Respiration in Plants
SUMMARY*****
Plants unlike animals have no special systems for breathing or gaseous exchange. Stomata and lenticels allow gaseous exchange by diffusion. Almost all living cells in a plant have their surfaces exposed to air.
The breaking of C-C bonds of complex organic molecules by oxidation cells leading to the release of a lot of energy is called cellular respiration. Glucose is the favoured substrate for respiration. Fats and proteins can also be broken down to yield energy. The initial stage of cellular respiration takes place in the cytoplasm. Each glucose molecule is broken through a series of enzyme catalysed reactions into two molecules of pyruvic acid. This process is called glycolysis. The fate of the pyruvate depends on the availability of oxygen and the organism. Under anaerobic conditions either lactic acid fermentation or alcohol fermentation occurs. Fermentation takes place under anaerobic conditions in many prokaryotes, unicellular eukaryotes and in germinating seeds. In eukaryotic organisms aerobic respiration 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 . Acetyl CoA then enters the tricarboxylic acid pathway or Krebs’ cycle operating in the matrix 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+ synthesised during glycolysis are used to synthesise ATP. This is accomplished through a system of electron carriers called electron transport system (ETS) located on the inner membrane of the mitochondria. The electrons, as they move through the system, release enough energy that are trapped to synthesise ATP. This is called oxidative phosphorylation. In this process O2 is the ultimate acceptor of electrons and it gets reduced to water. The respiratory pathway is an amphibolic pathway as it involves both anabolism and catabolism. The respiratory quotient depends upon the type of respiratory substance used during respiration.
Plants unlike animals have no special systems for breathing or gaseous exchange. Stomata and lenticels allow gaseous exchange by diffusion. Almost all living cells in a plant have their surfaces exposed to air.
The breaking of C-C bonds of complex organic molecules by oxidation cells leading to the release of a lot of energy is called cellular respiration. Glucose is the favoured substrate for respiration. Fats and proteins can also be broken down to yield energy. The initial stage of cellular respiration takes place in the cytoplasm. Each glucose molecule is broken through a series of enzyme catalysed reactions into two molecules of pyruvic acid. This process is called glycolysis. The fate of the pyruvate depends on the availability of oxygen and the organism. Under anaerobic conditions either lactic acid fermentation or alcohol fermentation occurs. Fermentation takes place under anaerobic conditions in many prokaryotes, unicellular eukaryotes and in germinating seeds. In eukaryotic organisms aerobic respiration 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 . Acetyl CoA then enters the tricarboxylic acid pathway or Krebs’ cycle operating in the matrix 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+ synthesised during glycolysis are used to synthesise ATP. This is accomplished through a system of electron carriers called electron transport system (ETS) located on the inner membrane of the mitochondria. The electrons, as they move through the system, release enough energy that are trapped to synthesise ATP. This is called oxidative phosphorylation. In this process O2 is the ultimate acceptor of electrons and it gets reduced to water. The respiratory pathway is an amphibolic pathway as it involves both anabolism and catabolism. The respiratory quotient depends upon the type of respiratory substance used during respiration.
EXERCISES
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 glycolysis? 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?
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 glycolysis? 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?
RESPIRATION VS. AEROBIC RESPIRATION
EXERCISES****
Q1. Differentiate between (a) Respiration and Combustion (b) Glycolysis and Krebs’ cycle (c) Aerobic respiration and FermentationAns. a. Respiration and Combustion: -
Respiration: It is a biochemical process. It occurs in the living cells.ATP is generatedEnzymes are requiredIt is a biologically controlled process.
Combustion: It is a physiochemical process.It does not occur in the living cells.ATP is not generatedEnzymes are not requiredIt is an uncontrolled process.
b. Glycolysis and Kreb's cycle
Glycolysis: - It is a linear pathway.It occurs in the cell cytoplasm.It occurs in both aerobic and anaerobic respiration.It generates 2 NADH and 2 ATP molecules on the breakdown of one glucose molecule.
Kreb's cycle: - It is a cyclic pathway.It occurs in the mitochondrial matrix.It occurs in aerobic respiration.It produces 6 NADH, 2FADH, and 2 ATP molecules on the breakdown of two acetyl - CoA molecules.
(c) Aerobic respiration and Fermentation
Aerobic respiration
Oxygen is used for deriving energyOccurs in the cytoplasm and mitochondriaEnd products are carbon dioxide and waterComplete oxidation of the respiratory substrate takes place About 36 ATP molecules are produced
Fermentation: -
Occurs in the absence of oxygenOccurs in the cytoplasmEnd products are ethyl alcohol and carbon dioxideIncomplete oxidation of the respiratory substrate takes placeOnly 2 ATP molecules are produced
Q2. What are respiratory substrates? Name the most common respiratory substrate.Ans. The compounds oxidised during the process of respiration are called respiratory substrates. Carbohydrates, especially glucose, act as respiratory substrates. Fats, proteins, and organic acids also act as respiratory substrates. Q3. Give the schematic representation of glycolysis?
Q4. What are the main steps in aerobic respiration? Where does it take place? Ans. The major steps in aerobic respiration and the sites where they occur are listed as belowGlycolysis: CytoplasmKrebs cycle: Matrix of the MitochondriaElectron Transport System : Inner Mitochondrial MembraneOxidative phosphorylation: F0-F1 particles in the inner mitochondrial membrane.
5. Give the schematic representation of an overall view of Krebs’ cycle.
6. Explain ETS. Ans. ETS or electron transport system is located in the inner mitochondrial membrane. It helps in releasing and utilizing the energy stored in NADH+H + and FADH2. NADH + H +, which is formed during glycolysis and citric acid cycle, gets oxidized by NADH dehydrogenase (complex I). The electrons so generated get transferred to ubiquinone through FMN. In a similar manner, FADH2 (complex II) generated during citric acid cycle gets transferred to ubiquinone. The electrons from ubiquinone are received by cytochrome bc1 (complex III) and further get transferred to cytochrome c. The cytochrome c acts as a mobile carrier between complex III and cytochrome c oxidase complex, containing cytochrome a and a3, along with copper centres (complex IV). During the transfer of electrons from each complex, the process is accompanied by the production of ATP from ADP and inorganic phosphate by the action ATP synthase (complex V). The amount of ATP produced depends on the molecule, which has been oxidized. 2 ATP molecules are produced by the oxidation of one molecule of NADH. One molecule of FADH2 , on oxidation, gives 3 ATP molecules.
Q7 . Distinguish between the following: (a) Aerobic respiration and Anaerobic respiration (b) Glycolysis and Fermentation (c) Glycolysis and Citric acid Cycle Ans. (a) Aerobic respiration and Anaerobic respiration
Aerobic respiration: - 1. It uses oxygen for deriving energy. 2. It occurs in cytoplasm and mitochondria. 3. The end products of aerobic respiration are carbon dioxide and water. 4. Complete oxidation of respiratory substrate takes place. 5. 36-38 ATP molecules are produced.
Anaerobic respiration: 1. It occurs in the absence of oxygen. 2. It occurs in cytoplasm. 3. The end products of fermentation are ethyl alcohol and carbon-dioxide.4. Incomplete oxidation of respiratory substrate takes place.5. Only 2 ATP molecules are produced
(b) Glycolysis and Fermentation
Glycolysis: - 1. Glycolysis occurs during aerobic and anaerobic respiration. 2. Pyruvic acid is produced as its end product.
Fermentation: -1. Fermentation is a type of anaerobic respiration.2. Ethanol or lactic acid is produced as its end product.
(c) Glycolysis and Citric acid Cycle
Glycolysis: - 1. It is a linear pathway. 2. It occurs in the cell cytoplasm. 3. It occurs in both aerobic and anaerobic respiration. 4. One glucose molecule breaks down to generate 2 NADH2and 2 ATP molecules.
Citric acid cycle: - 1. It is a cyclic pathway.2. It occurs in the mitochondrial matrix.3. It occurs in aerobic respiration. 4. It produces 6 NADH2,2 FADH2, and 2 ATP molecules on breakdown of two acetyl-coA molecules.
Q8. What are the assumptions made during the calculation of net gain of ATP? Ans. For theoretical calculation of ATP molecules, various assumptions are made, which are as follows. (a) It is assumed that various parts of aerobic respiration such as glycolysis, TCA cycle, and ETS occur in a sequential and orderly pathway.(b). NADH produced during the process of glycolysis enters into mitochondria to undergo oxidative phosphorylation. (c)Glucose molecule is assumed to be the only substrate while it is assumed that no other molecule enters the pathway at intermediate stages. (d)The intermediates produced during respiration are not utilized in any other process.
Q9. Discuss “The respiratory pathway is an amphibolic pathway.” Ans. Respiration is generally assumed to be a catabolic process because during respiration, various substrates are broken down for deriving energy. Carbohydrates are broken down to glucose before entering respiratory pathways. Fats get converted into fatty acids and glycerol whereas fatty acids get converted into acetyl CoA before entering the respiration. In a similar manner, proteins are converted into amino acids, which enter respiration after deamination. During synthesis of fatty acids, acetyl CoA is withdrawn from respiratory pathway. Also, in the synthesis of proteins, respiratory substrates get withdrawn. Thus, respiration is also involved in anabolism. Therefore, respiration can be termed as amphibolic pathway as it involves both anabolism and catabolism
Q10. Define RQ. What is its value for fats?Ans. Respiratory quotient (RQ) or respiratory ratio can be defined as the ratio of the volume of CO2 evolved to the volume of O2consumed during respiration. The value of respiratory quotient depends on the type of respiratory substrate. Its value is one for carbohydrates. However, it is always less than one for fats as fats consume more oxygen for respiration than carbohydrates. It can be illustrated through the example of tripalmitin fatty acid, which consumes 145 molecules of O2for respiration while 102 molecules of CO2are evolved. The RQ value for tripalmitin is 0.7.
Q11. What is oxidative phosphorylation? Ans. Oxidative phosphorylation is a process in which electrons are transferred from electron donors to oxygen, which acts as electron acceptor. The oxidation-reduction reactions are involved in the formation of proton gradient. The main role in oxidative phosphorylation is played by the enzyme ATP synthase (complex V). This enzyme complex consists of F0and F1components. The F1headpiece is a peripheral membrane protein complex and contains the site for ATP synthesis from ADP and inorganic phosphate. F0component is a part of membrane protein complex, which acts as a channel for crossing of the protons from inner mitochondrial membrane to the mitochondrial matrix. For every two protons passing through F0–F1complex, synthesis of one ATP molecule takes place.
Q12. What is the significance of step-wise release of energy in respiration? Ans. The process of aerobic respiration is divided into four phases –glycolysis, TCA cycle, ETS, and oxidative phosphorylation. It is generally assumed that the process of respiration and production of ATP in each phase takes place in a step-wise manner. The product of one pathway forms the substrate of the other pathway. Various molecules produced during respiration are involved in other biochemical processes. The respiratory substrates enter and withdraw from pathway on necessity. ATP gets utilized wherever required and enzymatic rates are generally controlled. Thus, the step-wise release of energy makes the system more efficient in extracting and storing energy.
Q1. Differentiate between (a) Respiration and Combustion (b) Glycolysis and Krebs’ cycle (c) Aerobic respiration and FermentationAns. a. Respiration and Combustion: -
Respiration: It is a biochemical process. It occurs in the living cells.ATP is generatedEnzymes are requiredIt is a biologically controlled process.
Combustion: It is a physiochemical process.It does not occur in the living cells.ATP is not generatedEnzymes are not requiredIt is an uncontrolled process.
b. Glycolysis and Kreb's cycle
Glycolysis: - It is a linear pathway.It occurs in the cell cytoplasm.It occurs in both aerobic and anaerobic respiration.It generates 2 NADH and 2 ATP molecules on the breakdown of one glucose molecule.
Kreb's cycle: - It is a cyclic pathway.It occurs in the mitochondrial matrix.It occurs in aerobic respiration.It produces 6 NADH, 2FADH, and 2 ATP molecules on the breakdown of two acetyl - CoA molecules.
(c) Aerobic respiration and Fermentation
Aerobic respiration
Oxygen is used for deriving energyOccurs in the cytoplasm and mitochondriaEnd products are carbon dioxide and waterComplete oxidation of the respiratory substrate takes place About 36 ATP molecules are produced
Fermentation: -
Occurs in the absence of oxygenOccurs in the cytoplasmEnd products are ethyl alcohol and carbon dioxideIncomplete oxidation of the respiratory substrate takes placeOnly 2 ATP molecules are produced
Q2. What are respiratory substrates? Name the most common respiratory substrate.Ans. The compounds oxidised during the process of respiration are called respiratory substrates. Carbohydrates, especially glucose, act as respiratory substrates. Fats, proteins, and organic acids also act as respiratory substrates. Q3. Give the schematic representation of glycolysis?
Q4. What are the main steps in aerobic respiration? Where does it take place? Ans. The major steps in aerobic respiration and the sites where they occur are listed as belowGlycolysis: CytoplasmKrebs cycle: Matrix of the MitochondriaElectron Transport System : Inner Mitochondrial MembraneOxidative phosphorylation: F0-F1 particles in the inner mitochondrial membrane.
5. Give the schematic representation of an overall view of Krebs’ cycle.
6. Explain ETS. Ans. ETS or electron transport system is located in the inner mitochondrial membrane. It helps in releasing and utilizing the energy stored in NADH+H + and FADH2. NADH + H +, which is formed during glycolysis and citric acid cycle, gets oxidized by NADH dehydrogenase (complex I). The electrons so generated get transferred to ubiquinone through FMN. In a similar manner, FADH2 (complex II) generated during citric acid cycle gets transferred to ubiquinone. The electrons from ubiquinone are received by cytochrome bc1 (complex III) and further get transferred to cytochrome c. The cytochrome c acts as a mobile carrier between complex III and cytochrome c oxidase complex, containing cytochrome a and a3, along with copper centres (complex IV). During the transfer of electrons from each complex, the process is accompanied by the production of ATP from ADP and inorganic phosphate by the action ATP synthase (complex V). The amount of ATP produced depends on the molecule, which has been oxidized. 2 ATP molecules are produced by the oxidation of one molecule of NADH. One molecule of FADH2 , on oxidation, gives 3 ATP molecules.
Q7 . Distinguish between the following: (a) Aerobic respiration and Anaerobic respiration (b) Glycolysis and Fermentation (c) Glycolysis and Citric acid Cycle Ans. (a) Aerobic respiration and Anaerobic respiration
Aerobic respiration: - 1. It uses oxygen for deriving energy. 2. It occurs in cytoplasm and mitochondria. 3. The end products of aerobic respiration are carbon dioxide and water. 4. Complete oxidation of respiratory substrate takes place. 5. 36-38 ATP molecules are produced.
Anaerobic respiration: 1. It occurs in the absence of oxygen. 2. It occurs in cytoplasm. 3. The end products of fermentation are ethyl alcohol and carbon-dioxide.4. Incomplete oxidation of respiratory substrate takes place.5. Only 2 ATP molecules are produced
(b) Glycolysis and Fermentation
Glycolysis: - 1. Glycolysis occurs during aerobic and anaerobic respiration. 2. Pyruvic acid is produced as its end product.
Fermentation: -1. Fermentation is a type of anaerobic respiration.2. Ethanol or lactic acid is produced as its end product.
(c) Glycolysis and Citric acid Cycle
Glycolysis: - 1. It is a linear pathway. 2. It occurs in the cell cytoplasm. 3. It occurs in both aerobic and anaerobic respiration. 4. One glucose molecule breaks down to generate 2 NADH2and 2 ATP molecules.
Citric acid cycle: - 1. It is a cyclic pathway.2. It occurs in the mitochondrial matrix.3. It occurs in aerobic respiration. 4. It produces 6 NADH2,2 FADH2, and 2 ATP molecules on breakdown of two acetyl-coA molecules.
Q8. What are the assumptions made during the calculation of net gain of ATP? Ans. For theoretical calculation of ATP molecules, various assumptions are made, which are as follows. (a) It is assumed that various parts of aerobic respiration such as glycolysis, TCA cycle, and ETS occur in a sequential and orderly pathway.(b). NADH produced during the process of glycolysis enters into mitochondria to undergo oxidative phosphorylation. (c)Glucose molecule is assumed to be the only substrate while it is assumed that no other molecule enters the pathway at intermediate stages. (d)The intermediates produced during respiration are not utilized in any other process.
Q9. Discuss “The respiratory pathway is an amphibolic pathway.” Ans. Respiration is generally assumed to be a catabolic process because during respiration, various substrates are broken down for deriving energy. Carbohydrates are broken down to glucose before entering respiratory pathways. Fats get converted into fatty acids and glycerol whereas fatty acids get converted into acetyl CoA before entering the respiration. In a similar manner, proteins are converted into amino acids, which enter respiration after deamination. During synthesis of fatty acids, acetyl CoA is withdrawn from respiratory pathway. Also, in the synthesis of proteins, respiratory substrates get withdrawn. Thus, respiration is also involved in anabolism. Therefore, respiration can be termed as amphibolic pathway as it involves both anabolism and catabolism
Q10. Define RQ. What is its value for fats?Ans. Respiratory quotient (RQ) or respiratory ratio can be defined as the ratio of the volume of CO2 evolved to the volume of O2consumed during respiration. The value of respiratory quotient depends on the type of respiratory substrate. Its value is one for carbohydrates. However, it is always less than one for fats as fats consume more oxygen for respiration than carbohydrates. It can be illustrated through the example of tripalmitin fatty acid, which consumes 145 molecules of O2for respiration while 102 molecules of CO2are evolved. The RQ value for tripalmitin is 0.7.
Q11. What is oxidative phosphorylation? Ans. Oxidative phosphorylation is a process in which electrons are transferred from electron donors to oxygen, which acts as electron acceptor. The oxidation-reduction reactions are involved in the formation of proton gradient. The main role in oxidative phosphorylation is played by the enzyme ATP synthase (complex V). This enzyme complex consists of F0and F1components. The F1headpiece is a peripheral membrane protein complex and contains the site for ATP synthesis from ADP and inorganic phosphate. F0component is a part of membrane protein complex, which acts as a channel for crossing of the protons from inner mitochondrial membrane to the mitochondrial matrix. For every two protons passing through F0–F1complex, synthesis of one ATP molecule takes place.
Q12. What is the significance of step-wise release of energy in respiration? Ans. The process of aerobic respiration is divided into four phases –glycolysis, TCA cycle, ETS, and oxidative phosphorylation. It is generally assumed that the process of respiration and production of ATP in each phase takes place in a step-wise manner. The product of one pathway forms the substrate of the other pathway. Various molecules produced during respiration are involved in other biochemical processes. The respiratory substrates enter and withdraw from pathway on necessity. ATP gets utilized wherever required and enzymatic rates are generally controlled. Thus, the step-wise release of energy makes the system more efficient in extracting and storing energy.
