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Module 1: Plant Cell Technology

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Photosynthesis and Photorespiration

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Let us study or revise what is photosynthesis.Photosynthesis we all know can be carried out by all green photosynthetic organismswhich have chlorophyll in them.So, including the autotrophs such organisms are autotrophs.Now, autotrophs means what the organisms which are capable of producing organic compoundsby using light energy and fixing the carbon dioxide.Now, an over view, so we all know in general what happens in photosynthesis, in the presenceof light energy it drives, it produces sugars and oxygen from carbon dioxide and water.So, what how does it happen inside the leaf?Why do plants look green?Rest of the light is absorbed and only green is transmitted and reflected that is why theylook green.So, what happens in photosynthesis, you know that it happens in chloroplast in plants.Now, chloroplast are their membranes are rich in a coloured, green coloured pigment whichis called as chlorophyll.Now, they have a number of granular stacks which are called as thylakoids .Now, this light reactions they happen in the membrane of these thylakoids, these granularstructures.So, and the carbon fixation happens at the stroma, the the carbon diffuses the carbondioxide diffuses in the stroma through the chloroplast, and it is transparent to lights,light can easily pass through the chloroplast membrane.So, what happens?The entire process of photosynthesis is divided into two parts as I said light reactions anddark reactions.In light reactions, the light energy is captured by the photosystems, so they these are calledas reaction centres . So, these reaction centres they have some 100 molecules few hundred moleculesof coloured pigments including chlorophyll.So, the reaction centres in these photo systems, photosystem 1 and photosystem 2, which iswhich forms the a cyclic phosphorylation cycle.Now, in bacteria you will find cyclic phosphorylation . So, I am now going to talk about the lightreactions of photosynthesis, where the water is broken down to produce oxygen the photophotons are captured so as to drive ATP and NADPH synthesis .So, now stroma is the liquid part of the chloroplast and grana are the stacks of the thylakoid.So, this entire light reaction of capturing the photons and then passing on the electronexcited electrons to the electron transport chain everything is present at the membraneof the thylakoid . So, what happens that as soon as the photon the light is incident onthese reaction centres, I will show you a picture.The two reaction centres I said photosystem 1 and photosystem 2 during evolution photosystem1 came first, and then photosystem 2, but with adaptations to plants to make it moreeconomical or more efficient, the two photosystems both were brought in together.Therefore, photosystem 2 comes first and then the photosystem 1 in that acyclic phosphorylation.So, when the photons are incident, the light energy is incident on the photos.So, first comes the photosystem 2 because it was discovered probably later.So, photosystem 2 it is incident, then there are number of pigment molecules, now the energywhich is absorbed by the pigment molecules the it keeps on getting transferred to neighbouringpigment molecules, still it reaches the reaction centre .What is this reaction centre, it is a chlorophyll A molecule which can absorb light at a wavelengthof 600 nano metres . So, it absorbs the energy and then which drives this energy drives oxidationreduction reaction, breaking down the water molecule releasing, oxygen and the electrons.Now, this excited electron is then taken up captured by the neighbouring electron acceptormolecule, specific electron acceptor molecule.And then it subsequently keeps transferring the electrons in the electron transport chainwhich is present at the thylakoid membrane because of which the hydrogen ion gets releasedin the thylakoid lumen.So, now once the hydrogen ion concentration in the thylakoid lumen increases, it has tobe the electrochemical gradient is there.So, this electrochemical gradient through chemiosmosis is then exploited by the ATPswhich is present in the thylakoid membrane now this is ATPs then generates ATP by drivingthe hydrogen ions out in the stroma.The hydrogen ions which are then let out in the stroma, they then combined with the cofactorNADP plus forming NADPH . So, this is how the light reactions give rise to ATP and theNADPH.Now, this ATP and the NADPH both are to be utilised for capturing CO 2 to convert itinto sugars.This we will see how does it happen . So, oxygen if you see it is an incidental by productbecause water is available, it captures break breaks down water and it is an by productwhich happens in photosynthesis . So, if you see the entire picture whateverI said would be very clear.These are the different, so you must have seen the electron transport even in the chainin your mitochondria . So, it is similar to that .So, the different steps I will repeat what happens in the thylakoid membrane the chlorophyllis organised along with other molecules to from the two photosystems . Now, if you willobserve that the photosystem 1 it was it is called as P700, photosystem 2 is P680 whichmeans there is a difference in the maximum absorption wavelength.Now, it is a fact that both the reaction centres are identically same chlorophyll moleculesA, but still there is a difference in the wavelength.Why do you think?This difference in absorption properties happens because it is in conjugation with other proteinspresent in the reaction centre . So, because it combines with the other proteins in thetwo photosystems, there is a difference in the absorptive properties.Then the photosystems are what as I said these are light harvesting units.So, again to repeat there are two photosystems.When a photon of light, it will struck, strike the photosystems, the electrons are released.These electrons being high energy states, they are transferred to a nearing electronacceptor molecule which again transfers it to the electron transport chain present inthe membrane, thereby releasing the hydrogen ions in the thylakoid lumen.This causes electrochemical gradient which is then exploited by AT phase to produce ATP,and the hydrogen ions sent back or diffusing through the membrane into the stroma, thenare used to reduce the NADP plus to form NADPH.Let us talk about the dark reactions.So, this was the picture which I showed in the beginning.So, we have now spoken about the light reactions.Light reactions have given rise to ATP and NADPH.Now, dark reaction is called as Calvin cycle.Now, why dark reactions in this reaction the CO 2 is captured.This CO 2 is combined with ribulose 1, 5-bisphosphate . Now, in order to combine this, you needan enzyme which is called as rubisco, it is the most abundant organic molecule presenton the earth . So, this enzyme then combines the two to give you two stable, one unstable6 carbon molecule which immediately breaks down to give you two stable molecules of phosphoglycericacids.Now, because these are the first stable products of the Calvin cycle, this cycle is also calledas C 3 cycle . So, in general Calvin cycle, we have threeportions, one is carboxylation, then reduction phase, and after the reduction phase regenerationphase.So, what happens in the carboxylation phase the carbon dioxide is reduced by rubisco,which is ribulose bisphosphate carboxylase.So, it combines carbon dioxide with ribulose 1, 5-bisphosphate to give you two moleculesof phosphoglyceric acid.Now, this phosphoglyceric acid is then reduced using ATP and NADPH and with if you rememberyour gluconeogenesis which is reverse of glycolysis, some of those it is similar to those reactionssuch reactions takes place to convert your phosphoglyceric acid to 6 molecules of 3 phosphoglycerate.Now, in this then one molecule is then diverted to form your sugars, and the remaining fiveis then again in the regeneration phase.Now, why it is called regeneration phase, because again the precursor molecular ribulosebisphosphate has to be regenerated.So, now, these five molecules are again regenerated to give you back ribulose bisphosphate whichcan then subsequently again combined with CO 2 to give you PGA.So, which enzymes are involved rubisco which is rubisco 1, 5-bisphosphate carboxylase.Then it catalyses the addition of CO 2 to ribulose 1, 5-bisphosphate as I said formingthe first stable two molecules of phosphoglyceric acid.Then comes the reduction phase I discussed three phosphoglyceric acid is reduced to glyceraldehyde-3-phosphate.If you remember glyceraldehyde-3-phosphate is gives rise to is a part of glycolysis . So,it is then reduced to give you glyceraldehyde-3-phosphate can then give you sugars . And there ATP andNADPH is needed to reduce PGA to glyceraldehyde-3-phosphate.Enzymes involved are what 3-phosphoglycerate kinase these are the major enzymes and glyceraldehyde-3-phosphatedehydrogenase.Now, comes the regeneration phase where the carbohydrates or fructose, and your glucose,and how does it happens with using enzymes trans transaldolase and transketolase.If you remember these are enzymes involved in pentose phosphate pathway.So, again similar reactions of pentose phosphate pathways are involved ones from glyceraldehyde-3-phosphateyou get fructose diphosphate.If you remember your gluconeogenesis, cycle of glycolysis .So, now coming back to we were saying talking about adaptations.Now, C 3 plants which are generally all the plants are follow C 3 metabolism.Now, in C 3 plants, the light reactions and the dark reactions both are taking place inthe same chloroplast, same cell.So, but what happens as I discussed earlier that in order to prevent excessive water loss,the stomata is closed.Now, once the stomata is closed the CO 2, now is anyways getting utilised in carbonfixation.So, then a time comes when the concept relative concentration of CO 2 to oxygen reduces.As soon as this happens rubisco now starts getting more associated with oxygen . Now,what happens now it starts acting as an oxygenise, and it will combine your ribulose bisphosphatewith oxygen.Now, this oxygen when it drives this reaction as oxygenise, it will give rise to I saida toxic molecule which is called as phosphoglycolate.I will be talking about this process.Now, once phosphoglycolate is formed, it is a highly toxic molecule.So, the cell drives certain reactions to convert this toxic molecule into a number of moleculeslike glycine, and then glycine gets converted to serine, and then serine thereby gives riseto many other molecules.Now, all these reactions takes place using organelles, paroxysm and mitochondria.Now, these reactions again happen at the expense of cells energy.So, ATP is involved, and there is a loss of CO 2 when the glycine is getting convertedto serine in mitochondria.So, effectively 30 percent of the carbon fixed is lost in salvaging this molecule.So, we will see what do plants to in that case.So, there is one adaptation which is called as C 4 metabolism . So, in C 4 plants, thephotosynthesis takes place in chloroplast.And I was mentioning that there are cells which surround the vascular bundle.These are called as bundle sheath cells.Now, why it is called C 4?Yes, so because the first stable compound formed is malate or aspartic acid.Now, this 4-carbon compound is then transported, now this is light reaction, is then transportedto bundle sheath cells where it again breaks down to release CO 2.Now, because it is now very close to vascular bundle, and in in the inner portions of theleaf of the tissue, it is set to be secured.Now, then once it releases CO 2 in the vicinity, the relative concentration of CO 2 to oxygenis then increased which then can cause rubisco to again carry out the dark reactions . So,this is what C 4 metabolism.Now, so there is a spatial change in the light and the dark reactions.Now, then there is one other kind of metabolism which is called as scam metabolism which isfound in succulent plants.Now, why succulent plants, these leaves are plump with lot of water content, now thereis a reason why do they become plump and with lot of water content because of the four moleculewhich was mallet.Now, in these scam metabolism rather than opening in early hours of the morning andtaking in CO 2 like C 4 plants and these carry out the photosynthesis and capturing of CO2 to form a 4-carbon molecule at night.And in the morning, in the same cells, this molecule is broken down and photosynthesisor the dark reactions fixing of the CO 2 which is released happens.So, it is a temporal shift between the light and the dark reactions.So, what happens in photorespiration, very hot days the stomata is closed.Once the stomata is closed, there is no gas exchange.The CO 2 is continuously getting utilised . Then again as I said relative concentrationof CO 2 to O 2 is reduced.So, now, rubisco is combining with O 2 to give you a toxic molecule phosphoglycolate.Now, this toxic molecules has to be broken down to prevent the cell damage.So, now, this whole process is called as photo respiration.So, as I said see when the CO 2 levels drop in the leaf less than 50 ppm, then the rubiscogets acts starts acting as an oxygenise.So, this is the reactions which take place in photorespiration.So, three organelles are involved chloroplast, peroxisomes and mitochondria.If you remember when I was discussing organelles I asked you what do peroxisomes, what is therole of peroxisomes . Peroxisomes play a crucial role in photorespiration . So. as I said thisglycolate when formed it is then sent to peroxisomes where the reaction to convert it into glycinehappens.Now, this happens by utilising oxygen.So, in the peroxisome, the oxygen is then taken into convert your glycolate to glycine.Now, once the glycine is formed, it is then transported to mitochondria and then the glycineis converted to serine.Now, this reaction happens by taking in by release of CO 2 . So, therefore, respiration.So, as I said oxygenise and carboxylase is it depends on their Km value.The Km although for rubisco to combine with CO 2 is will be very low or very high.In general rubisco combines with CO 2 . So, if it can combine with oxygen the Km valuefor CO 2 will be higher or lower, lower, more affinity towards CO 2 than with oxygen.So, what in nature, the strategy could be applied to prevent this process photorespiration,either the plant should have avoided rubisco there should have been another enzyme to carryout photosynthesis or fixing CO 2 in an environment shielded from O 2, this could be another choice.The third choice could be use of an enzyme that does not react with O 2 which is thesame.Do not use rubisco use another enzyme which does not react with O 2.So, this is what happened.CO 2 fixation we know it happens in the mesophyll cells, palisade cells, spongy cells with airgaps for gas exchange . So, now, CO 2 fixing enzyme is not rubisco, the adaptation happened.Then CO 2 fixing enzyme was then changed from rubisco to PEP carboxylase.Then rubisco still remains the same, but CO 2 fixing enzyme became for the first lightreactions it was changed to PEP carboxylase.So, we will see how does the malate is formed.So, in C 4 synthesis to prevent the wasteful effect of photorespirations, plants like yoursugarcane and corn they adopt to this metabolism.Now, what what happened their anatomy.Even in if you compare the anatomy between C 3 and C 4 plants, it is different . In C4 plants, you will see there there are bundle sheath cells around the vascular bundle, wellwhich is xylem and phloem.While in your C 3 plants and they all mesophyll cells are all palisade we saw coalminer cells,and then they were spongy cells, so the chloroplast in C 3 plants and all these cells can carryout light and dark both the reactions.So, now, C 4 pathway is this where it combine the PEP carboxylase converts PEP capture CO2 to make the first four carbon molecule which is called as malate.This malate is then transported to bundle sheath cells in C 4plants where it is againbroken down to release CO 2.Now, in this the rubisco is protected . It is not now on the surface to, so that it canbe affected by the oxygen.So, here again then rubisco combines with co two in dark reactions to give sugar molecules.Now, what happens in cam plants i have already spoken about it, it is temporally different.So, this CO 2 fixation into malate happens early hours of the morning where CO 2 is takenin converted into malate, and then this during the day hours when the stomata is closed,this malate is broken down to give you fixing the CO 2, and then CO 2 combines with rubiscoto give sugars.But then in cam plants at night photosynthesis these reactions happen, the CO 2 is fixedto give you malate, malate is stored in vacuoles.And then in the morning in the malate is then broken down to give you CO 2.Now, this happens in succulent plants I said.Now, why did I say succulent plants they are more plump with lot of water, there is a largeamount of malate present.So, lot of water is sucked in, so that is the reason why they become succulent in nature.And sometimes there is so much of pressure that the malate is forced to be broken downby such kind of plants.