

2. Introduction In all of the metabolic pathways just discussed, organisms obtain energy for cellular work by oxidizing organic compounds. But where do organisms obtain these organic compounds? Some, including animals and many microbes, feed on matter produced by other organisms. For example, bacteria may catabolize compounds from dead plants and animals, or may obtain nourishment from a living host. Other organisms synthesize complex organic compounds from simple inorganic substances. The major mechanism for such synthesis is a process called photosynthesis. which is used by plants and many microbes.

3. Essentially, photosynthesis is the conversion of light energy from the sun into chemical energy. The chemical energy is then used to convert CO 2 from the atmosphere to more reduced carbon compounds, primarily sugars. This synthesis of sugars by using carbon atoms from CO 2 gas is also called carbon fixation.

4. Photosynthesis, in bacteria, is defined as “the synthesis of carbohydrates by the chlorophyll in the presence of sunlight, CO 2 and reductants taken from air and oxygen do not evolve as by product, except in cynobacteria .

5. The most important biological process on Earth is photosynthesis, the conversion of light energy to chemical energy. Organisms that carry out photosynthesis are called phototrophs . 1

6. Most phototrophic organisms are also autotrophs , capable of growing with CO 2 as the sole carbon source. Energy from light is used in the reduction of CO 2 to organic compounds ( photoautotrophy ). However, some phototrophs use organic carbon as their carbon source; this lifestyle is called photoheterotrophy .

7. Photoautotrophy requires that two distinct sets of reactions operate in parallel: (1) ATP production and (2) CO 2 reduction to cell material. For autotrophic growth, energy is supplied from ATP, and electrons for the reduction of CO 2 come from NADH (or NADPH). The latter are produced by the reduction of NAD+ (or NADP+) by electrons originating from various electron donors. Some phototrophic bacteria obtain reducing power from electron donors in their environment, such asreduced sulfur sources, for example hydrogen sulfide (H 2 S), or from hydrogen (H 2 ). By contrast, green plants, algae, and cyanobacteria use electrons from water (H 2 O) as reducing power.

8. The oxidation of H 2 O produces molecular oxygen (O 2 ) as a by-product. Because O 2 is produced, photosynthesis in these organisms is called oxygenic photosynthesis. However, in many phototrophic bacteria H 2 O is not oxidized and O 2 is not produced, and thus the process is called anoxygenic photosynthesis.

9. Photosynthetic Microorganisms All life can be divided into three domains, Archaea , Bacteria and Eucarya , which originated from a common ancestor. Historically, the term photosynthesis has been applied to organisms that depend on chlorophyll (or bacteriochlorophyll ) for the conversion of light energy into chemical free energy. These include organisms in the domains Bacteria (photosynthetic bacteria) and Eucarya (algae and higher plants). The most primitive domain, Archaea , includes organisms known as halobacteria , that convert light energy into chemical free energy. However, the mechanism by which halobacteria convert light is fundamentally different from that of higher organisms because there is no oxidation/reduction chemistry and halobacteria cannot use CO 2 as their carbon source. Consequently some biologists do not consider halobacteria as photosynthetic.

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12. Classification of photosynthetic bacteria Two broad groups: Anoxygenic photosynthetic bacteria Oxygenic photosynthetic bacteria

13. Anoxygenic photosynthetic bacteria Some photosynthetic bacteria can use light energy to extract electrons from molecules other than water. These organisms are of ancient origin, presumed to have evolved before oxygenic photosynthetic organisms. Anoxygenic photosynthetic organisms occur in the domain Bacteria and have representatives in four phyla – Purple- Sulphur Bacteria, Purple non- Sulphur Bacteria, Green-Sulfur Bacteria, Green non-Sulfur Bacteria.

14. Anoxygenic photosynthesis depends on electron donors such as reduced sulphur compounds, molecular hydrogen or organic compounds. They are found in fresh water, brackish water, marine and hypersaline water. Anoxygenic photosynthetic bacteria have been divided into three groups on the basis of pigmentation: purple bacteria, green bacteria and heliobacteria .

15. Purple Bacteria The anoxygenic phototrophs grow under anaerobic conditions in the presence of light and do not use water as electron donor as higher plants. They grow autotrophically with CO 2 and hydrogen or reduced sulphur compounds act as electron donor. The pigment synthesis is repressed by O 2.

16. Purple bacteria contain Bchl a and b as photosynthetic pigment. The colour of purple bacteria shows brown, pink brown-red, purple-violet based on carotenoid contents. The photosynthetic pigments are innfluenced by light intensity. At high intensity, photo-apparatus is inhibited. Carotenoids give rise to purple colour ; mutants lack carotenoids are blue green reflecting the actual colour of BChl a. Purple Bacteria are of two types: purple- suphur bacteria and purple non- sulphur bacteria.

17. Purple- sulphur bacteria Family: Chromatiaceae They are gram negative bacteria which contain BChl a and b and grow chemolithotrophically in dark with thiosulphate as electron donor. They are also chemoorganotrophs , utilize acetate, pyruvate and few other compounds. The mole % of G+C varies from 46-70.

18. The cells of purple- sulphur bacteria are larger than green bacteria and packed with intracellular sulfide deposition. They are found in anoxic zone of lakes and sulphur springs. They are photolithotrophs and motile in nature e.g. Ectothiorhodospira , Chromatiium , Thiocapsa , Thiospirillum , Thiodictyon , Thiopedia etc.

19. Purple non- sulphur Bacteria Family: Ectothiorhodospiraceae They also contain BChl a and b and use low concentration of sulphide . The concn of sulphide utilized by purple sulphur bacteria proved toxic to this category of bacteria. Earlier, scientists thought that these bacteria are unable to use sulphide as ele donor for reduction of CO 2 to cell material, thus named them non- sulphur .

20. Some non- sulphur bacteria grow anaerobically in the dark using fermentative metabolism, while others can grow anaerobically in the dark by respiration in which ele donor may be an organic/inorganic compound as H 2 . This group is most versatile energetically due to broad requirements and are photoorganotrophs i.e. use organic acids, amino acids, benzoate and ethanol. They also grow as chemoorganotrophs and require vitamins. The DNA base composition is 61-73 mole % (G+C) and the sulphur granules are formed outside the cell. Eg , Rhodomicrobium , Rhodopseudomonmas , Rhodospirillum , Rhodocyclus , etc.

21. Green Bacteria Instead of green in colour , these are brown due to the presence of carotenoids components. They are gram-negative. They contain BChl c, d and e plus small amount of Bchl a. The photosyntheic apparatus is chlorosomes . They do not require vitamins for their growth. Green bacteria are of two types: green sulphur bacteria and green non- sulphur bacteria.

22. Green Sulphur Bacteria Family: Chlorobiaceae They are non-motile, rods, spiral and cocci . Chlorosomes are present in the cell. They are strictly anaerobic and obligate phototroph . Deposit sulphur extracellularly . Mol % G+C is 45-58. Eg ; Chlorobium , Prostheochloris , Pelodictyon , Chloroherpeton

23. Green non- Sulphur Bacteria The Green non- Sulphur Bacteria are filamentous, gliding bacteria and thermophilic in nature. The pigments are Bchl a, Bchl c, and carotenes. Chlorosomes are present when grown anaerobically . They are photoheterotrophic and photoautotrophic and show gliding movement. They do not deposit sulphur . The mol % G+C vary 53-55. Eg , Chloroflexus

24. Heliobacteria Based on 16S rRNA sequencing and other morphological and biochemical characters, helicobacter are quite different with other anoxygenic photosynthetic bacteria. They are gram-positive, rod shaped, motile either by gliding or by means of flagella. The mol % G+C is between 50-55. Eg , Heliobacterium , Helophilum , Heliobacillus

25. Most of them produce endospores and grow up to 42°C. The heliobacteria are green in colour . Most of the heliobacteria are found in tropical soils of paddy fields. They contain BChl g.

26. Oxygenic Photosynthetic Bacteria The Oxygenic Photosynthetic Bacteria are unicellular or multicellular and possess bacteriochlorophyll a and carry out oxygenic photosynthesis. They are mostly represented by gram-negative cynobacteria . Carboxysomes and gas vesicles are present and also show gliding movement. Photosynthesis is oxygenic and autotrophic. Photosynthates get accumulated in the form of glycogen.

27. Photosynthetic Pigments Because of presence of carotenoids in all photosynthetic tissues their role is anticipated in photosynthesis. The cells rich in carotenoids devoid of clorophyll do not photosynthesize. Light energy absorbed by carotenoids appears to be transferred to chlorophyll a or bacterio chlorophyll a and utilized in photosynthesis.

28. Chlorophylls and Bacteriochlorophylls Phototrophic organisms contain some form of chlorophyll (oxygenic phototrophs ) or bacteriochlorophyll ( anoxygenic hototrophs ). Chlorophyll a is green in color because it absorbs red and blue light preferentially and transmits green light. 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).

29. There are a number of different chlorophylls and bacteriochlorophylls , and each is distinguished by its unique absorption spectrum. Chlorophyll b, for instance, absorbs maximally at 660 nm rather than the 680-nm absorbance maximum of chlorophyll a. All plants contain chlorophylls a and b. Some prokaryotes contain chlorophyll d, while chlorophyll c is found only in certain eukaryotic phototrophs .

30. Among prokaryotes, cyanobacteria produce chlorophyll a and prochlorophytes produce chlorophylls a and b. Anoxygenic phototrophs , such as the phototrophic purple and green bacteria, produce one or more bacteriochlorophylls . Bacteriochlorophyll a, present in most purple bacteria absorbs maximally between 800 and 925 nm, depending on the species.

31. Why do different phototrophs have different forms of chlorophyll or bacteriochlorophyll that absorb light of different wavelengths? This allows phototrophs to make better use of the available energy in the electromagnetic spectrum. Only light energy that is absorbed is useful for energy conservation. By having different pigments with different absorption properties, different phototrophs can coexist in the same habitat, each organism using wavelengths of light that others are not using. Thus, pigment diversity has ecological significance for the successful coexistence of different phototrophs in the same habitat.

32. Prokaryotes do not contain chloroplasts. Their photosynthetic pigments are integrated into internal membrane systems. These systems arise (1) from invagination of the cytoplasmic membrane (purple bacteria); (2) from the cytoplasmic membrane itself ( heliobacteria ); (3) in both the cytoplasmic membrane and specialized structures enclosed in a nonunit membrane, called chlorosomes (green bacteria) or (4) in thylakoid membranes ( cyanobacteria )

33. The chlorosome of green sulfur and green nonsulfur bacteria. (a) Transmission electron micrograph of a cross-section of a cell of the green sulfur bacterium Chlorobaculum tepidum . Note the chlorosomes (arrows). (b) Model of chlorosome structure. The chlorosome (green) lies appressed to the inside surface of the cytoplasmic membrane. Antenna bacteriochlorophyll ( Bchl ) molecules are arranged in tubelike arrays inside the chlorosome , and energy is transferred from these to reaction center (RC) Bchl a in the cytoplasmic membrane (blue) through a protein called FMO. Base plate (BP) proteins function as connectors between the chlorosome and the cytoplasmic membrane. 4

34. Carotenoids and Phycobilins Although chlorophyll or bacteriochlorophyll is required for photosynthesis, phototrophic organisms contain an assortment of accessory pigments as well. These include, in particular, the carotenoids and phycobilins . Carotenoids primarily play a photoprotective role in both anoxygenic and oxygenic phototrophs , while phycobilins function in energy metabolism as the major light-harvesting pigments in cyanobacteria .

35. Carotenoids The most widespread accessory pigments in phototrophs are the carotenoids . Carotenoids are hydrophobic light-sensitive pigments that are firmly embedded in the photosynthetic membrane. Carotenoids are typically yellow, red, brown, or green in color and absorb light in the blue region of the spectrum. Carotenoids are closely associated with bacteriochlorophyll in photosynthetic complexes, and energy absorbed by carotenoids can be transferred to the reaction center.

36. Nevertheless, carotenoids primarily function in phototrophic organisms as photoprotective agents. Bright light can be harmful to cells; Carotenoids quench toxic oxygen species by absorbing much of this harmful light and prevent these dangerous photooxidations .

37. Phycobiliproteins and Phycobilisomes Cyanobacteria and the chloroplasts of red algae contain phycobiliproteins , which are the main light-harvesting systems of these phototrophs . The red phycobiliprotein , called phycoerythrin , absorbs most strongly at wavelengths around 550 nm, whereas the blue phycobiliprotein , phycocyanin , absorbs most strongly at 620 nm. Phycobiliproteins assemble into aggregates called phycobilisomes . In a fashion similar to how light-harvesting systems function in anoxygenic phototrophs , phycobilisomes facilitate energy transfer to allow cyanobacteria to grow at fairly low light intensities.

38. Photosynthetic Electron Transport System In the photosynthetic light reactions, electrons travel through a series of electron carriers arranged in a photosynthetic membrane in order of their increasingly more electropositive reduction potential (E ). This generates a proton motive force that drives ATP synthesis. Anoxygenic photosynthesis occurs in at least five phyla of Bacteria: the proteobacteria (purple bacteria); green sulfur bacteria; green nonsulfur bacteria; the gram-positive bacteria ( heliobacteria ); and the acidobacteria .

39. Photosynthetic Reaction Centers The photosynthetic apparatus of purple bacteria has been best studied. Membrane vesicles, sometimes called chromatophores , or membrane stacks called lamellae are common membrane morphologies.

40. Membranes in anoxygenic phototrophs . (a) Chromatophores . Section through a cell of the purple bacterium Rhodobacter showing vesicular photosynthetic membranes. The vesicles are continuous with and arise by invagination of the cytoplasmic membrane. A cell is about 1 micro m wide. (b) Lamellar membranes in the purple bacterium Ectothiorhodospira . A cell is about 1.5 micro m wide. These membranes are also continuous with and arise from invagination of the cytoplasmic membrane, but instead of forming vesicles, they form membrane stacks. 5

41. Photosynthetic Electron Flow in Purple Bacteria Pohotosynthetic reaction centers are surrounded by antenna pigments that function to funnel light energy to the reaction center. The energy of light is transferred from the antenna to the reaction center in packets called excitons . The light reactions begin when exciton energy strikes the special pair of bacteriochlorophyll a molecules. The absorption of energy excites the special pair, converting it from a relatively weak to a very strong electron donor. Once this strong donor has been produced, the remaining steps in electron flow simply conserve the energy released when electrons flow through a membrane from carriers of low E to those of high E , generating a proton motive force.

42. Before excitation, the purple bacterial reaction center, which is called P870, has an E of about +0.5 V; after excitation, it has a potential of about -1.0 V The excited electron within P870 proceeds to reduce a molecule of bacteriochlorophyll a within the reaction center. This transition takes place incredibly fast, taking only about three-trillionths (3X10 -12 ) of a second.

43. Once reduced, bacteriochlorophyll a reduces bacteriopheophytin a and the latter reduces quinone molecules within the membrane. These transitions are also very fast, taking less than one-billionth of a second. Relative to what has happened in the reaction center, further electron transport reactions proceed rather slowly, on the order of microseconds to milliseconds.

44. 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 bc 1 and cytochrome c 2 . Cytochrome c 2 is a periplasmic cytochrome that functions as an electron shuttle between the membrane-bound bc 1 complex and the reaction center.

45. Electron flow in anoxygenic photosynthesis in a purple bacterium. Only a single light reaction occurs. Note how light energy converts a weak electron donor, P870, into a very strong electron donor, P870*. Bph , bacteriopheophytin ; QA, QB, intermediate quinones ; Q pool, quinone pool in membrane; Cyt, cytochrome. 6

46. Arrangement of protein complexes in the purple bacterium reaction center. The light-generated proton gradient is used in the synthesis of ATP by the ATP synthase ( ATPase ). LH, lightharvesting bacteriochlorophyll complexes; RC, reaction center; Bph , bacteriopheophytin ; Q, quinone ; FeS , iron–sulfur protein; bc1, cytochrome bc1 complex; c2, cytochrome c2. 7

47. Photophosphorylation ATP is synthesized during photosynthetic electron flow from the activity of ATPase that couples the proton motive force to ATP formation. Electron flow is completed when cytochrome c 2 donates an electron to the special pair. This returns these bacteriochlorophyll molecules to their original ground-state potential (E = +0.5 V). The reaction center is then capable of absorbing new energy and repeating the process. This mechanism of ATP synthesis is called photophosphorylation , specifically cyclic photophosphorylation , because electrons move within a closed loop.

48. Autotrophy in Purple Bacteria: Electron Donors and Reverse Electron Flow For a purple bacterium to grow as a photoautotroph, the formation of ATP is not enough. Reducing power (NADH or NADPH) is also necessary so that CO2 can be reduced to the redox level of cell material. As previously mentioned, reducing power for purple sulfur bacteria comes from hydrogen sulfide (H 2 S), although sulfur (S ), thiosulfate (S 2 O 3 2- ), ferrous iron (Fe 2+ ), nitrite (NO 2- ), and arsenite (AsO 3 2- ) can also be used by one or another species. When H 2 S is the electron donor in purple sulfur bacteria, globules of S are stored inside the cells.

49. Phototrophic purple and green sulfur bacteria. (a) Purple bacterium, Chromatium okenii . Notice the sulfur granules deposited inside the cell (arrows). (b) Green bacterium, Chlorobium limicola . The refractile bodies are sulfur granules deposited outside the cell (arrows). In both cases the sulfur granules arise from the oxidation of H2S to obtain reducing power. Cells of C. okenii are about 5 m in diameter, and cells of C. limicola are about 0.9 m in diameter. Both micrographs are brightfield images. 8

50. Reduced substances used as photosynthetic electron donors are oxidized and electrons eventually end up in the “ quinone pool” of the photosynthetic membrane. However, the E of quinone (about 0 volts) is insufficiently electronegative to reduce NAD+ (- 0.32 V) directly. Instead, electrons from the quinone pool travel backwards against the thermodynamic gradient to eventually reduce NAD(P)+ to NAD(P)H. This energy-requiring process, called reverse electron transport, is driven by the energy of the proton motive force If NADPH is needed as a reductant instead of NADH, it can be produced from NADH by enzymes called transhydrogenases .

51. Photosynthesis in Other Anoxygenic Phototrophs Our discussion of photosynthetic electron flow has thus far focused on the process as it occurs in purple bacteria. Although similar membrane reactions drive photophosphorylation in other anoxygenic phototrophs , there are differences in certain details. The reaction centers of green nonsulfur bacteria and purple bacteria are structurally quite similar; however, the reaction centers of green sulfur bacteria and heliobacteria differ significantly from those of purple and green nonsulfur bacteria.

52. A comparison of electron flow in purple bacteria, green sulfur bacteria, and heliobacteria . Note that reverse electron flow in purple bacteria is necessary to produce NADH because the primary acceptor ( quinone , Q) is more positive in potential than the NAD+/NADH couple. In green and heliobacteria , ferredoxin ( Fd ), whose E is more negative than that of NADH, is produced by light-driven reactions for reducing power needs. Bchl , Bacteriochlorophyll ; BPh , bacteriopheophytin . P870 and P840 are reaction centers of purple and green bacteria, respectively, and consist of Bchl a. The reaction center of heliobacteria (P798) contains Bchl g, and the reaction center of Chloroflexus is of the purple bacterial type. Note that forms of chlorophyll a are in the reaction centers of green bacteria and heliobacteria . 9

53. In green bacteria and heliobacteria the excited state of the reaction center bacteriochlorophylls resides at a significantly more electronegative E than in purple bacteria and that actual chlorophyll a (green bacteria) or a structurally modified form of chlorophyll a, called hydroxychlorophyll a ( heliobacteria ), is present in the reaction center. Thus, unlike in purple bacteria, where the first stable acceptor molecule ( quinone ) has an E of about 0 V, the acceptors in green bacteria and heliobacteria iron–sulfur ( FeS ) proteins have a much more electronegative E than does NADH. In green bacteria a protein called ferredoxin (reduced by the FeS protein) is the direct electron donor for CO 2 fixation. This has a major effect on reducing power synthesis in these organisms, as reverse electron flow, necessary in purple bacteria, is not required in green sulfur bacteria or heliobacteria .

54. Oxygenic Photosynthesis In contrast to electron flow in anoxygenic phototrophs , electron flow in oxygenic phototrophs proceeds through two distinct but interconnected series of light reactions. The two light systems are called photosystem I and photosystem II, each photosystem having a spectrally distinct form of reaction center chlorophyll a. Photosystem I (PSI) chlorophyll, called P700, absorbs light at long wavelengths (far red light), whereas PSII chlorophyll, called P680, absorbs light at shorter wavelengths (near red light). Oxygenic phototrophs use light to generate both ATP and NADPH, the electrons for the latter arising from the splitting of water into oxygen and electrons.

55. Electron Flow in Oxygenic Photosynthesis The path of electron flow in oxygenic phototrophs resembles the letter Z turned on its side, and Figure outlines this so-called “Z scheme” of photosynthesis. The reduction potential of the P680 chlorophyll a molecule in PSII is very electropositive, even more positive than that of the O 2 /H 2 O couple. This facilitates the first step in oxygenic electron flow, the splitting of water into oxygen and electrons.

56. Light energy converts P680 into a strong reductant which reduces pheophytin a (chlorophyll a minus its magnesium atom), a molecule with an E of about -0.5 V. An electron from water is then donated to the oxidized P680 molecule to return it to its ground-state reduction potential. From the pheophytin the electron travels through several membrane carriers of increasingly more positive E that include quinones , cytochromes , and a copper-containing protein called plastocyanin ; the latter donates the electron to PSI.

57. The electron is accepted by the reaction center chlorophyll of PSI, P700, which has previously absorbed light energy and donated an electron that will eventually lead to the reduction of NADP+. Electrons transferred through several intermediates terminating with the reduction of NADP+ to NADPH.

58. ATP Synthesis in Oxygenic Photosynthesis Besides the net synthesis of reducing power (that is, NADPH), other important events take place while electrons flow in the photosynthetic membrane from PSII to PSI. Electron transport generates a proton motive force from which ATP can be produced by ATPase . This mechanism for ATP synthesis is called noncyclic photophosphorylation because electrons do not cycle back to reduce the oxidized P680, but instead are used in the reduction of NADP+.

59. Electron flow in oxygenic photosynthesis, the “Z” scheme. Electrons flow through two photosystems , PSI and PSII. Ph, pheophytin ; Q, quinone ; Chl , chlorophyll; Cyt , cytochrome ; PC, plastocyanin ; FeS , nonheme iron–sulfur protein; Fd , ferredoxin ; Fp , flavoprotein ; P680 and P700 are the reaction center chlorophylls of PSII and PSI, respectively. 10

60. The Calvin-Benson cycle The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration. The Calvin cycle has three stages. In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule, 3-PGA. In stage 2, the organic molecule is reduced using electrons supplied by NADPH. In stage 3, RuBP , the molecule that starts the cycle, is regenerated so that the cycle can continue. Only one carbon dioxide molecule is incorporated at a time, so the cycle must be completed three times to produce a single three-carbon GA3P molecule, and six times to produce a six-carbon glucose molecule.

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62. Stage 1: Fixation in addition to CO 2 ,two other components are present to initiate the light-independent reactions: an enzyme called ribulose bisphosphate carboxylase ( RuBisCO ) and three molecules of ribulose bisphosphate ( RuBP ). RuBP has five atoms of carbon, flanked by two phosphates . RuBisCO catalyzes a reaction between CO 2 and RuBP . For each CO 2 molecule that reacts with one RuBP , two molecules of 3-phosphoglyceric acid (3-PGA) form. 3-PGA has three carbons and one phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3-PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the reactions (3 atoms from 3CO 2 + 15 atoms from 3RuBP = 18 atoms in 3 molecule of 3-PGA). This process is called carbon fixation because CO 2 is "fixed" from an inorganic form into organic molecules.

63. Stage 2: Reduction ATP and NADPH are used to convert the six molecules of 3-PGA into six molecules of a chemical called glyceraldehyde 3-phosphate (G3P). This is a reduction reaction because it involves the gain of electrons by 3-PGA. Recall that a reduction is the gain of an electron by an atom or molecule. Six molecules of both ATP and NADPH are used. For ATP, energy is released with the loss of the terminal phosphate atom, converting it to ADP; for NADPH, both energy and a hydrogen atom are lost, converting it into NADP + . Both of these molecules return to the nearby light-dependent reactions to be reused and reenergized.

64. Stage 3: Regeneration At this point, only one of the G3P molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. Because the G3P exported from the chloroplast has three carbon atoms, it takes three "turns" of the Calvin cycle to fix enough net carbon to export one G3P. But each turn makes two G3Ps, thus three turns make six G3Ps. One is exported while the remaining five G3P molecules remain in the cycle and are used to regenerate RuBP , which enables the system to prepare for more CO 2 to be fixed. Three more molecules of ATP are used in these regeneration reactions.

65. Conclusion The Calvin cycle refers to the light-independent reactions in photosynthesis that take place in three key steps. Although the Calvin Cycle is not directly dependent on light, it is indirectly dependent on light since the necessary energy carriers (ATP and NADH) are products of light-dependent reactions. In fixation, the first stage of the Calvin cycle, light-independent reactions are initiated; CO2 is fixed from an inorganic to an organic molecule. In the second stage, ATP and NADPH are used to reduce 3-PGA into G3P; then ATP and NADH are converted to ADP and NADP+, respectively. In the last stage of the Calvin Cycle, RuBP is regenerated, which enables the system to prepare for more CO2to be fixed. For each molecule of carbon dioxide that is fixed, two molecules of NADPH and three molecules of ATP from the light reactions are required. The overall reaction can be represented as follows:

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67. References Reading Brock biology of microorgamism (13 th edition) by Madigan, Martinko , Stahl, Clark Microbiology (10 th edition) by Tortora , Funke and Case Microbiology (5 th edition) by Prescott Images 1-12: Brock biology of microorgamism (13 th edition) by Madigan, Martinko , Stahl, Clark