CLASS 11 BIOLOGY CHAPTER- 13 RESPIRATION IN PLANTS CELLULAR RESPIRATION: The process of conversion of the chemical energy of organic substances into a metabolically usable energy within living cells is called cellular respiration. TYPES OF CELLULAR RESPIRATION: (i) Aerobic respiration: The process of respiration which requires molecular oxygen. (ii) Anaerobic respiration: The process of respiration which does not require molecular oxygen and occurs in the cytoplasm. MECHANISM OF RESPIRATION: Following are the steps- 1. GLYCOLYSIS / EMP Pathway: It involves a series of closely integrated reactions in which hexose sugars(usually glucose) are converted into pyruvic acid. It is common in both aerobic and anaerobic reactions. It occurs in the cytoplasm. It does not require oxygen. Gollowing are the steps of GLYCLOLYSIS: (i) Conversion of glucose to Fructose-1,6-diphosphate: First phosphorylation: Glucose is converted to Glucose -6-phosphate in the presence of enzyme hexokinase and Mg++ ions and energy in the form of ATP. Isomerization: Glucose-6-phosphate is converted to Fructose-6-phosphate in the presence of phosphohexoisomerase. Second phosphorylation: Fructose-6-phosphate is converted to Fructose-1,6- diphosphate by the use of energy in the form of ATP. (ii) Formation of pyruvic acid from fructose -1,6-diphosphate: Cleavage: Fructose-1,6-diphosphate splits into 3-phosphoglyceraldehyde and Dihydroxyacetone phosphate in the presence of enzyme aldolase. Phosphorylation and oxidative dehydrogenase: 3-phosphoglyceraldehyde is converted to 1,3-biphosphoglyceric acid. ATP generation(first): 1,3-biphosphoglyceric acid is converted to 3-phosphoglyceric acid in the presence of Mg++ and phosphoglycerokinase . Isomerization: 3-phosphoglyceric acid is converted to 2-phosphoglyceric acid in the presence of Mg++. Dehydration: 2-phosphoglyceric acid is converted to 2-phosphoenolpyruvic acid. ATP generation(second): 2-phosphoenolpyruvic acid is converted to pyruvic acid in the presence of ADP, pyruvate kinase and Mg++. 2. KREBS CYCLE CYCLE / TCA( Tricarboxylic Acid Cycle) / Citric Acid Cycle: Formation of citric acid: Acetyl CoA reacts with Oxaloacetic acid to form Citric acid in the presence of citrate synthase. Dehydration: Citric acid is converted to cis-aconitic acid in the presence of aconitase. Hydration I: Cis-aconitic acid is converted to Isocitric acid in the presence of acotinase. Dehydriogenation I: Isocitric acid is converted to Oxalosuccinic acid in the presence of CoA and NAD+ . Decarboxylation I: Oxalosuccinic acid is converted to alpha-ketoglutaric acid in the presence of carboxylase. Dehydrogenation II and decarboxylation II: (i) alpha-ketoglutaric acid is converted to Succinyl-CoA in the presence of NAD+ and CoA . (ii) Succinyl-CoA is converted to Succinic acid in the presence of succinic thiokinase and GDP. (iii) Dehydrogenation III: Succinic acid is converted to Fumaric acid in the presence of succinic dehydrogenase and FAD. Hydration II: Fumaric acid is converted to Malic acid in the presence of fumarase. Dehydrogenation IV: Malic acid is converted to Oxaloacetic acid by the use of NAD+. ELECTRON TRANSPORT SYSTEM /ETS: It is a chain of carriers located in the F1 particles of mitochondria .It consists of the following components : (i) NAD (Nicotinamide adenine dinucleotide) (ii) FAD and FMN (flavin nucleotides) (iii) Coenzyme Q (CoQ or Ubiquinone) (iv) Cytochromes (cyt b, c, a, a3) OXIDATIVE PHOSPHORYLATION: The synthesis of ATP by electron flow through the ETS, with oxygen as the terminal electron acceptor. (i) At three sites along the chain each electron pair loses so much of free energy that each site a phosphate utilizes this energy in binding with an ADP. (ii) An electron pair which originates from NADH yields three molecules of ATP. (iii) An electron pair which originates from FADH2 yields two molecules of ATP. (iv) ATP synthesis in mitochondria is brought about by ATP-synthesizing enzyme complexes. (v) Each complex has two major components-F0 and F1 . (vi) F1 protudes like a knob into the matrix from inner membrane of mitochondria. (vii) F1 is attached by a stalk which is embedded in the membrane. ANAEROBIC RESPIRATION : FERMENTATION: (i) In fermentation, carbohydrates are degraded into two or more simpler molecules without the utilization of oxygen. (ii) Micro-organisms involved in fermentation are called anaerobes. (iii) During the process of fermentation, glucose is first converted to pyruvic acid. (iv) In the next step pyruvic acid is converted to ethanol and CO2 /lactic acid. (v) Lactic acid bacteria convert pyruvic acid to lactic acid.In this process , NADH produced by glycolysis is reoxidised to NAD+ donating one hydrogen ion and two electrons to pyruvic acid which yields lactic acid. (vi) No ATP is produced by this process. (vii) Alcoholic fermentation by yeast. Pyruvic acid is further changed to acetaldehyde by releasing a molecule of CO2. Acetaldehyde oxidizes NADH and is reduced tio ethanol. The net gain of ATP per molecule of glucose fermented is two. AMPHIBOLIC PATHWAY: (i) All carbohydrates convert into glucose and other substances like fats, proteins act as respiratory substrates but they do not enter the respiratory pathway. (ii) If fatty acids are to be respired they are degraded to acetyl CoA to enter trhe pathway. (iii) Glycerol enter the pathway by converting to PGAL. (iv) Proteins are degraded by proteases and the individual amino acids enter the pathway (kreb’s cycle) as pyruvate/acetyl CoA. (v) Since the respiratory process involves the breakdown of substrates therefore it is also called catabolic pathway. (vi) Now when the organism need to synthesise fatty acids, acetyl CoA is withdrawn from the respiratory pathway. (vii) As both catabolic and anabolic process occurs in respiratory pathway, it is called amphibolic pathway. RESPIRATORY QUOTIENT: The molar ratio of CO2 evolved to O2 absorbed in respiration is known as respiratory quotient. (i) When carbohydrates are completely oxidized the value of R.Q. is unity(one). (ii) When fats and proteins are the substrates , the value of R.Q. is less than unity(0.5-0.9). (iii) When the substrates are organic compounds ,the value is more than unity(1.3 –4.0). (iv) In succulents likeOpuntia and Bryophyllum where there is incomplete oxidation of carbohydrates, no CO2 is produced and hence the value of R.Q is zero. (v) R.Q is measured by Ganong’s respirometer . (vi) R.Q in humans is 0.85. QUESTIONS-ANSWERS: Q1. What are respiratory substrates? Name the most common respiratory substrate. A1. The compounds that are oxidized during the process of respoiration are known as respiratory substrates, Carbohydrates are the most common respiratory substrate. Q2. Name the connecting link between glycolysis and the kreb’s cycle. A2. Formation of acetyl coenzyme A. Q3. What is the other name of glycolytic pathway? A3. Embden-Meyerhof-Parnas(EMP) pathway. Q4. Where are the enzymes of the electron transport system found? A4. In cristae of mitochondria. Q5. What is used and produced in glycolysis? A5. Used : 1 Glucose, 4 ADP, 4 phosphate groups, 2 ATP, 2NAD+. Produced: 2 pyruvic acid, 4 ATP, 2 phosphate groups, 2 ADP, 2 NADH+H+, 2H20. Q6. Explain the major steps of glycolysis. A6. Q7. Explain amphibolic pathway. A7. (i) All carbohydrates convert into glucose and other substances like fats, proteins act as respiratory substrates but they do not enter the respiratory pathway. (ii) If fatty acids are to be respired they are degraded to acetyl CoA to enter trhe pathway. (iii) Glycerol enter the pathway by converting to PGAL. (iv) Proteins are degraded by proteases and the individual amino acids enter the pathway (kreb’s cycle) as pyruvate/acetyl CoA. (v) Since the respiratory process involves the breakdown of substrates therefore it is also called catabolic pathway. (vi) Now when the organism need to synthesise fatty acids, acetyl CoA is withdrawn from the respiratory pathway. (vii) As both catabolic and anabolic process occurs in respiratory pathway, it is called amphibolic pathway. ACTIVITIES: 1. LEARN THE CHAPTER THOROUGHLY. 2. DRAW THE FLOW CHARTS OF KREB’ CYCLE, EMP PATHWAY ETC. .