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National Council of Educational Research and Training (NCERT) Book for Class XI
Chapter: Chapter 14 – Respiration in Plants
Class XI NCERT Biology Text Book Chapter 14 Respiration in Plants is given below.
All of us breathe to live, but why is breathing so essential to life? Whathappens when we breathe? Also, do all living organisms, including plantsand microbes, breathe? If so, how?
All living organisms need energy for carrying out daily life activities,be it absorption, transport, movement, reproduction or even breathing.Where does all this energy come from? We know we eat food for energy –but how is this energy taken from food? How is this energy utilised? Doall foods give the same amount of energy? Do plants ‘eat’? Where do plantsget their energy from? And micro-organisms – for their energyrequirements, do they eat ‘food’?
You may wonder at the several questions raised above – they mayseem to be very disconnected. But in reality, the process of breathing isvery much connected to the process of release of energy from food. Let ustry and understand how this happens.
All the energy required for ‘life’ processes is obtained by oxidation ofsome macromolecules that we call ‘food’. Only green plants andcyanobacteria can prepare their own food; by the process of photosynthesisthey trap light energy and convert it into chemical energy that is stored inthe bonds of carbohydrates like glucose, sucrose and starch. We mustremember that in green plants too, not all cells, tissues and organsphotosynthesise; only cells containing chloroplasts, that are most oftenlocated in the superficial layers, carry out photosynthesis. Hence, evenin green plants all other organs, tissues and cells that are non-green,need food for oxidation. Hence, food has to be translocated to all nongreenparts. Animals are heterotrophic, i.e., they obtain food from plants directly (herbivores) or indirectly (carnivores). Saprophytes like fungi aredependent on dead and decaying matter. What is important to recogniseis that ultimately all the food that is respired for life processes comes fromphotosynthesis. This chapter deals with cellular respiration or themechanism of breakdown of food materials within the cell to releaseenergy, and the trapping of this energy for synthesis of ATP.
Photosynthesis, of course, takes place within the chloroplasts (in theeukaryotes), whereas the breakdown of complex molecules to yield energytakes place in the cytoplasm and in the mitochondria (also only ineukaryotes). The breaking of the C-C bonds of complex compoundsthrough oxidation within the cells, leading to release of considerableamount of energy is called respiration. The compounds that are oxidisedduring this process are known as respiratory substrates. Usuallycarbohydrates are oxidised to release energy, but proteins, fats and evenorganic acids can be used as respiratory substances in some plants, undercertain conditions. During oxidation within a cell, all the energy containedin respiratory substrates is not released free into the cell, or in a singlestep. It is released in a series of slow step-wise reactions controlled byenzymes, and it is trapped as chemical energy in the form of ATP. Hence,it is important to understand that the energy released by oxidation inrespiration is not (or rather cannot be) used directly but is used tosynthesise ATP, which is broken down whenever (and wherever) energyneeds to be utilised. Hence, ATP acts as the energy currency of the cell.This energy trapped in ATP is utilised in various energy-requiringprocesses of the organisms, and the carbon skeleton produced duringrespiration is used as precursors for biosynthesis of other molecules inthe cell.
14.1 DO PLANTS BREATHE?
Well, the answer to this question is not quite so direct. Yes, plants requireO2 for respiration to occur and they also give out CO2. Hence, plants havesystems in place that ensure the availability of O2. Plants, unlike animals,have no specialised organs for gaseous exchange but they have stomataand lenticels for this purpose. There are several reasons why plants canget along without respiratory organs. First, each plant part takes care ofits own gas-exchange needs. There is very little transport of gases fromone plant part to another. Second, plants do not present great demandsfor gas exchange. Roots, stems and leaves respire at rates far lower thananimals do. Only during photosynthesis are large volumes of gasesexchanged and, each leaf is well adapted to take care of its own needsduring these periods. When cells photosynthesise, availability of O2 is nota problem in these cells since O2 is released within the cell. Third, the distance that gases must diffuse even in large, bulky plants is not great.Each living cell in a plant is located quite close to the surface of the plant.‘This is true for leaves’, you may ask, ‘but what about thick, woody stemsand roots?’ In stems, the ‘living’ cells are organised in thin layers insideand beneath the bark. They also have openings called lenticels. The cellsin the interior are dead and provide only mechanical support. Thus, mostcells of a plant have at least a part of their surface in contact with air. Thisis also facilitated by the loose packing of parenchyma cells in leaves, stemsand roots, which provide an interconnected network of air spaces.
The complete combustion of glucose, which produces CO2 and H2O as end products, yields energy most of which is given out as heat.
If this energy is to be useful to the cell, it should be able to utilise it tosynthesise other molecules that the cell requires. The strategy that theplant cell uses is to catabolise the glucose molecule in such a way thatnot all the liberated energy goes out as heat. The key is to oxidise glucosenot in one step but in several small steps enabling some steps to be justlarge enough such that the energy released can be coupled to ATPsynthesis. How this is done is, essentially, the story of respiration.During the process of respiration, oxygen is utilised, and carbondioxide, water and energy are released as products. The combustionreaction requires oxygen. But some cells live where oxygen may or maynot be available. Can you think of such situations (and organisms) where
O2 is not available? There are sufficient reasons to believe that the first cells on this planet lived in an atmosphere that lacked oxygen. Evenamong present-day living organisms, we know of several that are adaptedto anaerobic conditions. Some of these organisms are facultativeanaerobes, while in others the requirement for anaerobic condition isobligate. In any case, all living organisms retain the enzymatic machineryto partially oxidise glucose without the help of oxygen. This breakdownof glucose to pyruvic acid is called glycolysis.
The term glycolysis has originated from the Greek words, glycos for sugar,and lysis for splitting. The scheme of glycolysis was given by GustavEmbden, Otto Meyerhof, and J. Parnas, and is often referred to as theEMP pathway. In anaerobic organisms, it is the only process in respiration.Glycolysis occurs in the cytoplasm of the cell and is present in all livingorganisms. In this process, glucose undergoes partial oxidation to formtwo molecules of pyruvic acid. In plants, this glucose is derived fromsucrose, which is the end product of photosynthesis, or from storage
carbohydrates. Sucrose is converted into glucoseand fructose by the enzyme, invertase, and thesetwo monosaccharides readily enter the glycolyticpathway. Glucose and fructose are
phosphorylated to give rise to glucose-6-phosphate by the activity of the enzymehexokinase. This phosphorylated form of glucosethen isomerises to produce
fructose-6-phosphate. Subsequent steps ofmetabolism of glucose and fructose are same.The various steps of glycolysis are depicted inFigure 14.1. In glycolysis, a chain of tenreactions, under the control of different enzymes,takes place to produce pyruvate from glucose.While studying the steps of glycolysis, please notethe steps at which utilisation (ATP energy) orsynthesis of ATP or (in this case of) NADH + H+take place.
ATP is utilised at two steps: first in theconversion of glucose into glucose 6-phosphateand second in the conversion of fructose6-phosphate to fructose 1, 6-diphosphate.The fructose 1, 6-diphosphate is splitinto dihydroxyacetone phosphate and3-phosphoglyceraldehyde (PGAL). We findthat there is one step where NADH + H+ isformed from NAD+; this is when
3-phosphoglyceraldehyde (PGAL) is convertedto 1, 3-bisphosphoglycerate (DPGA). Tworedox-equivalents are removed (in the form oftwo hydrogen atoms) from PGAL and transferredto a molecule of NAD+. PGAL is oxidised andwith inorganic phosphate to get converted intoDPGA. The conversion of DPGA to3-phosphoglyceric acid (PGA), is also an energyyielding process; this energy is trapped by theformation of ATP. Another ATP is synthesisedduring the conversion of PEP to pyruvic acid.Can you then calculate how many ATPmolecules are directly synthesised in thispathway from one glucose molecule?Pyruvic acid is then the key product ofglycolysis. What is the metabolic fate of
pyruvate? This depends on the cellular need. There are three majorways in which different cells handle pyruvic acid produced by glycolysis.These are lactic acid fermentation, alcoholic fermentation and aerobicrespiration. Fermentation takes place under anaerobic conditions in manyprokaryotes and unicellular eukaryotes. For the complete oxidation ofglucose to CO2 and H2O, however, organisms adopt Krebs’ cycle which isalso called as aerobic respiration. This requires O2 supply.
In fermentation, say by yeast, the incomplete oxidation of glucose isachieved under anaerobic conditions by sets of reactions where pyruvicacid is converted to CO2 and ethanol. The enzymes, pyruvic aciddecarboxylase and alcohol dehydrogenase catalyse these reactions. Otherorganisms like some bacteria produce lactic acid from pyruvic acid. Thesteps involved are shown in Figure 14.2. In animal cells also, like musclesduring exercise, when oxygen is inadequate for cellular respiration pyruvicacid is reduced to lactic acid by lactate dehydrogenase. The reducingagent is NADH+H+ which is reoxidised to NAD+ in both the processes.
In both lactic acid and alcoholfermentation not much energy is released; lessthan seven per cent of the energy in glucoseis released and not all of it is trapped as highenergy bonds of ATP. Also, the processes arehazardous – either acid or alcohol isproduced. What is the net ATPs that issynthesised (calculate how many ATP aresynthesised and deduct the number of ATPutilised during glycolysis) when one moleculeof glucose is fermented to alcohol or lacticacid? Yeasts poison themselves to death whenthe concentration of alcohol reaches about 13per cent. What then would be the maximumconcentration of alcohol in beverages thatare naturally fermented? How do you thinkalcoholic beverages of alcohol content greaterthan this concentration are obtained?What then is the process by which
organisms can carryout complete oxidation ofglucose and extract the energy stored tosynthesise a larger number of ATP molecules needed for cellular metabolism? In eukaryotes these steps take placewithin the mitochondria and this requires O2. Aerobic respiration is theprocess that leads to a complete oxidation of organic substances in thepresence of oxygen, and releases CO2, water and a large amount of energypresent in the substrate. This type of respiration is most common in higherorganisms. We will look at these processes in the next section.
14.4 AEROBIC RESPIRATION
For aerobic respiration to take place within the mitochondria, the finalproduct of glycolysis, pyruvate is transported from the cytoplasm intothe mitochondria. The crucial events in aerobic respiration are:
The complete oxidation of pyruvate by the stepwise removal of all
What is interesting to note is that the first process takes place in thematrix of the mitochondria while the second process is located on theinner membrane of the mitochondria.
Pyruvate, which is formed by the glycolytic catabolism ofcarbohydrates in the cytosol, after it enters mitochondrial matrixundergoes oxidative decarboxylation by a complex set of reactionscatalysed by pyruvic dehydrogenase. The reactions catalysed by pyruvicdehydrogenase require the participation of several coenzymes, includingNAD+ and Coenzyme A.
During this process, two molecules of NADH are produced from themetabolism of two molecules of pyruvic acid (produced from one glucosemolecule during glycolysis).
The acetyl CoA then enters a cyclic pathway, tricarboxylic acid cycle,more commonly called as Krebs’ cycle after the scientist Hans Krebs whofirst elucidated it.
14.4.1 Tricarboxylic Acid Cycle
The TCA cycle starts with the condensation of acetyl group with oxaloaceticacid (OAA) and water to yield citric acid (Figure 14.3). The reaction iscatalysed by the enzyme citrate synthase and a molecule of CoA is released.Citrate is then isomerised to isocitrate. It is followed by two successivesteps of decarboxylation, leading to the formation of α-ketoglutaric acid
and then succinyl-CoA. In the remaining stepsof citric acid cycle, succinyl-CoA is oxidised toOAA allowing the cycle to continue. During theconversion of succinyl-CoA to succinic acid amolecule of GTP is synthesised. This is asubstrate level phosphorylation. In a coupledreaction GTP is converted to GDP with thesimultaneous synthesis of ATP from ADP. Alsothere are three points in the cycle where NAD+is reduced to NADH + H+ and one point whereFAD+ is reduced to FADH2. The continuedoxidation of acetic acid via the TCA cyclerequires the continued replenishment ofoxaloacetic acid, the first member of the cycle.In addition it also requires regeneration of NAD+and FAD+ from NADH and FADH2 respectively.The summary equation for this phase ofrespiration may be written as follows:
We have till now seen that glucose has been broken down to releaseCO2 and eight molecules of NADH + H+; two of FADH2 have beensynthesised besides just two molecules of ATP. You may be wonderingwhy we have been discussing respiration at all – neither O2 has come intothe picture nor the promised large number of ATP has yet beensynthesised. Also what is the role of the NADH + H+ and FADH2 that issynthesised? Let us now understand the role of O2 in respiration and howATP is synthesised.
14.4.2 Electron Transport System (ETS) and OxidativePhosphorylation
The following steps in the respiratory process are to release and utilisethe energy stored in NADH+H+ and FADH2. This is accomplished whenthey are oxidised through the electron transport system and the electronsare passed on to O2 resulting in the formation of H2O. The metabolicpathway through which the electron passes from one carrier to another,is called the electron transport system (ETS) (Figure 14.4) and it ispresent in the inner mitochondrial membrane. Electrons from NADH
produced in the mitochondrial matrix duringcitric acid cycle are oxidised by an NADHdehydrogenase (complex I), and electrons arethen transferred to ubiquinone locatedwithin the inner membrane. Ubiquinone alsoreceives reducing equivalents via FADH2(complex II) that is generated duringoxidation of succinate in the citric acid cycle.The reduced ubiquinone (ubiquinol) is thenoxidised with the transfer of electrons tocytochrome c via cytochrome bc1 complex(complex III). Cytochrome c is a small proteinattached to the outer surface of the innermembrane and acts as a mobile carrier fortransfer of electrons between complex III andIV. Complex IV refers to cytochrome c oxidasecomplex containing cytochromes a and a3,and two copper centres.
When the electrons pass from one carrierto another via complex I to IV in the electrontransport chain, they are coupled to ATPsynthase (complex V) for the production ofATP from ADP and inorganic phosphate. Thenumber of ATP molecules synthesiseddepends on the nature of the electron donor.Oxidation of one molecule of NADH gives riseto 3 molecules of ATP, while that of onemolecule of FADH2 produces 2 molecules ofATP. Although the aerobic process ofrespiration takes place only in the presenceof oxygen, the role of oxygen is limited to theterminal stage of the process. Yet, thepresence of oxygen is vital, since it drives thewhole process by removing hydrogen from the system. Oxygen acts asthe final hydrogen acceptor. Unlike photophosphorylation where it is thelight energy that is utilised for the production of proton gradient requiredfor phosphorylation, in respiration it is the energy of oxidation-reductionutilised for the same process. It is for this reason that the process is calledoxidative phosphorylation.
You have already studied about the mechanism of membrane-linkedATP synthesis as explained by chemiosmotic hypothesis in the earlierchapter. As mentioned earlier, the energy released during the electron
transport system is utilised in synthesising ATPwith the help of ATP synthase (complex V). Thiscomplex consists of two major components, F1and F0 (Figure 14.5). The F1 headpiece is aperipheral membrane protein complex andcontains the site for synthesis of ATP from ADPand inorganic phosphate. F0 is an integralmembrane protein complex that forms thechannel through which protons cross the innermembrane. The passage of protons through thechannel is coupled to the catalytic site of the F1component for the production of ATP. For eachATP produced, 2H+ passes through F0 from theintermembrane space to the matrix down theelectrochemical proton gradient.
14.5 THE RESPIRATORY BALANCE SHEET
It is possible to make calculations of the net gain of ATP for every glucosemolecule oxidised; but in reality this can remain only a theoretical exercise.These calculations can be made only on certain assumptions that:
There is a sequential, orderly pathway functioning, with onesubstrate forming the next and with glycolysis, TCA cycle and ETSpathway following one after another.
But this kind of assumptions are not really valid in a living system; allpathways work simultaneously and do not take place one after another;substrates enter the pathways and are withdrawn from it as and whennecessary; ATP is utilised as and when needed; enzymatic rates arecontrolled by multiple means. Yet, it is useful to do this exercise toappreciate the beauty and efficiency of the living system in extractionand storing energy. Hence, there can be a net gain of 36 ATP moleculesduring aerobic respiration of one molecule of glucose.
Now let us compare fermentation and aerobic respiration:Fermentation accounts for only a partial breakdown of glucosewhereas in aerobic respiration it is completely degraded to CO2 andH2O.
14.6 AMPHIBOLIC PATHWAY
Glucose is the favoured substrate for respiration. All carbohydrates areusually first converted into glucose before they are used for respiration.Other substrates can also be respired, as has been mentioned earlier, butthen they do not enter the respiratory pathway at the first step. See Figure14.6 to see the points of entry of different substrates in the respiratorypathway. Fats would need to be broken down into glycerol and fatty acidsfirst. If fatty acids were to be respired they would first be degraded toacetyl CoA and enter the pathway. Glycerol would enter the pathwayafter being converted to PGAL. The proteins would be degraded byproteases and the individual amino acids (after deamination) dependingon their structure would enter the pathway at some stage within the Krebs’cycle or even as pyruvate or acetyl CoA.
Since respiration involves breakdown of substrates, the respiratoryprocess has traditionally been considered a catabolic process and therespiratory pathway as a catabolic pathway. But is this understandingcorrect? We have discussed above, at which points in the respiratorypathway different substrates would enter if they were to be respired andused to derive energy. What is important to recognise is that it is these verycompounds that would be withdrawn from the respiratory pathway for thesynthesis of the said substrates. Hence, fatty acids would be broken down to acetyl CoA before entering the respiratory pathway when it is used as a substrate. But when the organism needs to synthesise fatty acids, acetyl CoA would be withdrawn from the respiratory pathway for it. Hence, the respiratory pathway comes into the picture both during breakdown and synthesis of fatty acids. Similarly, during breakdown and synthesis of protein too, respiratory intermediates form the link. Breaking down processes within the living organism is catabolism, and synthesis is anabolism. Because the respiratory pathway is involved in both anabolism and catabolism, it would hence be better to consider the respiratory pathway
as an amphibolic pathway rather than as a catabolic one.
14.7 RESPIRATORY QUOTIENT
The respiratory quotient depends upon the type of respiratorysubstrate used during respiration.When carbohydrates are used as substrate and are completely
When proteins are respiratory substrates the ratio would be about0.9.What is important to recognise is that in living organisms respiratorysubstances are often more than one; pure proteins or fats are never used asrespiratory substrates.
Plants unlike animals have no special systems for breathing or gaseous exchange.Stomata and lenticels allow gaseous exchange by diffusion. Almost all living cellsin a plant have their surfaces exposed to air.
The breaking of C-C bonds of complex organic molecules by oxidation cellsleading to the release of a lot of energy is called cellular respiration. Glucose is thefavoured substrate for respiration. Fats and proteins can also be broken down toyield energy. The initial stage of cellular respiration takes place in the cytoplasm.Each glucose molecule is broken through a series of enzyme catalysed reactionsinto two molecules of pyruvic acid. This process is called glycolysis. The fate of thepyruvate depends on the availability of oxygen and the organism. Under anaerobicconditions either lactic acid fermentation or alcohol fermentation occurs.Fermentation takes place under anerobic conditions in many prokaryotes,unicellular eukaryotes and in germinating seeds. In eukaryotic organisms aerobicrespiration occurs in the presence of oxygen. Pyruvic acid is transported into the mitochondria where it is converted into acetyl CoA with the release of CO2. AcetylCoA then enters the tricarboxylic acid pathway or Krebs’ cycle operating in thematrix of the mitochondria. NADH + H+ and FADH2 are generated in the Krebs’cycle. The energy in these molecules as well as that in the NADH + H+ synthesisedduring glycolysis are used to synthesise ATP. This is accomplished through asystem of electron carriers called electron transport system (ETS) located on theinner membrane of the mitochondria. The electrons, as they move through thesystem, release enough energy that are trapped to synthesise ATP. This is calledoxidative phosphorylation. In this process O2 is the ultimate acceptor of electronsand it gets reduced to water.
The respiratory pathway is an amphibolic pathway as it involves both anabolismand catabolism. The respiratory quotient depends upon the type of respiratorysubstance used during respiration.
1. Differentiate between
(a) Respiration and Combustion
(b) Glycolysis and Krebs’ cycle
(c) Aerobic respiration and Fermentation
2. What are respiratory substrates? Name the most common respiratory substrate.
3. Give the schematic representation of glyolysis?
4. What are the main steps in aerobic respiration? Where does it take place?
5. Give the schematic representation of an overall view of Krebs’ cycle.
6. Explain ETS.
7. Distinguish between the following:
(a) Aerobic respiration and Anaerobic respiration
(b) Glycolysis and Fermentation
(c) Glycolysis and Citric acid Cycle
8. What are the assumptions made during the calculation of net gain of ATP?
9. Discuss “The respiratory pathway is an amphibolic pathway.”
10. Define RQ. What is its value for fats?
11. What is oxidative phosphorylation?
12. What is the significance of step-wise release of energy in respiration?
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