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IGHER, HOTOSYNTHESIS IN HIGHI, , 207, , PLANTS, , ou may have carried out, Another experin, , half-leaf, s, , Hosed, , eclase, , in, , a, , where, , experime, , t, , test tube, , The, to air., , part of a, , leafis, , containing s o m e KOH soaked, , sorbs, , 1(which, x t o n, , a, , CO,.)., , while the other halfis, , in, setup is then placed, , light for, , nosed, for starch later in the two, me time. On testing must have found that the, kalves of the leaf. you, for starch, the leat tested positive, DOsed part of that w a s in the tube, tested, hile the portion, w a s required for, eative. This showed that CO,, photosynthesis., , Can you, , explain, , how this, , be drawn?, 2onclusion could, , (a), 7, , 13.2 EARLY EXPERIMENT, interesting to, periments that led, , it is, , (b), , simple, gradual development in, , learn about those, to, , a, , ur understanding of photosynthesis., (1733-1804) in 1770, of experiments that revealed the, , oseph Priestley, performed a series, , ssentialrole of airin the growth of greenplants., , miestley, you may recall, discovered oxygen in, 774 Priestley observed that a candle burning in, 2Closed space a bell jar, soon gets extinguished, -, , gure 13.1 a. b, c, d). Similarly, a mouse would, , Tos 177, (d), (c), suffocate, in, that, a, He, concluded, closed, space., n, Figure 13.1 Priestley's experiment, candle or an animal that breathe the air,, Riestly auggested that, gih, somehow, damage the air. But when he placed a mint plant in the, eukini camdle ama baat, e bell jar, he found that the mouse stayed aliveand the candle, J l i a e bad a u, u e d to burn. Priestley hypothesised as follows Plantsre_toreto amumal, caled Phlogsten wua, , Durning, , r whatever breathing animals and burning candles removey, , you imagine how Priestley would have conducted the experiment, USiny, candle and a plant? Remember, he would need to rekindle the, e, , test whether it burns after a few days. How many different, , l, , ou think ofto ight the candle without disturbing the set-up?, , e, , ing a similar setup as the one used by Priestley, but by placing it, in the, , dark, , ved that, , in the sunlight(Jan Ingenhousz)(1730-1799), and once, that somehow, is, the, , plant process, Purifies the sunlight, air fouled by ourning candles, or breathing animals., , les 1, , ential to, , hbrnght1Ousz in an elegantexperime, , with an aquatic plant showed that, , sunlight, small bubbles were formed around the green parts, , nthe dark they did, they, , not. Later he identified these bubbles to, , bethatof, flencehe, showed, that, it, is, only, the, green, part, of, the, plants, ld release oxygen), , Puteol bplonts wi tt, , dephlbg'ston, maghijyighss len
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208, Tst, , BioLoGY, , pdt ophotd. Glucoae, =, , tha Julius, , von Sachs provided evido., SucHOse for production of glucose when plants grow. Glucose is usually store, , Stable, * J * siau /visbl, , It was not until about 1854, , S tach. Starch. His later studies showed that the green substance in nia, , (chlorophyll as we know it now) is located in special bodies (later called, chloroplasts) within plant cells. He found that the gueen parts in plants, that the glucose is, where, , glucoseis made, and, , usuallystored as tarch), , Now consider the interesting experiments done by T.W Engelmann, (1843 1909). Using a prism he split light into its specfral componen., , SPikoata, and then illuminated a green alga. Cladophora, placed in a suspensinm, of aerobic bacteria. The bactería were used to detect the sites of, evolution. He observed that the bacteria accumulated mainly in the region, of, , blue and red light of the split spectrum. A first, , action, , spectrum of, , photosymthesis was thus de_cribed. It resembles roughly the absorption, spectra of chlorophyll a and b(discussed in section 13.4)., , By the middle of the nineteenth century the key features of plant, , photosynthesis were known, namely, that plants could use light enerev m, to make carbohydrates from, CO, and water. The, , empirical equation in, oxygen evolving de, , representing, , the total process of, photosynthesis for, organisms was then understood as:, , CO +H,O, where, , sugar)., , [CH,O] represented, , a, , ICH,O]+0,, carbohydrate (e.g., glucose, a six-carbon, , A milestone, , contribution to the understanding of photosynthesis was, that made by a, microbiologist, Cormelius van Niel (1897-1985). who,, based on his studies of, purple and green bacteria, demonstrated that, , photosynthesis is essentially a light-dependent reaction in which, hydrogen from a suitable oxidisable compound reduces carbon dioxide, to, , carbohydrates. This can be expressed by:, , 2H,A+CO, Light2A +CH,O+H,O, In green plants, , H,O is the hydrogen donor and is oxidised to O,. 01, organisms, do release O, during photosynthesis. When H., is the, hydrogen donor for purple and green, sulphur bacteeem, Oxidation' product is, or, sulphur, on the, sulphate, depending, and not O,. Hence, he, orgaplant, inferred that the O, evolved by, the, gree, comes from H,O, not from carbon dioxide. This was, Dy, later, proved, he, the, radioisotopic techniques) The correct equation, that would, represet, overall process of, not, , ol, , photosynthesis is therefore:, , (, , where, , 6CO, +12H,O, , tCH,,0 +6H,0+60,, , ODu dakOm, , C H2 O, represents glucose. The O,, released is from, was proved using radio, singe, isotope, that this is nota, this, , water:, , techniquesNote, , is, , T0
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PHOTO, , but description ol a multisteP process called photosynthesis., explain why twelve molecules of water as substrate are used, , reaction b u t d e s ., , given above? For treddat e bneos eog eoHzbeor g, Gaation, 6 CO,RN D°tane, req, n the, , oso ifoorGED, , u, , 83 WHERE DOES PHOTOSYNTHESIS TAKE PLACE?, , 13.3, , l d of course answer: in "the green leaf or you may add, 'in the, arnlasts' based on what you earlier read in Chapter 8. You are, , You, , nitely right. Photosynthesis does take place in the green leaves of plants, it does so also in other green parts of the plants. Can you name some, , butit, , ther parts vhere you think photosynthesis may pccur?, , You would recollect fom previous unit that the mesophyll cells in the, , large number of chloroplasts. Usually the chloroplasts align, themselvesalong the walls of the mesophyll cells, such that they get the, bAes,, , have a, , oa.tnoPhe, , optimum quantity of the incident light) When do you think the, , chloroplasts will be aligned with their flat súrfaces parallel to the walls?, When would they be perpendicular to the incident, light?, You have studied the structure of, chloroplast in Chapter 8. Within, the chloroplast there is the, , membranous systenm consisting of grana, the, stToma lamellae, and the fluid stroma (Figure T3.2).There is a clear, division, oflabour within the chloroplast. The membrane, is responsible for, system, the, , trapping light energy and also for the synthesis of ATPand NADPH., n stroma, enzymatic reactions incorporate CO, into the plant, leading tQ, the, synthesis of sugar, which in turn forms starch. The former set of, Pactions, since they are directly light driven are called, light, Ihe latter are not directly light driven but are dependent orn thereactions., products, a ight reactions (ATP and NADPH). Hence, to, the latter they, distinguish, are, called, by convention, as dark reactions. However, this should not be, Onstrued to, , mean, , Gependent., , that they occur in darkness, , or, , that they, , are, , not, , 7mamboraKL Stem, stdama, , L, , uid stsr, , lightOuter membrane, -Inner membranne, , Stromal lamella, , PS7, PS.T, , -Grana, , Aamal, , PS, S, , Stroma, -Ribosomes, 00, Starch granule, , Lipid droplet, Bure 3.2 Diagrammatic representation of an electron micrograph of a section of, , chloroplast, 4,, , O
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IN PHOTOSYNTHHESIS?, , VED, ED, , Looking at plants have you ever wondered, , why, and hoOw there are so many shades of, green in, their leaves even in the same, plant? We can, look for an answer to this, question by trying to, separate the leaf pigments of any green, -, , through, , plant, , paper, , chromatography., , A, , chromatographic separation of the leaf pigments, shows that the colour that, not. due to a, , we see, , in leaves, , Is, , single pigment but due to fourI, pigments: Chlorophyll a (bright or blue green, in the chromatogram), chlorophyll b (yelow, 8reen), Xanthophylls yellow) and carOtep s, elewto yelloW-orange).) Let usnow BEe Wild, roles yariou1, CDMe, , ais
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YNTHESIS IN HIGHER, , 2, , PLANTS, , celIs(in mitochondria and, ohloroplasts, is amed phosphorylation,Photo-, , Poo, , ynthesisedbye, , ATPis, , Photosystem I, , cnhorylation is the synthesis of ATP from, phos, , Dand inorganicphosphatein thepresence of, hen the two photosystems work in a, , e acceptor, , ADPa, , series., , first PSland then, , the, , SI, aprocess called, , (Light, , -CYclic photo-phosphorylation occurs. The, nHD, , photosystems, , are, , connected, , electron transport chain,, 7 scheme., , as seen, , through, , an, , Electron, , earlier- in the, , Both ATP and NADPH, , +, , H', , transport, system, , are, , amthesised by this kind of electronflow (Figure, , 13.5)., When only PS I is functional, the electron is, dreulated within the photosystem and the, , Chlorophyll, P 700, , phosphorylation occurS due to cyclic flowof, , Figure 13.6 Cyclic photophosphorylation, electrons (Figure 13.6). A possible location, where this could be happening is in the stroma, amellae. While the membrane or lamellae of the grana have both PSI, and PS II the stroma lamellae membranes lack PS II as well as NADP., Teductase enzyme. The excited electron does not pass on to NADP" but is, gycled back to the PS I complex through the electron transport chain, Pigure 13.6). The cyclic flow hence, results only in the synthesis of ATP,, , Dutnot of NADPH +H'. Cyclic.photophosphorylation also occurs when, , mly light of wavelengths beyond 680 nm are available for excitationa., , 13.6.3 Chemiosrmotic Hypothesis, t us now try and understand how actually ATP is synthesised in the, plast.The chemiosmotic hypothesis has been put forward to explain, , mechanism. Like in respiration., in photosynihesis too. ATPsynthesisis, kcd to development of a proton gradient across a membrane. This time, , thes are membranes the, thylakoid. There is one difference though, here, ese, of, he, i.e., in the, , OLOn accumulation is towards the inside ofthe membrane,, intermembrane space of, nen. In, respiration, protons accumulate in the, the, he, mitochondria when electrons move through the ETS (Chapter 14)., s, , understand what causes the proton gradient across the, , Mermbr, ane. We necd to consider again the processes that take place during, , the tivation of electrons and their transport to determine the steps that, , ause a pro, , aprolon, S, , gradient to develop (Figure 13.7)., the inner side of, , splitting of the water molecule takes place on, embrane, the protons or hydrogen lons that are produced by, , the, Splitting of water accumulate within, , the lumen of the thylakoids.
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Light reaction, The, , light, , reaction takes, , place, , in the grana, , f the chloroplast. Here, light energy gets, onverted to chemical energy as ATP and, , ADPH., , In this very, , light reaction,, , the, , ddition of phosphate in the presence of, ght or the synthesizing of ATP by cells is, known as photophosphorylation., , Dark reaction, While in the dark reaction, the energy, , produced previously in the light reaction is, utilized, , to, , fix, , carbon, , dioxide, , into, , carbohydrates. The location where this, happens is the stroma of the chloroplasts., , Photophosphorylation, Photophosphorylation is the process of, utilizing light energy from photosynthesis, to convert ADP to ATP. It is the process of, , synthesizing energy-rich ATP molecules by, transferring the phosphate group into ADP, molecule in the presence of light.
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Cyclic, , Non-Cyclic, , Photophosphorylation Photophosphorylation, Only Photosystem lis, , Both Photosystem|, , involved., , and ll are involved., , P700 is the active, , P680 is the active, , reaction centre., , reaction centre., , Electronstravel, , in, , a, , Electrons travel in a, , cyclic manner., , non, , cyclic manner., , Electrons revert to, Photosystem I, , Electrons from, Photosystem I are, accepted by NADP, , ATP molecules are, , Both NADPH and ATPP, , produced., , molecules are, , produced., Water is not, , required., , Photolysis of water is, present., , NADPH is not, synthesized., , synthesized., , Oxygen is not evolved, , Oxygen is evolved as a, , as the by-product, , by-product., , This process is, , This process is, , redominant only in, , BOOK teria., , Free Class, , NADPH is, , predominant in all, green plants.
