CHAPTER 13 PHOTOSYNTHESIS H P
206BIOLOGYC HAPTER 13PHOTOSYNTHESIS IN HIGHER PLANTS13.1 What do weKnow?All animals including human beings depend on plants for their food. Haveyou ever wondered from where plants get their food? Green plants, in fact,have to make or rather synthesise the food they need and all other organisms13.2 Earlydepend on them for their needs. The green plants make or rather synthesiseExperimentsthe food they need through photosynthesis and are therefore called autotrophs.13.3 Where doesYou have already learnt that the autotrophic nutrition is found only in plantsPhotosynthesis and all other organisms that depend on the green plants for food aretake place?heterotrophs. Green plants carry out ‘photosynthesis’, a physico-chemicalprocess by which they use light energy to drive the synthesis of organic13.4 How manycompounds. Ultimately, all living forms on earth depend on sunlight forPigments areenergy. The use of energy from sunlight by plants doing photosynthesis isinvolved inPhotosynthesis? the basis of life on earth. Photosynthesis is important due to two reasons: itis the primary source of all food on earth. It is also responsible for the release13.5 What is Lightof oxygen into the atmosphere by green plants. Have you ever thought whatReaction?would happen if there were no oxygen to breath? This chapter focusses on13.6 The Electronthe structure of the photosynthetic machinery and the various reactionsTransportthat transform light energy into chemical energy.13.7 Where are the13.1 W HAT D O W E KNOW?ATP and NADPHUsed?Let us try to find out what we already know about photosynthesis. Some13.8The C4 Pathway simple experiments you may have done in the earlier classes have shown13.9 Photorespiration13.10 FactorsaffectingPhotosynthesisthat chlorophyll (green pigment of the leaf), light and CO2 are required forphotosynthesis to occur.You may have carried out the experiment to look for starch formationin two leaves – a variegated leaf or a leaf that was partially covered withblack paper, and exposed to light. On testing these leaves for the presenceof starch it was clear that photosynthesis occurred only in the green partsof the leaves in the presence of light.2021-22
PHOTOSYNTHESIS IN HIGHER PLANTS207Another experiment you may have carried outwhere a part of a leaf is enclosed in a test tubecontaining some KOH soaked cotton (whichabsorbs CO2), while the other half is exposed to air.The setup is then placed in light for some time. Ontesting for the presence of starch later in the twoparts of the leaf, you must have found that theexposed part of the leaf tested positive for starchwhile the portion that was in the tube, testednegative. This showed that CO2 was required forphotosynthesis. Can you explain how thisconclusion could be drawn?(a)(b)13.2 E ARLY EXPERIMENTSIt is interesting to learn about those simpleexperiments that led to a gradual development inour understanding of photosynthesis.Joseph Priestley (1733-1804) in 1770performed a series of experiments that revealed theessential role of air in the growth of green plants.Priestley, you may recall, discovered oxygen in1774. Priestley observed that a candle burning ina closed space – a bell jar, soon gets extinguished(Figure 13.1 a, b, c, d). Similarly, a mouse would(c)(d)soon suffocate in a closed space. He concluded thatFigure 13.1 Priestley’s experimenta burning candle or an animal that breathe the air,both somehow, damage the air. But when he placed a mint plant in thesame bell jar, he found that the mouse stayed alive and the candlecontinued to burn. Priestley hypothesised as follows: Plants restore tothe air whatever breathing animals and burning candles remove.Can you imagine how Priestley would have conducted the experimentusing a candle and a plant? Remember, he would need to rekindle thecandle to test whether it burns after a few days. How many differentways can you think of to light the candle without disturbing the set-up?Using a similar setup as the one used by Priestley, but by placing itonce in the dark and once in the sunlight, Jan Ingenhousz (1730-1799)showed that sunlight is essential to the plant process that somehowpurifies the air fouled by burning candles or breathing animals.Ingenhousz in an elegant experiment with an aquatic plant showed thatin bright sunlight, small bubbles were formed around the green partswhile in the dark they did not. Later he identified these bubbles to be ofoxygen. Hence he showed that it is only the green part of the plants thatcould release oxygen.2021-22
208BIOLOGYIt was not until about 1854 that Julius von Sachs provided evidencefor production of glucose when plants grow. Glucose is usually stored asstarch. His later studies showed that the green substance in plants(chlorophyll as we know it now) is located in special bodies (later calledchloroplasts) within plant cells. He found that the green parts in plants iswhere glucose is made, and that the glucose is usually stored as starch.Now consider the interesting experiments done by T.W Engelmann(1843 – 1909). Using a prism he split light into its spectral componentsand then illuminated a green alga, Cladophora, placed in a suspensionof aerobic bacteria. The bacteria were used to detect the sites of O2evolution. He observed that the bacteria accumulated mainly in the regionof blue and red light of the split spectrum. A first action spectrum ofphotosynthesis was thus described. It resembles roughly the absorptionspectra of chlorophyll a and b (discussed in section 13.4).By the middle of the nineteenth century the key features of plantphotosynthesis were known, namely, that plants could use light energyto make carbohydrates from CO2 and water. The empirical equationrepresenting the total process of photosynthesis for oxygen evolvingorganisms was then understood as:LightCO2 H2 O [CH2 O] O2where [CH2O] represented a carbohydrate (e.g., glucose, a six-carbonsugar).A milestone contribution to the understanding of photosynthesis wasthat made by a microbiologist, Cornelius van Niel (1897-1985), who,based on his studies of purple and green bacteria, demonstrated thatphotosynthesis is essentially a light-dependent reaction in whichhydrogen from a suitable oxidisable compound reduces carbon dioxideto carbohydrates. This can be expressed by:Light2H2 A CO2 2 A CH2 O H2 OIn green plants H2O is the hydrogen donor and is oxidised to O2. Someorganisms do not release O2 during photosynthesis. When H2S, insteadis the hydrogen donor for purple and green sulphur bacteria, the‘oxidation’ product is sulphur or sulphate depending on the organismand not O2. Hence, he inferred that the O2 evolved by the green plantcomes from H2O, not from carbon dioxide. This was later proved by usingradioisotopic techniques. The correct equation, that would represent theoverall process of photosynthesis is therefore:Light6CO2 12H2 O C6 H12 O6 6H2 O 6O2where C6 H12 O6 represents glucose. The O2 released is from water; thiswas proved using radio isotope techniques. Note that this is not a single2021-22
PHOTOSYNTHESIS IN HIGHER PLANTS209reaction but description of a multistep process called photosynthesis.Can you explain why twelve molecules of water as substrate are usedin the equation given above?13.3 WHEREDOESPHOTOSYNTHESISTAKEPLACE?You would of course answer: in ‘the green leaf’ or ‘in the chloroplasts’,based on what you earlier read in Chapter 8. You are definitely right.Photosynthesis does take place in the green leaves of plants but it does soalso in other green parts of the plants. Can you name some other partswhere you think photosynthesis may occur?You would recollect from previous unit that the mesophyll cells in theleaves, have a large number of chloroplasts. Usually the chloroplasts alignthemselves along the walls of the mesophyll cells, such that they get theoptimum quantity of the incident light. When do you think thechloroplasts will be aligned with their flat surfaces parallel to the walls?When would they be perpendicular to the incident light?You have studied the structure of chloroplast in Chapter 8. Withinthe chloroplast there is membranous system consisting of grana, thestroma lamellae, and the matrix stroma (Figure 13.2). There is a cleardivision of labour within the chloroplast. The membrane system isresponsible for trapping the light energy and also for the synthesis of ATPand NADPH. In stroma, enzymatic reactions synthesise sugar, which inturn forms starch. The former set of reactions, since they are directly lightdriven are called light reactions (photochemical reactions). The latterare not directly light driven but are dependent on the products of lightreactions (ATP and NADPH). Hence, to distinguish the latter they are called,by convention, as dark reactions (carbon reactions). However, this shouldnot be construed to mean that they occur in darkness or that they are notlight-dependent.Outer membraneInner membraneStromal lamellaGranaStromaRibosomesStarch granuleLipid dropletFigure 13.2 Diagrammatic representation of an electron micrograph of a section ofchloroplast2021-22
210BIOLOGY13.4 HOW MANY TYPES OF PIGMENTS AREINVOLVED IN PHOTOSYNTHESIS?Figure 13.3a Graph showing the absorptionspectrum of chlorophyll a, b andthe carotenoidsFigure 13.3b Graphshowingactionspectrum of photosynthesisFigure 13.3c Graphshowingactionspectrum of photosynthesissuperimposed on absorptionspectrum of chlorophyll aLooking at plants have you ever wondered whyand how there are so many shades of green intheir leaves – even in the same plant? We canlook for an answer to this question by trying toseparate the leaf pigments of any green plantthroughpaperchromatography.Achromatographic separation of the leaf pigmentsshows that the colour that we see in leaves isnot due to a single pigment but due to fourpigments: Chlorophyll a (bright or blue greenin the chromatogram), chlorophyll b (yellowgreen), xanthophylls (yellow) and carotenoids(yellow to yellow-orange). Let us now see whatroles various pigments play in photosynthesis.Pigments are substances that have an abilityto absorb light, at specific wavelengths. Can youguess which is the most abundant plantpigment in the world? Let us study the graphshowing the ability of chlorophyll a pigment toabsorb lights of different wavelengths (Figure13.3 a). Of course, you are familiar with thewavelength of the visible spectrum of light aswell as the VIBGYOR.From Figure 13.3a can you determine thewavelength (colour of light) at which chlorophylla shows the maximum absorption? Does itshow another absorption peak at any otherwavelengths too? If yes, which one?Now look at Figure 13.3b showing thewavelengths at which maximum photosynthesisoccurs in a plant. Can you see that thewavelengths at which there is maximumabsorption by chlorophyll a, i.e., in the blue andthe red regions, also shows higher rate ofphotosynthesis. Hence, we can conclude thatchlorophyll a is the chief pigment associatedwith photosynthesis. But by looking at Figure13.3c can you say that there is a completeone-to-one overlap between the absorptionspectrum of chlorophyll a and the actionspectrum of photosynthesis?2021-22
PHOTOSYNTHESIS IN HIGHER PLANTS211These graphs, together, show that most of the photosynthesis takesplace in the blue and red regions of the spectrum; some photosynthesisdoes take place at the other wavelengths of the visible spectrum. Let ussee how this happens. Though chlorophyll is the major pigmentresponsible for trapping light, other thylakoid pigments like chlorophyllb, xanthophylls and carotenoids, which are called accessory pigments,also absorb light and transfer the energy to chlorophyll a. Indeed, theynot only enable a wider range of wavelength of incoming light to be utilisedfor photosyntesis but also protect chlorophyll a from photo-oxidation.13.5 WHATISLIGHT REACTION?Light reactions or the ‘Photochemical’ phaseinclude light absorption, water splitting, oxygenPrimary acceptorrelease, and the formation of high-energychemical intermediates, ATP and NADPH.Several protein complexes are involved in theprocess. The pigments are organised into twodiscrete photochemical light harvestingcomplexes (LHC) within the Photosystem I (PSReactionPhotonI) and Photosystem II (PS II). These are namedcentrein the sequence of their discovery, and not inthe sequence in which they function during thePigmentlight reaction. The LHC are made up ofmoleculeshundreds of pigment molecules bound toproteins. Each photosystem has all the pigments(except one molecule of chlorophyll a) forminga light harvesting system also called antennaeFigure 13.4 The light harvesting complex(Figure 13.4). These pigments help to makephotosynthesis more efficient by absorbingdifferent wavelengths of light. The single chlorophyll a molecule formsthe reaction centre. The reaction centre is different in both thephotosystems. In PS I the reaction centre chlorophyll a has an absorptionpeak at 700 nm, hence is called P700, while in PS II it has absorptionmaxima at 680 nm, and is called P680.13.6 THE ELECTRON TRANSPORTIn photosystem II the reaction centre chlorophyll a absorbs 680 nmwavelength of red light causing electrons to become excited and jumpinto an orbit farther from the atomic nucleus. These electrons are pickedup by an electron acceptor which passes them to an electrons transport2021-22
212BIOLOGYsystem consisting of cytochromes (Figure13.5). This movement of electrons is downhill,in terms of an oxidation-reduction or redoxNADPH potential scale. The electrons are not used upe- acceptorLight e- acceptoras they pass through the electron transportNADP ATPADP iPchain, but are passed on to the pigments ofphotosystem PS I. Simultaneously, electronsElectrontransportin the reaction centre of PS I are also excitedsystemwhen they receive red light of wavelength 700nm and are transferred to another acceptermolecule that has a greater redox potential.LHCThese electrons then are moved downhillagain, this time to a molecule of energy-richNADP . The addition of these electrons reducesLHC2e- 2H [O]H 2ONADP to NADPH H . This whole scheme oftransfer of electrons, starting from the PS II,uphill to the acceptor, down the electronFigure 13.5 Z scheme of light reactiontransport chain to PS I, excitation of electrons,transfer to another acceptor, and finally down hill to NADP reducing it toNADPH H is called the Z scheme, due to its characterstic shape (Figure13.5). This shape is formed when all the carriers are placed in a sequenceon a redox potential scale.Photosystem IIPhotosystem I13.6.1 Splitting of WaterYou would then ask, How does PS II supply electrons continuously? Theelectrons that were moved from photosystem II must be replaced. This isachieved by electrons available due to splitting of water. The splitting ofwater is associated with the PS II; water is split into 2H , [O] and electrons.This creates oxygen, one of the net products of photosynthesis. Theelectrons needed to replace those removed from photosystem I are providedby photosystem II.2H2 O 4H O2 4e We need to emphasise here that the water splitting complex is associatedwith the PS II, which itself is physically located on the inner side of themembrane of the thylakoid. Then, where are the protons and O2 formedlikely to be released – in the lumen? or on the outer side of the membrane?13.6.2 Cyclic and Non-cyclic Photo-phosphorylationLiving organisms have the capability of extracting energy from oxidisablesubstances and store this in the form of bond energy. Special substances likeATP, carry this energy in their chemical bonds. The process through which2021-22
PHOTOSYNTHESIS IN HIGHER PLANTS213ADP iPATPATP is synthesised by cells (in mitochondria andPhotosystem Ichloroplasts) is named phosphorylation. Photophosphorylation is the synthesis of ATP fromADP and inorganic phosphate in the presence ofe - acceptorlight. When the two photosystems work in aLightseries, first PS II and then the PS I, a process callednon-cyclic photo-phosphorylation occurs. Thetwo photosystems are connected through anElectronelectron transport chain, as seen earlier – in thetransportsystemZ scheme. Both ATP and NADPH H aresynthesised by this kind of electron flow (Figure13.5).When only PS I is functional, the electron isChlorophyllP 700circulated within the photosystem and thephosphorylation occurs due to cyclic flow ofFigure 13.6 Cyclic photophosphorylationelectrons (Figure 13.6). A possible locationwhere this could be happening is in the stromalamellae. While the membrane or lamellae of the grana have both PS Iand PS II the stroma lamellae membranes lack PS II as well as NADPreductase enzyme. The excited electron does not pass on to NADP but iscycled back to the PS I complex through the electron transport chain(Figure 13.6). The cyclic flow hence, results only in the synthesis of ATP,but not of NADPH H . Cyclic photophosphorylation also occurs whenonly light of wavelengths beyond 680 nm are available for excitation.13.6.3 Chemiosmotic HypothesisLet us now try and understand how actually ATP is synthesised in thechloroplast. The chemiosmotic hypothesis has been put forward to explainthe mechanism. Like in respiration, in photosynthesis too, ATP synthesis islinked to development of a proton gradient across a membrane. This timethese are the membranes of thylakoid. There is one difference though, herethe proton accumulation is towards the inside of the membrane, i.e., in thelumen. In respiration, protons accumulate in the intermembrane space ofthe mitochondria when electrons move through the ETS (Chapter 14).Let us understand what causes the proton gradient across themembrane. We need to consider again the processes that take place duringthe activation of electrons and their transport to determine the steps thatcause a proton gradient to develop (Figure 13.7).(a) Since splitting of the water molecule takes place on the inner side ofthe membrane, the protons or hydrogen ions that are produced bythe splitting of water accumulate within the lumen of the thylakoids.2021-22
214BIOLOGYFigure 13.7 ATP synthesis through chemiosmosis(b) As electrons move through the photosystems, protons are transportedacross the membrane. This happens because the primary accepter ofelectron which is located towards the outer side of the membranetransfers its electron not to an electron carrier but to an H carrier.Hence, this molecule removes a proton from the stroma whiletransporting an electron. When this molecule passes on its electronto the electron carrier on the inner side of the membrane, the protonis released into the inner side or the lumen side of the membrane.(c) The NADP reductase enzyme is located on the stroma side of themembrane. Along with electrons that come from the acceptor ofelectrons of PS I, protons are necessary for the reduction of NADP toNADPH H . These protons are also removed from the stroma.Hence, within the chloroplast, protons in the stroma decrease innumber, while in the lumen there is accumulation of protons. This createsa proton gradient across the thylakoid membrane as well as a measurabledecrease in pH in the lumen.Why are we so interested in the proton gradient? This gradient isimportant because it is the breakdown of this gradient that leads to thesynthesis of ATP. The gradient is broken down due to the movement ofprotons across the membrane to the stroma through the transmembrane2021-22
PHOTOSYNTHESIS IN HIGHER PLANTS215channel of the CF0 of the ATP synthase. The ATP synthase enzyme consistsof two parts: one called the CF0 is embedded in the thylakoid membraneand forms a transmembrane channel that carries out facilitated diffusionof protons across the membrane. The other portion is called CF1 andprotrudes on the outer surface of the thylakoid membrane on the sidethat faces the stroma. The break down of the gradient provides enoughenergy to cause a conformational change in the CF1 particle of the ATPsynthase, which makes the enzyme synthesise several molecules of energypacked ATP.Chemiosmosis requires a membrane, a proton pump, a protongradient and ATP synthase. Energy is used to pump protons across amembrane, to create a gradient or a high concentration of protons withinthe thylakoid lumen. ATP synthase has a channel that allows diffusion ofprotons back across the membrane; this releases enough energy to activateATP synthase enzyme that catalyses the formation of ATP.Along with the NADPH produced by the movement of electrons, theATP will be used immediately in the biosynthetic reaction taking place inthe stroma, responsible for fixing CO2, and synthesis of sugars.13.7 WHEREARE THEATPANDNADPH USED?We learnt that the products of light reaction are ATP, NADPH and O2. Ofthese O2 diffuses out of the chloroplast while ATP and NADPH are used todrive the processes leading to the synthesis of food, more accurately, sugars.This is the biosynthetic phase of photosynthesis. This process does notdirectly depend on the presence of light but is dependent on the productsof the light reaction, i.e., ATP and NADPH, besides CO2 and H2O. You maywonder how this could be verified; it is simple: immediately after lightbecomes unavailable, the biosynthetic process continues for some time,and then stops. If then, light is made available, the synthesis starts again.Can we, hence, say that calling the biosynthetic phase as the darkreaction is a misnomer? Discuss this amongst yourselves.Let us now see how the ATP and NADPH are used in the biosyntheticphase. We saw earlier that CO2 is combined with H2O to produce (CH2O)nor sugars. It was of interest to scientists to find out how this reactionproceeded, or rather what was the first product formed when CO2 is takeninto a reaction or fixed. Just after world war II, among the several effortsto put radioisotopes to beneficial use, the work of Melvin Calvin isexemplary. The use of radioactive 14C by him in algal photosynthesisstudies led to the discovery that the first CO2 fixation product was a3-carbon organic acid. He also contributed to working out the completebiosynthetic pathway; hence it was called Calvin cycle after him. Thefirst product identified was 3-phosphoglyceric acid or in short PGA.How many carbon atoms does it have?2021-22
216BIOLOGYScientists also tried to know whether all plants have PGA as the firstproduct of CO2 fixation, or whether any other product was formed inother plants. Experiments conducted over a wide range of plants led tothe discovery of another group of plants, where the first stable product ofCO2 fixation was again an organic acid, but one which had 4 carbonatoms in it. This acid was identified to be oxaloacetic acid or OAA. Sincethen CO2 assimilation during photosynthesis was said to be of two maintypes: those plants in which the first product of CO2 fixation is a C3 acid(PGA), i.e., the C3 pathway, and those in which the first product was a C4acid (OAA), i.e., the C4 pathway. These two groups of plants showedother associated characteristics that we will discuss later.13.7.1 The Primary Acceptor of CO2Let us now ask ourselves a question that was asked by the scientists whowere struggling to understand the ‘dark reaction’. How many carbon atomswould a molecule have which after accepting (fixing) CO2, would have 3carbons (of PGA)?The studies very unexpectedly showed that the acceptor moleculewas a 5-carbon ketose sugar – ribulose bisphosphate (RuBP). Did anyof you think of this possibility? Do not worry; the scientists also tooka long time and conducted many experiments to reach this conclusion.They also believed that since the first product was a C3 acid, the primaryacceptor would be a 2-carbon compound; they spent many years tryingto identify a 2-carbon compound before they discovered the 5-carbonRuBP.13.7.2 The Calvin CycleCalvin and his co-workers then worked out the whole pathway and showedthat the pathway operated in a cyclic manner; the RuBP was regenerated.Let us now see how the Calvin pathway operates and where the sugar issynthesised. Let us at the outset understand very clearly that the Calvinpathway occurs in all photosynthetic plants; it does not matter whetherthey have C3 or C4 (or any other) pathways (Figure 13.8).For ease of understanding, the Calvin cycle can be described underthree stages: carboxylation, reduction and regeneration.1. Carboxylation – Carboxylation is the fixation of CO2 into a stable organicintermediate. Carboxylation is the most crucial step of the Calvin cyclewhere CO2 is utilised for the carboxylation of RuBP. This reaction iscatalysed by the enzyme RuBP carboxylase which results in the formationof two molecules of 3-PGA. Since this enzyme also has an oxygenationactivity it would be more correct to call it RuBP carboxylase-oxygenaseor RuBisCO.2021-22
PHOTOSYNTHESIS IN HIGHER PLANTS217AtmosphereRibulose-1,5bisphosphateC02 TPReductionATP NADPHTriosephosphateADP Pi NADP Sucrose, starchFigure 13.8 The Calvin cycle proceeds in three stages : (1) carboxylation, during whichCO2 combines with ribulose-1,5-bisphosphate; (2) reduction, during whichcarbohydrate is formed at the expense of the photochemically made ATPand NADPH; and (3) regeneration during which the CO2 acceptor ribulose1,5-bisphosphate is formed again so that the cycle continues2. Reduction – These are a series of reactions that lead to the formationof glucose. The steps involve utilisation of 2 molecules of ATP forphosphorylation and two of NADPH for reduction per CO2 moleculefixed. The fixation of six molecules of CO2 and 6 turns of the cycle arerequired for the formation of one molecule of glucose from the pathway.3. Regeneration – Regeneration of the CO2 acceptor molecule RuBP iscrucial if the cycle is to continue uninterrupted. The regenerationsteps require one ATP for phosphorylation to form RuBP.2021-22
218BIOLOGYHence for every CO2 molecule entering the Calvin cycle, 3 moleculesof ATP and 2 of NADPH are required. It is probably to meet this differencein number of ATP and NADPH used in the dark reaction that the cyclicphosphorylation takes place.To make one molecule of glucose 6 turns of the cycle are required.Work out how many ATP and NADPH molecules will be required to makeone molecule of glucose through the Calvin pathway.It might help you to understand all of this if we look at what goes inand what comes out of the Calvin cycle.InSix CO218 ATP12 NADPHOutOne glucose18 ADP12 NADP13.8 THE C4 PATHWAYPlants that are adapted to dry tropical regions have the C4 pathwaymentioned earlier. Though these plants have the C4 oxaloacetic acid asthe first CO2 fixation product they use the C3 pathway or the Calvin cycleas the main biosynthetic pathway. Then, in what way are they differentfrom C3 plants? This is a question that you may reasonably ask.C4 plants are special: They have a special type of leaf anatomy, theytolerate higher temperatures, they show a response to high light intensities,they lack a process called photorespiration and have greater productivityof biomass. Let us understand these one by one.Study vertical sections of leaves, one of a C3 plant and the other of a C4plant. Do you notice the differences? Do both have the same types ofmesophylls? Do they have similar cells around the vascular bundle sheath?The particularly large cells around the vascular bundles of the C4plants are called bundle sheath cells, and the leaves which have suchanatomy are said to have ‘Kranz’ anatomy. ‘Kranz’ means ‘wreath’ andis a reflection of the arrangement of cells. The bundle sheath cells mayform several layers around the vascular bundles; they are characterisedby having a large number of chloroplasts, thick walls impervious togaseous exchange and no intercellular spaces. You may like to cut asection of the leaves of C4 plants – maize or sorghum – to observe theKranz anatomy and the distribution of mesophyll cells.It would be interesting for you to collect leaves of diverse species ofplants around you and cut vertical sections of the leaves. Observe underthe microscope – look for the bundle sheath around the vascularbundles. The presence of the bundle sheath would help you identifythe C4 plants.2021-22
PHOTOSYNTHESIS IN HIGHER PLANTS219Now study the pathway shown in Figure 13.9. This pathway that hasbeen named the Hatch and Slack Pathway, is again a cyclic process. Letus study the pathway by listing the steps.The primary CO2 acceptor is a 3-carbon molecule phosphoenolpyruvate (PEP) and is present in the mesophyll cells. The enzymeresponsible for this fixation is PEP carboxylase or PEPcase. It is importantto register that the mesophyll cells lack RuBisCO enzyme. The C4 acidOAA is formed in the mesophyll cells.It then forms other 4-carbon compounds like malic acid or asparticacid in the mesophyll cells itself, which are transported to the bundlesheath cells. In the bundle sheath cells these C4 acids are broken downto release CO2 and a 3-carbon molecule.The 3-carbon molecule is transported back to the mesophyll where itis converted to PEP again, thus, completing the cycle.The CO2 released in the bundle sheath cells enters the C3 or the Calvinpathway, a pathway common to all plants. The bundle sheath cells areFigure 13.9 Diagrammatic representation of the Hatch and Slack Pathway2021-22
220BIOLOGYrich in an enzyme Ribulose bisphosphate carboxylase-oxygenase(RuBisCO), but lack PEPcase. Thus, the basic pathway that results inthe formation of the sugars, the Calvin pathway, is common to the C3 andC4 plants.Did you note that the Calvin pathway occurs in all the mesophyllcells of the C3 plants? In the C4 plants it does not take place in themesophyll cells but does so only in the bundle sheath cells.13.9 PHOTORESPIRATIONLet us try and understand one more process that creates an importantdifference between C3 and C4 plants – Photorespiration. To understandphotorespiration we have to know a little bit more about
photosynthesis. Hence, we can conclude that chlorophyll is the chief pigment associateda with photosynthesis. But by looking at Figure 13.3c can you say that there is a complete one-to-one overlap between the absorption spectrum of chlorophyll a and the action spectrum of photosynthesis? Figure 13.3aGraph showing the absorption
Part One: Heir of Ash Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18 Chapter 19 Chapter 20 Chapter 21 Chapter 22 Chapter 23 Chapter 24 Chapter 25 Chapter 26 Chapter 27 Chapter 28 Chapter 29 Chapter 30 .
photosynthesis 1/2-5/2 8/2 S1-5 Picnic Day 9/2-19/2 Lunar New Year Holiday photosynthesis 11A 4/6 Chapter 21 Photosynthesis o Basic concepts of photosynthesis o Requirements for photosynthesis o Site of The process of photosynthesis oPre Lesson Worksheet o Communication s
Photosynthesis Chapter 8 2 Photosynthesis Overview Energy for all life on Earth ultimately comes from photosynthesis. 6CO 2 12H 2O C 6H 12O 6 6H 2O 6O 2 Oxygenic photosynthesis is carried out by: cyanobacteria, 7 groups of algae, all land plants 3 Photosynthesis Overview Photosynt
Photosynthesis takes place in autotrophs. [brown] Oxygen is a product of photosynthesis. [black] Photosynthesis takes place in the nucleus of plantcells. [blue] 12. Which of the followingstatements is true? Photosynthesis takes place primarily in plant leaves. [light red] Plants obtain their ”food” from the soil. [red] Photosynthesis .
TO KILL A MOCKINGBIRD. Contents Dedication Epigraph Part One Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 Part Two Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18. Chapter 19 Chapter 20 Chapter 21 Chapter 22 Chapter 23 Chapter 24 Chapter 25 Chapter 26
UNIT 1 PHOTOSYNTHESIS AND RESPIRATION Photosynthesis is the process whereby plants use carbon dioxide, water and light energy in a series of chemical reactions to produce glucose (food). Photosynthesis Microorganisms Interactions and interdependencies Life and living Photosynthesis and respiration Photosynthesis Respiration
photosynthesis of an aquatic plant [Elodea]. The rate of photosynthesis can be determined by measuring the concentration of dissolved oxygen as the plant undergoes photosynthesis. There are multiple methods for measuring the rate of photosynthesis including: The uptake of CO 2 The production and release of O 2
In this live Gr 11 Life Sciences show we take a look at Photosynthesis. In this lesson we study the process of photosynthesis looking at the light and dark phases. We discuss the importance of photosynthesis as well as consider the effects of varying amou nts of light, carbon dioxide and temperature on the rate of photosynthesis.
