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12.PHOTOSYNTHESIS, , .c, om, , INTRODUCTION, •, , All living organisms require energy to carry out their different life activities. This energy directly or indirectly, comes from the sun., • The green plants are unique in the respect that they synthesize food in the presence of sunlight from simple, , substances like carbon dioxide and water., , lo, g, , sp, ot, , "Photosynthesis is a Physicochemical or photo-biochemical process (anabolic & endergonic) in which organic, compounds (carbohydrates) are synthesised from the inorganic raw material (H2O & CO2) in presence of light, & pigments. O2is evolved as a byproduct"., , p., b, , Light energy is conserved into chemical energy by photosynthesis., • 90% of total photosynthesis is carried out by aquatic plants., , • First true and oxygenic photosynthesis started in cyanobacteria (Blue - green algae)., • In the Cuscuta (parasitic) and fungi (achlorophyllous), photosynthesis is absent., , gy, ch, am, , • Euglena is a photosynthetic organism and is the link between animals and plants. Though, they are, photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by, predating on other smaller organisms., • Roots of Tinospora and Trapa are assimilatory or photosynthetic., • Atmosphere contains only about 0.03percent carbon dioxide by volume. This small percentage, represents 2200 billion tons of CO2in the atmosphere. The oceans contain over 50 times by amount of, atmospheric CO2 in the form of dissolved gas or carbonates. From these two sources, about 70 billion, tons of carbon is fixed by the green plants annually., , w, , w, , w, , .b, , io, , lo, , Chloroplasts / site of photosynthesis., , www.biologychamp.blogspot.com, , 1

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.c, om, , lo, , gy, ch, am, , p., b, , lo, g, , sp, ot, , Chloroplasts are green plastids. These are the special protoplasmic organelles present only in the green cells of, plants. Chloroplasts act as photosynthetic apparatus., The entire process of photosynthesis is completed in each chloroplast. Hence these are the site of photosynthetic, reactions., i), It is the structural and functional unit of photosynthesis which is studied by Willstatter et al., ii) It is bounded by double lipoprotenous membranes called peristomium or chloroplast envelope (40 -60 A°), iii) The outer membrane is thin and permeable to no. of solute and metabolites, iv) The inner membrane is thick and selectively permeable., v) In between these two membranes a space is present known as periplastidal space (100 -200 A°), vi) Inside the peristomium a space or cavity is present which is filled by coloureless hydrophilic matrix called, as stroma., vii) The matrix having no. of granules, enzymes, DNA, RNA, 70’S Ribosome., viii) The DNA of Chloroplasts is called chloroplast DNA or ctDNA or Plastidome., ix) Here, stroma is the site for the dark reaction., x) Inside the stroma 40 to 100 grana are present, xi) Each granum is unit of plates which are arranged one upon another and such grana are connected with each, other by thin lamellae called as intergrana lamellae or stroma lamellae or fret channels having the enzyme, carboxylase useful for CO2 fixation., xii) Each disc like structure of grana separately called as Thylakoids. In prokaryotes like cyanobacteria, purple, bacteria, etc., thylakoids are present but they lie naked in the cytoplasm, xiii) According to the quantasomal theory the thylakoid having a membrane called as thylakoid membrane or fret, membrane., xiv) The thylakoid membrane is made up of ultra-microscopic hemispherical units called quantasome., xv) The term quantasome is given by Park and Biggins., xvi) Each quantasome internally having 200 – 250 pigment systems like –, Chlorophyll, Carotenoids, Phycobilins, • Chlorophylls are tetrapyrrolic magnesium (Mg) porphyrin compounds., • Chemically chlorophyll molecule consists of two parts head of tetrapyrrol the Porphyrin ring and a long, hydrocarbon tail called phytol attached to the porphyrin group., • Chlorophyll molecule has a Mg-porphyrin head and an alcoholic phytol tail. Head is hydrophilic and phytol tail is, hydrophobic in nature., , io, , PHOTOSYNTHETIC PIGMENTS, , w, , w, , w, , .b, , Chlorophyll a – C55H72O5N4Mg (CH3 group at IIIrd C of IInd pyrrole ring), Chlorophyll b – C55H70O6N4Mg (CHO group at IIIrd C of IInd pyrrole), Chlorophyll c – C55H32O5N4Mg,, Chlorophyll d – C54H70O6N4Mg,, Carotenes – C40H56, Xanthophylls – C40H56O2., • Chlorophyll-a and carotenes are universal pigments, which are found in all O 2 liberating cells., • Chlorophylls are soluble only in organic solvents like ketones, ethers etc., • Stroma lamellae/stroma thylakoids lack PS II and enzyme NADP reductase., • By paper chromatography/chromatogram, produces different pigment colours which appear as:, • Chlorophyll a - blue-green., • Chlorophyll b - yellow green., • Xanthophyll - yellow, • Carotenoids - yellow to yellow – orange, , www.biologychamp.blogspot.com, , 2

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•, , p., b, , lo, g, , sp, ot, , .c, om, , Carotenoids absorb light energy and transfer it to Chlorophyll- a and thus, act as accessory pigments., They protect the chlorophyll molecules from photo-oxidation by picking up nascent oxygen and, converting it into harmless molecular stage. Carotenoids can be classified into two groups namely, carotenes and xanthophylls., • Carotenes: They are orange red in colour and have general formula C 40H56. They are isolated from, carrot. They are found in all groups of plants i.e., from algae to angiosperms. Some of the common, carotenes are α, β, γ and δ carotene; phytotene, lycopene, neurosporene etc. Lycopene is a red pigment, found in ripe tomatoes and red pepper fruits. The β-carotene on hydrolysis gives vitamin A, hence the, carotenes are also called provitamin A. β-carotene is black yellow pigment of carrot roots., •, Xanthophylls: They are yellow coloured carotenoid also called as xanthols or carotenols. They contain, oxygen also along with carbon and hydrogen and have general formula C40H56O2., Lutein (C40H56O2) is a widely distributed xanthophyll which is responsible for yellow colour in autumn, foliage. Fucoxanthin (C40H56O6) is another important xanthophyll present in Phaeophyceae (brown, algae)., •, Phycobilins: These pigments are mainly found in blue-green algae (cyanobacteria) and red algae., These pigments have open tetrapyrrolic structure and do not bear magnesium and phytol chain., •, Blue-green algae have higher quantity of phycocyanin and red algae have more phycoerythrin., Phycocyanin and phycoerythrin together form phycobilins. These water-soluble pigments are thought, to be associated with small granules attached with lamellae. Like carotenoids, phycobilins are accessory, pigments i.e., they absorb light and transfer it to chlorophyll a., All photosynthetic plants have these pigments that absorb light between the red and blue region of the, spectrum. Carotenoids found mainly in higher plants absorb primarily in the violet to blue regions of the, spectrum. They not only absorb light energy and transfer it to chlorophyll but also protect the chlorophyll, molecule from photo-oxidation., Nature of Light:, , w, , .b, , io, , lo, , gy, ch, am, , Light is a form of energy. It travels as stream of tiny particles called photons. A photon contains a, quantum of light. Light has different wavelengths having different colors. One can see electromagnetic radiation, with wavelengths ranging from 390nm to 730nm.This part of the spectrum is called the Visible light. It lies, between wavelengths of ultraviolet and infra-red., , w, , w, , ABSORPTION AND ACTION SPECTRA, The curve representing the light absorbed at each wavelength by pigment is called absorption spectrum., Curve showing rate of photosynthesis at different wavelengths of light is called action spectrum., , Absorption spectrum, Absorption spectrum is studied with the help of spectrophotometer. The absorption spectrum of, chlorophyll a and chlorophyll b indicate that these pigments mainly absorb blue, violet and red lights, (430 nm, and 662 nm for chlorophyll a, 455 nm and 644 nm for chlorophyll b)., , www.biologychamp.blogspot.com, , 3

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.c, om, , The absorption spectrum of chlorophyll-a and b clearly shows that more light energy is absorbed at blue,, violet and red wavelengths of the visible spectrum., , sp, ot, , Action spectrum of photosynthesis, , gy, ch, am, , p., b, , lo, g, , Action spectrum shows that maximum photosynthesis takes place in blue and red regions of the, spectrum. The first action spectrum of photosynthesis was studied by T.W. Engelmann (1882) using green alga, Spirogyra and oxygen seeking bacteria., , .b, , io, , lo, , Photoexcitation of chlorophyll-a, , Ground state, , Excited state, , Ionised state, , w, , w, , w, , 1. Initially chlorophyll a is at the ground state., 2. When chlorophyll a absorb the photons from sunlight, its electron gets activated(charged), and moves to the, higher energy level called as excited state., 3. Such exited stage electron of chlorophyll a having extra amount of energy., 4. When such electron transported through electron accepter system, it loses its energy where ATP is produced, i.e. light energy is converted into chemical energy., 5. When such electron is transported the chlorophyll a is without electron having +ve charge, known as ionised, state., 6. Such chlorophyll a does not remain without electron more than 10-9 seconds, hence it gets its own electron or, from the photolysis of water., The oxygen evolved contain18O2 thereby proving Van Neil’s hypothesis that oxygen evolved in, photosynthesis comes from water. This leads to the currently accepted general equation of photosynthesis –, Light, 6CO2+ 12H218O, C6H12O6+ 6H2O + 618O2, , www.biologychamp.blogspot.com, , 4

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.c, om, , Reaction center:, The light absorbing pigments are located in the thylakoid membranes. They are arranged in clusters of, chlorophyll and accessory pigments along with special types of chlorophyll molecules P680and P700(the letter, P stands for Pigment and 680 and 700 for the wavelengths of light at which these molecules show maximum, absorbance). P680 and P700molecules form the Reaction centers or Photocenters., The accessory pigments and other chlorophyll molecule harvest solar energy and pass it on the reaction, centers. These are Called Light harvesting or Antenna molecule. They function to absorb light energy, which they, transmit at a very high rate to the reaction center where the photochemical act occurs., , Hill reaction / Photolysis of water., , lo, g, , sp, ot, , Firstly, it is considered that during the photosynthesis the oxygen which is given outside the plant comes, from the carbon dioxide and not from water, but in 1930 Van Niel by using green sulpher bacteria showed that, the oxygen is produced from the break down (lysis) of water to produce oxygen and hydrogen., In 1939 Robert hill carried out a experiment in which it is shown that the oxygen comes from the, photolysis of water in the presence of sunlight such experiment is known as Hills Reaction., In this expt. He isolated the chloroplast from green spinach leaves and suspended inside the carbon, dioxide free medium in which ferric salt and haemoglobin are added and all such content is exposed to sunlight., After sometime the found ferrous salts and oxyhaemoglobin., This expt. indicates that in the presence of light and chlorophyll water molecule gets splits into hydrogen, and oxygen which are accepted by ferric salts and haemoglobin., Here, the photolysis or water is shown by reaction., , gy, ch, am, , p., b, , Where, (A = hydrogen acceptor), During the experiment. Hills is unable to tell the name of Acceptor (A) but in 1959 Arnon discovered the, acceptor for hydrogen is NADP., Importance of Hills Reaction., i) Hills reaction shows that the oxygen which is give during photosynthesis comes form the splitting of water., ii) The NADP is the Hydrogen Acceptor which accept the hydrogen to form NADPH2 such hydrogen also comes, from photolysis of water., Confirmation of Hills reaction., In 1941 Ruben Martin and Kamen confirmed that the water is only. Source for oxygen in the, photosynthesis. It is confirmed by using radio- active isotope as a trace element as follows, 1) When carbon dioxide having O 18 type of oxygen (CO2 18) is given to the plant it gives out normal oxygen, outside the plant., , io, , lo, , 2) When water having O18 type of oxygen (H2O18) is given to the plant it gives out O18 type of oxygen outside, the plant., , From such expt. It is confirmed that the oxygen during photosynthesis comes from the photolysis of water., , .b, , Phosphorylation, , w, , w, , w, , Formation of ATP by the addition of one inorganic phosphate in the ADP is known as phosphorylation., The phosphorylation is of two types, 1) Photophosphorylation, 2) Oxidative phosphorylation., 1) Photophosphorylation, Formation of ATP from ADP and inorganic phosphate in the presence of light is known as, photophosphorylation., , ADP + iP (H3PO4), , light, ⎯⎯⎯⎯⎯, →, chlorophyll − a, , ATP, , (ADP = adenosine diphosphate), 2) Oxidative phosphorylation, Formation of ATP from ADP and inorganic phosphate in the presence of oxidation of food or chemicals, is called oxidative phosphorylation., , www.biologychamp.blogspot.com, , 5

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Mechanism of Photosynthesis, , .c, om, , Photosynthesis consists of two successive series of reactions., • Light phase or photochemical reactions or light dependent reactions or Hill's reactions:, During this stage, energy from sunlight is absorbed and converted to chemical energy which is stored in, ATP and NADPH + H+., • Dark phase or chemical dark reactions or light independent reactions or Blackman’s reaction or biosynthetic, phase:, During this stage, carbohydrates are synthesized from carbon dioxide using the energy stored in the ATP, and NADPH formed in the light dependent reactions., , Chemiosmotic Hypothesis, , .b, , io, , lo, , gy, ch, am, , p., b, , lo, g, , sp, ot, , The Chemiosmotic hypothesis is given by Dr. Peter Mitchell in 1961., 1. The moment of ions across a selectively permeable membrane by their electro chemical gradient is called as, chemiosmosis., 2. Hydrogen ions are more across a membrane during their cellular respiration in mitochondria & during, photosynthesis in chloroplast which leads to generate ATP., 3. In photosynthesis, the membrane is thylakoid and the protons accumulate in the lumen grana while in respiration,, protons accumulate in the intermembrane space of mitochondria., 4. When hydrogen ion diffuses from an area of higher photon concentration area to an area of lower photon, concentration, electrochemical concentration gradient of proton is developed., 5. This electrochemical concentration gradient of protons across a membrane could be sufficient of make ATP, 6. ATP synthetase is the enzyme that make ATP by Chemiosmosis., 7. It allows proton to pass through the membrane using the kinetic energy to phosphorylate ADP producing ATP., 8. Due to splitting of water molecule on the inner side of the membrane, hydrogen ions accumulate within the lumen, of thylakoids., 9. The NADP reductase enzyme is located on the stroma side of the membrane., 10. For reduction of NADP to NADPH2 protons are required along with electron that come from ferrodoxin., 11. Hence, within the chloroplast, protons in the stroma decrease in number, while in the lumen there is increase in, number of protons., 12. This created a proton gradient across the thylakoid membrane. There is subsequent spontaneous movement of, protons which produced energy useful for the production of ATP synthesis., , w, , Light reaction:, , w, , w, , Light reaction / photochemical reaction / primary process of photosynthesis, The light reaction or photochemical reaction is carried inside the grana part of chloroplast in the presence of, sunlight., During this process the chlorophyll. A absorbs the sunlight energy by which the electron of it gets excite or, charged after the charging electron is circulated through the electron acceptor cytochrome system. During which electron, loses its charging and comes at the ground stage. Here, during circulating of electron through electron acceptor, cytochrome system ADP is converted in to ATP and NADP is converted in ADDPH2., Here, ATP is produced from the ADP & Inorganic phosphate in the presence of light known as, photophosphorylation., Photophosphorylation was discovered by Arnon et al in 1954., , www.biologychamp.blogspot.com, , 6

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p., b, , lo, g, , sp, ot, , .c, om, , For the photophosphorylation two photo systems are required, i) Photo System – I, ii) Photo System – II, 1) Photo system I, It having the chlorophyll which absorbs the sunlight of 700 nano meters, hence called as chlorophyll – 700., 2) Photo system – II, It having the chlorophyll which absorbs the sunlight of 680 nano meter hence called as chlorophyll – 680., There are two types of photophosphorylation., A) Cyclic Photophosphorylation., B) Non – Cyclic Photophosphorylation., A) Cyclic Photophosphorylation., , gy, ch, am, , 1) It is the type of photophosphorylation in which high energy having electron emitted from excited chlorophyll, – A. 700 of PS – I return again to chlorophyll A 700, 2) In cyclic photophosphorylation pigment system-I having chlorophyll 700 in involved, 3) When chlorophyll 700 absorb photon energy the electron of it gets excite and such charged electron is tossed, to circulate through electron acceptor system of cytochrome., 4) During this process ATP is produced from ADP and inorganic phosphate., 5) During the circulation of electron, the cytochrome system having firstly ferredoxin reducing substance (z), from which electron is given to ferredoxin during the process 1 ATP is produced from ferredoxin electron is, given to cytochrome b6. From cytochrome b6 electron is given to cytochrome f. Here 2nd ATP is produced, from cytochrome f electron is given to plastocyanin, at plastocyanin level electron comes at ground state, which is again given to chlorophyll 700 of photo system – I for excitement which is circulated again and again, hence, the type is called cyclic Photophosphorylation., , w, , w, , w, , .b, , io, , lo, , ii. Non-cyclic photophosphorylation:, , In this system both the photo systems i.e. photo stem I and II required., i) In non – cyclic photophosphorylation, firstly chlorophyll A 680 of photo system II absorb photon from sunlight., ii) When the photon is absorbed the electron of chlorophyll A 680 gets excited and tossed which is accepted by the, primary electron acceptor Q or PQRS (Plastoquinone reducing substance), iii) Now, from Q acceptor the excited electron shifted to the Plastoquinone then to the cytochrome b6 then to, cytochrome f then to the plastocyanin (PC) and finally given to the chlorophyll-a 700 of photo system – I. During, such process of transports of the electron only one ATP is produced., , www.biologychamp.blogspot.com, , 7

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lo, g, , sp, ot, , .c, om, , iv) When the chlorophyll A 700 of Ps-I accept the electron. Such electron is again charged by the photons of sunlight., When the electron circulated through the second electron accepting system which contains the acceptor (z ) or, ferredoxin reducing substance (F.R.S.) Ferredoxin, NADP., v) When the electron is circulated through the acceptor system of chlorophyll A -700 the photolysis of water is carried, out by which H + and OH – ions are produced., vi) The H+ ions are accepted by NADP to form NADPH2. NADPH2 is source of high energy which produces 3 ATP., vii) The OH ions liberate electron to produce 4e which are accepted by ch-680 of photosystem II in non- cyclic manner., viii) After giving electron OH ions are converted into H2O (water) and O2 (Oxygen) as a byproduct of photosynthesis., ix) For the photolysis of water manganese, calcium and chloride ions are required., Significance of non – cyclic photophosphorylation., i) The ATP is produced which is supplied to dark reaction., ii) The NADPH2 is also supplied to the dark reaction as a hydrogen donor., iii) The non – Cyclic photophosphorylation carry photolysis of water which gives hydrogen and oxygen., iv) The oxygen is given outside as the byproduct which is useful for purification of the air., Cyclic Photophosphorylation, Non cyclic Photophosphorylation, 1) emitted electron returns to the chlorophyll, 1) emitted electron does not return to the chlorophyll, 2) chlorophyll A receives its own electron, 2) Chlorophyll A receives electron from photolysis of water., 3)it having photosystem I, 3) it having photosystem I and II, 4) only ATP is produced, 4) ATP and NADPH2 produced, 5) chlorophyll 700 is present, 5) chlorophyll 700 and 680 are present, 6) comparatively less energy is produced, 6) comparatively more energy is produced, 7)photolysis of water does not carried out, 7) photolysis of water is carried out, 8) oxygen is not given out, 8) oxygen is given out, , Dark reaction, , w, , w, , w, , .b, , io, , lo, , CALVIN CYCLE / C3-CYCLE, , gy, ch, am, , p., b, , Carbon fixation occurs in the stroma by a series of enzyme catalyzed steps. Molecules of ATP and NADPH, produced in the thylakoids (light reaction) come in the stroma where carbohydrates are synthesized., The path of carbon fixation in dark reaction through intermediate compounds leading to the formation of sugar, and starch was worked out by Calvin, Benson and their co -workers. For this, Calvin was awarded Nobel Prize in 1961., Path of carbon was studied with the help of radioactive tracer technique using Chlorella, a unicellular green alga, and radioactive14CO2.With the help of radioactive carbon, it becomes possible to trace the intermediate steps of fixation, of14CO2., , www.biologychamp.blogspot.com, , 8

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gy, ch, am, , p., b, , lo, g, , sp, ot, , .c, om, , The various steps in the dark reactions (Calvin cycle / C-3 pathway) are as follows:, 1. Carboxylation :, CO2reduction starts with a 5-carbon sugar, ribulose-1,5-bisphosphate (RuBP). It is a 5-carbon sugar, (pentose) with two phosphate groups attached to it., RuBP reacts with CO2to produce a short - lived 6-carbon intermediate in the presence of an enzyme, RuBP carboxylase or Rubisco and immediately splits into 3-carbon compound,3-phosphoglyceric acid (3-PGA)., Rubisco is a large protein molecule and comprises 16% of the chloroplast proteins., 2. Glycolytic Reversal:, Molecules of 3-PGA form 1,3-diphosphoglyceric acid utilizing ATP molecules. These are reduced to, glyceraldehyde-3-phosphate (3-PGAL) by NADPH supplied by the light reactions of photosynthesis., For the Calvin cycle to run continuously, there must be sufficient amount of RuBP which accepts CO2and, a regular supply of AT and NADPH. Out of each of 12 molecules of 3-phosphoglyceraldehyde (3-PGAL), 2, molecules are used for synthesis of one glucose molecule and remaining 10 molecules are used for regeneration, of 6 molecules of RuBP., 3. Regeneration of RuBP:, During the reduction stage 12 molecules of 3 phosphoglyceraldehyde are produced out of which 10, molecules are used for regeneration of 6 molecules of ribulose mono-phosphate (RUMP), 10(3C) ═ 6(5C), through the intermediate products like erythrose 4 phosphate, seudoheptulase 7 phosphate., 4. Synthetic phase:, 2 molecules of 3 PGAL produced in the reduction stage enters inside the synthetic phase where they are, converted into single molecule of hexose sugar glucose useful for the plant metabolism & for the production of, starch. Such conversion of 3 phosphoglyceraldehyde into glucose is carried out through intermediate products, like fructose 1, 6 diphosphate, fructose 6 phosphate, glucose 6 phosphate., For the synthesis of one molecule of glucose 6 turns of Calvin cycle are required or 6 molecules of Co 2, are required and 1/6 part of PGAL (phospho glyceraldehyde) i.e. out of 12 molecules 2 molecules are used for, the synthesis of glucose., SIGNIFICANCE OF C3 PATHWAY, 1) The atmospheric carbon dioxide (CO2) is taken inside the plants and converted into carbohydrates., 2) ATP and NADH2 which are produced in light reaction are used in the dark reaction while NADH and ADP are, given to light reaction, Thus, the Calvin cycle regenerates ADP and NADP required for the light reaction., , w, , .b, , io, , lo, , Photorespiration:, , Photorespiration occurs under the conditions like high temperature, bright light, high oxygen and low, CO2concentration. RUBISCO is most abundant enzyme in the world & its active side can bind by co2 & O2, (oxygen), RUBISCO is thermolabile and requires higher concentration of CO2 for its activity., At higher temperature the stomata get closed partially so availability of CO2 becomes low., At higher temperature and at low CO2 RUBISCO functions as oxygenase and brings oxidation of RUBP, instead of carboxylation., At high temperature, highlight intensity &at low carbon dioxide concentration oxidation of RUBP takes by oxygen, which results in the formation of one molecule of 2 carbon compound Phosphoglycolate and one molecule of 3, carbon (PGA)., RUBP, phosphoglycolate + PGA, , w, , w, , 1, , 2, 3, 4, 5, , www.biologychamp.blogspot.com, , 9

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PGA gets enter in Calvin cycle(C3 cycle) while phosphoglycolate dephosphorylated to remove inorganic, phosphate to form glycolate in the chloroplast, 1. Glycolate gets diffuse inside the peroxisome where it is oxidized to glycoxylate and then gets converted into, an amino acid called glycine (2C), Glycolate, glycoxylate, Glycoxylate, , 3., , 4., 5., 6., 7., , Glycine gets entered inside the mitochondria where 2 molecules gets combine form one molecule of, serine(3c) and one molecule of CO2, Glycine + Glycine, serine + CO2, Glycine gets entered inside the peroxide and gets phosphorylated to convert into glycerate., ATP, ADP, , .c, om, , 2., , Glycine, , Glycine, Glycerate, The Glycerate enters into chloroplast and gets phosphorylated to form PGA which is utilized inside C3 cycle., Here 75% of the carbon lost by oxygenation of RUBP is recovered but 25 % CO2 is lost as release of one, molecule of CO2., Photorespiration is also called photosynthetic carbon oxidation cycle., Photorespiration protects the C3 plants from photooxidative damage., , sp, ot, , 6, , lo, g, , C4 CYCLE (HATCH & SLACK PATHWAY), , w, , w, , w, , .b, , io, , lo, , gy, ch, am, , p., b, , In certain tropical plants at low concentration of Co2 RUBISCO does not directly absorb the carbon di oxide ,, hence another pathway is developed where the first stable compound having four carbon atoms (oxalo acetic acid) is, produced hence known as C4 pathway., The c4 pathway is discovered by three scientists Hatch, Slack, Kortschak hence it is known as H.S.K. pathway., The H.S.K. pathway is carried inside the tropical monocot plants like sugarcane, maize, jowar in which the, vascular bundle is covered by bundle sheath., ., , The c4 plants having an anatomical modification in which the chloroplast is present both in, mesophyll and bundle sheath but both chloroplasts are dimorphic in nature., i) The chloroplast which is present in the mesophyll cells having grana., ii) The chloroplast which is present in bundle sheath is without grana., Such type of structure shown by the chloroplast is known as dimorphic form of chloroplast and it, is also known as Kranz anatomy., , www.biologychamp.blogspot.com, , 10

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.c, om, , The C4 plants are more efficient than C3 plants Because ---i. The C4 plant can grow well in less amount of water than the C3 plants., ii. In C3 plants, for the photosynthesis the amount of CO2 required is 50 ppm but when CO2 amount less, than it C3 plants unable to take CO2 for photosynthesis while C4 plants are able to absorb the CO2 in, less amount even at 1 to 2 ppm. i.e. C3 plants are unable to carry photosynthesis at less CO2, concentration but C4 plants are able., iii. The C4 plants are able to carry out photosynthesis in excess heat and high light intensity where C3, plants are unable to do it., Anatomical and Physiological Peculiarities of C4 Plants, , C4 pathway, 1) c4 pathway takes place in mesophyll and bundle sheath, cells, 2)Kranz anatomy is not observed, 2) Kranz anatomy is observed, 3)chloroplast is grana type, 3) both grana and agranal chloroplast are present, 4)co2 acceptor is RUDP, 4) co2 acceptor is PEP, 5) first stable compound is phosphoglyceric acid, 5)first stable compound is oxalo acetic acid, 6)photosynthesis takes place at high concentration of co2 6)photosynthesis takes place at low concentration of co2, 7) e.g. rice, wheat, potato, 7) e.g. maize, sugarcane, jawar, Since this conversion results in the formation of AMP (not ADP), two ATP are required to regenerate ATP, from AMP. Thus, C4pathway needs 12 additional ATP. The C3pathway requires 18 ATP for the synthesis of one, glucose molecule, whereas C4pathway requires 30 ATP., Thus, c4 plants are better photosynthesizers and there is no photorespiration in these plants., , gy, ch, am, , C3 pathway, 1)c3 pathway takes place in only mesophyll cells, , p., b, , lo, g, , sp, ot, , 1) Anatomical Peculiarities, i) The leaves of C4 plants shows the Kranz anatomy., ii) The leaves of C4 plants having dimorphic chloroplast., iii) The chloroplast present inside the mesophyll having grana part., iv) The chloroplast present inside the bundle sheath is without grana part., 2) Physiological Peculiarities, i) The dark reaction of HSK plants shows C4 and C3 cycles reaction., ii) The C4 cycle reaction occurs inside the grana of chloroplast., iii) The C3 cycle reaction occurs inside the stroma part of chloroplast present inside the bundle sheath., iv) The first CO2 acceptor in C4 cycle is PEP to produce 4 carbon compound oxalo acetic acid which is unstable, and it is converted in C4 acids, v) The CO2 acceptor in C3 cycle is RUDP., vi) The C4 plants can grow in dry and hot climatic condition., vii) The C4 plants are physiologically adapted to high light intensity and high temp., Distinguish between c3 and c4 pathway, , lo, , CAM (CRASSULACEAN ACID METABOLISM) PLANTS, , w, , w, , w, , .b, , io, , In certain plants of family Crassulaceae which are succulant in nature, grows in dry climatic condition (Xerophytic)., , www.biologychamp.blogspot.com, , 12

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.c, om, , In thèse plants the stomata remains closed during the day time while stomata gets open during the night time to, prevent the loss of water by transpiration. In this plants modified dark reaction is observed known as CAM pathway., During the night time the stomata gets open where PEPA fix the atmospheric CO2 in the presence of enzyme PEP, carboxylase to form oxalic acetic acid., The oxalo acetic acid is reduced into malic acid in the presnenc of enzyme malate dehydrogenase. Such malic acid, accumilate during night time., During the day time to prevent the transpiration stomata gets closed, malate (malic acid )undergoes decarboxylatin, gradually and converted into pyruvate(pyruvic acid).Co2 thus released enters in calcin cycle and sugar is produced. Where,, Pyruvate is converted into starch which accumulate during day time., During night time starch is converted into PEPA which is utilized during the night time., Examples of CAM plants: Kalanchoe, Opuntia, Aloe etc., The Chemical reactions of the carbon di-oxide fixation and its assimilation are similar to that of C4 plants., , Interdependence of light and dark reaction., , p., b, , lo, g, , sp, ot, , The two phases of photosynthesis i.e. light and dark reaction are interdependent to each other., The products of light reaction ATP and NADPH2 are required for dark reaction for the reduction of Co 2 into glucose., While during Dark reaction ADP, iP & NADP are produced which are used for the synthesis of ATP & NADPH2, , 1) During light reaction ATP and NADPD2 are produced which is supplied to dark reaction., 2) In dark reaction ADP and NADP are produced which is supplied to light reaction for the production of ATP and NADPH2, , gy, ch, am, , FACTORS AFFECTING PHOTOSYNTHESIS, , Like all other physiological processes, photosynthesis is also influenced by a number of factors., , EXTERNAL FACTORS, Light:, , w, , w, , w, , .b, , io, , lo, , It is an essential factor as it supplies the energy necessary for photosynthesis. Both quality and intensity of light affect, photosynthesis. Highest rate of photosynthesis takes place in the red rays and then comes the blue rays. In a forest canopy the, rate of photosynthesis decreases considerably in plants growing under the it., In most of the plants, photosynthesis is maximum in bright diffused sunlight. It decreases in strong light and again slows down, in the light of very low intensity. It has also been found that uninterrupted and continuous photosynthesis for relatively long, periods of time may be sustained without any visible damage to the plant., Carbon dioxide:, The main source of CO2in land plants is the atmosphere, which contains only 0.3% of the gas. Under normal conditions, of temperature and light, carbon dioxide acts as a limiting factor in photosynthesis. An increase in concentration of CO2increases, the photosynthesis. The increase in CO2to about 1% is generally advantageous to most of the plants. Higher concentration of, the gas has an inhibitory effect on photosynthesis., Temperature:, Like all other physiological processes, photosynthesis also needs a suitable temperature. In the presence of plenty of, light and carbon dioxide, photosynthesis increases with the rise of temperature till it becomes maximum. After that there is a, decrease or fall in the rate of the process., The optimum temperature at which the photosynthesis is maximum is 25 – 300C, though in certain plants like Opuntia,, photosynthesis takes place at as high as 550C. This is known as the maximum temperature. The temperature, at which the process just starts is the minimum temperature., Water:, Being one of the raw materials, water is also necessary for the photosynthetic process. An increase in water content, of the leaf results in the corresponding increase in the rate of photosynthesis. Thus, the limiting effect of water is not direct but, indirect. It is mainly due to the fact that it helps in maintaining the turgidity of the assimilatory cells and the proper hydration, of their protoplasm., , INTERNAL FACTORS, , Though the presence of chlorophyll is essential for photosynthesis but the rate of photosynthesis is proportional to, the quantity of chlorophyll present. It is because of the fact that chlorophyll merely acts as a biocatalyst and hence a small, quantity is quite enough to maintain the large bulk of the reacting substances., The final product in the photosynthesis reaction is sugar and its accumulation in the cells slow down the process of, photosynthesis. The thickness of cuticle and epidermis of the leaf, the size and distribution of intercellular spaces and the, distribution of the stomata and the development of chlorenchyma and other tissues also affects the rate of photosynthesis., , www.biologychamp.blogspot.com, , 13

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LAW OF LIMITING FACTORS (BLACKMAN), , sp, ot, , .c, om, , Blackman’s law of limiting factors:, The Blackman’s law of limiting factors states that when a process is conditioned as to its rapidity by a number of, separate factors, the rate of the process is controlled by the pace of the “slowest factor”., The slowest factor is that factor which is present in the lowest or minimum concentration in relation to others. The, law of limiting factor can be explained by taking two external factors such as carbon dioxide and light. Suppose a plant, photosynthesizing at a fixed light intensity sufficient to utilize 10mg of CO2per hour only., On increasing the CO2 concentration, the photosynthetic rate also goes on increasing. Now, if the CO2concentration, is further increased, no increase in the rate of photosynthesis will be noted. Thus, in this case light becomes the limiting factor., Under such circumstances, the rate of photosynthesis can be increased only by increasing the light intensity. This evidently, shows that the photosynthetic rate responds to one factor alone at a time and there would be a sharp break in the curve and a, plateau formed exactly at the point where another factor becomes limiting. If any one of the other factors which is kept constant, (say, light) is increased, the photosynthetic rate increases again reaching and optimum where again another factor become, limiting., , SIGNIFICANCE OF PHOTOSYNTHESIS, , w, , w, , w, , .b, , io, , lo, , gy, ch, am, , www.biologychamp.blogspot.com, , lo, g, , 4., , This anabolic process uses inorganic substances and produces food for all life directly or indirectly., This process transforms solar energy into chemical energy., The released byproduct O2is necessary not only for aerobic respiration in living organisms but also used in forming, protective ozone layer around earth., This process is also helping us in providing fossil fuels, coals, petroleum and natural gas., , p., b, , 1., 2., 3., , www.biologychamp.blogspot.com, , 14