Respiration in plants notes for neet

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RESPIRATION IN PLANTS

Introduction

  • All living organisms need energy for carrying out daily life activities, be it absorption, transport, movement, reproduction or even breathing. the process of breathing is very much connected to the process of release of energy from food.
  •  All the energy required for ‘life’ processes is obtained by oxidation of some macromolecules that we call ‘food’.
  •  Only green plants and cyanobacteria can prepare their own food; by the process of photosynthesis they trap light energy and convert it into chemical energy that is stored in the bonds of carbohydrates like glucose, sucrose and starch.
  •  We must remember that in green plants too, not all cells, tissues and organs photosynthesize; only cells containing chloroplasts, that are most often located in the superficial layers, carry out photosynthesis. Hence, even in green plants all other organs, tissues and cells that are non-green, need food for oxidation. Hence, food has to be translocated to all non-green parts
  • The breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to release of considerable amount of energy is called respiration.
  • The compounds that are oxidised during this process are known as respiratory substrates.



Respiration in Plants Notes pdf


Respiration in plants notes for neet

Do Plants breathe?

  • Usually carbohydrates are oxidised to release energy, but proteins, fats and even organic acids can be used as respiratory substances in some plants, under certain conditions.
  • Yes, plants require O2 for respiration to occur and they also give out CO2 . Hence, plants have systems in place that ensure the availability of O2 .
  • There are several reasons why plants can get along without respiratory organs. First, each plant part takes care of its own gas-exchange needs.
  • There is very little transport of gases from one plant part to another. Second, plants do not present great demands for gas exchange. Roots, stems and leaves respire at rates far lower than animals do.
  • Only during photosynthesis are large volumes of gases exchanged and, each leaf is well adapted to take care of its own needs during these periods.
  • The ‘living’ cells are organised in thin layers inside and beneath the bark. They also have openings called lenticels
  • The complete combustion of glucose, which produces CO2 and H2O as end products, yields energy most of which is given out as heat. 
  • During the process of respiration, oxygen is utilised, and carbon dioxide, water and energy are released as products.
  • The combustion reaction requires oxygen. But some cells live where oxygen may or may not be available.
  • In any case, all living organisms retain the enzymatic machinery to partially oxidise glucose without the help of oxygen. This breakdown of glucose to pyruvic acid is called glycolysis.

 Respiration in plants Class 11 NEET

Respiration in plants notes for neet

GLYCOLYSIS

  • The term glycolysis has originated from the Greek words, glycos for sugar, and lysis for splitting.
  • The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the EMP pathway.
  • Glycolysis occurs in the cytoplasm of the cell and is present in all living organisms. In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid.
  • In plants, this glucose is derived from sucrose, which is the end product of photosynthesis, or from storage carbohydrates
  • Sucrose is converted into glucose and fructose by the enzyme, invertase, and these two monosaccharides readily enter the glycolytic pathway.
  • ATP is utilised at two steps: first in the conversion of glucose into glucose 6-phosphate and second in the conversion of fructose 6-phosphate to fructose 1, 6-bisphosphate
  • We find that there is one step where NADH + H+ is formed from NAD+ ; this is when 3-phosphoglyceraldehyde (PGAL) is converted to 1, 3-bisphosphoglycerate (BPGA).
  • Two redox-equivalents are removed (in the form of two hydrogen atoms) from PGAL and transferred to a molecule of NAD+ .
  • PGAL is oxidised and with inorganic phosphate to get converted into BPGA.
  • The conversion of BPGA to 3-phosphoglyceric acid (PGA), is also an energy yielding process; this energy is trapped by the formation of ATP.
  • Another ATP is synthesised during the conversion of PEP to pyruvic acid.
  • Pyruvic acid is then the key product of glycolysis.
  • There are three major ways in which different cells handle pyruvic acid produced by glycolysis. These are lactic acid fermentation, alcoholic fermentation and aerobic respiration.
  • Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes. For the complete oxidation of glucose to CO2 and H2O, however, organisms adopt Krebs’ cycle which is also called as aerobic respiration. This requires O2 supply.




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FERMENTATION

  • In fermentation, say by yeast, the incomplete oxidation of glucose is achieved under anaerobic conditions by sets of reactions where pyruvic acid is converted to CO2 and ethanol.
  • In animal cells also, like muscles during exercise, when oxygen is inadequate for cellular respiration pyruvic acid is reduced to lactic acid by lactate dehydrogenase.
  • The reducing agent is NADH+H+ which is reoxidised to NAD+ in both the processes. In both lactic acid and alcohol fermentation not much energy is released; less than seven per cent of the energy in glucose is released and not all of it is trapped as high energy bonds of ATP.
  • Also, the processes are hazardous – either acid or alcohol is produced.when one molecule of glucose is fermented to alcohol or lactic acid.
  • Yeasts poison themselves to death when the concentration of alcohol reaches about 13 per cent.
  • In eukaryotes these steps take place within the mitochondria and this requires O2.
  • Aerobic respiration is the process that leads to a complete oxidation of organic substances in the presence of oxygen, and releases CO2 , water and a large amount of energy present in the substrate.
  • This type of respiration is most common in higher organisms.



AEROBIC RESPIRATION

  • For aerobic respiration to take place within the mitochondria, the final product of glycolysis, pyruvate is transported from the cytoplasm into the mitochondria.
  • The crucial events in aerobic respiration are:
  • • The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of CO2 .
  •  • The passing on of the electrons removed as part of the hydrogen atoms to molecular O2 with simultaneous synthesis of ATP.
  • The crucial events in aerobic respiration are:
  • • The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of CO2 .
  • • The passing on of the electrons removed as part of the hydrogen atoms to molecular O2 with simultaneous synthesis of ATP.
  • The reactions catalysed by pyruvic dehydrogenase require the participation of several coenzymes, including NAD+ and Coenzyme A..




  • During this process, two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid (produced from one glucose molecule during glycolysis)
  • The acetyl CoA then enters a cyclic pathway, tricarboxylic acid cycle, more commonly called as Krebs’ cycle after the scientist Hans Krebs who first elucidated it.
Respiration in Plants Class 11 NCERT PDF


Tricarboxylic Acid Cycle

  • The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid.
  • The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof, and J. Parnas
  • The reaction is catalysed by the enzyme citrate synthase and a molecule of CoA is released. Citrate is then isomerised to isocitrate.
  • It is followed by two successive steps of decarboxylation, leading to the formation of α-ketoglutaric acid.
  • In the remaining steps of citric acid cycle, succinyl-CoA is oxidised to OAA allowing the cycle to continue.
  •  During the conversion of succinyl-CoA to succinic acid a molecule of GTP is synthesised.
  •  This is a substrate level phosphorylation. In a coupled reaction GTP is converted to GDP with the simultaneous synthesis of ATP from ADP.
  • Also there are three points in the cycle where NAD+ is reduced to NADH + H+ and one point where FAD+ is reduced to FADH2
  • In addition it also requires regeneration of NAD+ and FAD+ from NADH and FADH2
  • We have till now seen that glucose has been broken down to release CO2 and eight molecules of NADH + H+ ; two of FADH2 have been synthesised besides just two molecules of ATP in TCA cycle.



 

Respiration in plants notes for neet


Also check: 

 

  • The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS)and it is present in the inner mitochondrial membrane
  • Electrons from NADH  produced in the mitochondrial matrix during citric acid cycle are oxidised by an NADH dehydrogenase (complex I), and electrons are then transferred to ubiquinone located within the inner membrane.
  • Ubiquinone also receives reducing equivalents via FADH2 (complex II) that is generated during oxidation of succinate in the citric acid cycle.
  • The reduced ubiquinone (ubiquinol) is then oxidised with the transfer of electrons to cytochrome c via cytochrome bc1 complex (complex III).
  • Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV.
  • Complex IV refers to cytochrome c oxidase complex containing cytochromes a and a3, and two copper centres.
  •  When the electrons pass from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase (complex V) for the production of ATP from ADP and inorganic phosphate.
  • The number of ATP molecules synthesised depends on the nature of the electron donor.
  • Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while that of one molecule of FADH2 produces 2 molecules of ATP.
  •  Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process.
  • Yet, the presence of oxygen is vital, since it drives the whole process by removing hydrogen from the system.
  • Oxygen acts as the final hydrogen acceptor. Unlike photophosphorylation where it is the light energy that is utilised for the production of proton gradient required for phosphorylation, in respiration it is the energy of oxidation-reduction utilised for the same process. It is for this reason that the process is called oxidative phosphorylation
  • The energy released during the electron Electron Transport System (ETS) transport system is utilised in synthesising ATP with the help of ATP synthase (complex V).

Respiration in plants class 11 NCERT pdf 2021






  • This complex consists of two major components, F1 and F0 The F1 headpiece is a peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate.
  •  F0 is an integral membrane protein complex that forms the channel through which protons cross the inner membrane.
  • The passage of protons through the channel is coupled to the catalytic site of the F1 component for the production of ATP. For each ATP produced, 4H+ passes through F0 from the intermembrane space.



Respiration in plants notes for neet

AMPHIBOLIC PATHWAY

 

  • Glucose is the favoured substrate for respiration. All carbohydrates are usually first converted into glucose before they are used for respiration.
  • Fats would need to be broken down into glycerol and fatty acids first. If fatty acids were to be respired they would first be degraded to acetyl CoA and enter the pathway.
  •  Glycerol would enter the pathway after being converted to PGAL. The proteins would be degraded by proteases and the individual amino acids (after deamination) depending on their structure would enter the pathway at some stage within the Krebs’ cycle or even as pyruvate or acetyl CoA in the respiratory pathway different substrates would enter if they were to be respired and used to derive energy
  • These very compounds that would be withdrawn from the respiratory pathway for the synthesis 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.
  • 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.



Respiration in Flowering Plants neet notes

RESPIRATORY QUOTIENT

  • As you know, during aerobic respiration, O2 is consumed and CO2 is released. The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.
  •  RQ volume of CO evolved volume of O consumed = 2 2
  • The respiratory quotient depends upon the type of respiratory substrate used during respiration.
  • When carbohydrates are used as substrate and are completely oxidised, the RQ will be 1, because equal amounts of CO2 and O2 are evolved and consumed, respectively, as shown in the equation below :


  • When fats are used in respiration, the RQ is less than 1.



  • When proteins are respiratory substrates the ratio would be about 0.9

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