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1 Cellular respiration Glycolysis:-It is defined as the sequence of reaction converting glucose to pyruvate, with the production of ATP. Glycolysis occurs in the cytoplasm of virtually all living cells, both in the absence and presence of O2. Probably it was the first energy releasing process in organisms when the life evolved. The oxidative respiration in mitochondria of eukaryotic cells became possible only after molecular oxygen had accumulated in the earth’s atmosphere as a result of photosynthesis by cyanobacteria. Reaction of glycolysis. 1. Glucose is phosphorylated to glucose 6 phosphate by enzyme hexokinase. This is a Mg+2 activated enzyme. 2. Glucose 6 phosphate is isomerised by enzyme glucose 6 phosphate isomerase into fructose 6 phosphate. 3. Fructose 6 phosphate is phosphorylated by enzyme phosphofructokinase to fructose 1, 6 diphosphate. 4. Fructose 1, 6 diphosphate is cleaved by enzyme aldolase into two interconvertible triosephosphates, Glyceraldehyde 3 phosphate and Dihydroxy acetone phosphate. 5. The enzyme triosephosphate isomerase catalyses the reversible inter conversion of glyceraldehyde 3 phosphate and dihydroxyacetone phosphate, thus two molecules of glyceraldehyde 3 phosphate are obtained from one molecule of glucose. 6. Glyceraldehyde 3 phosphate is oxidized in presence of enzyme glyceraldehydes 3 phosphate dehydrogenase to 1, 3 biphosphoglycerate, it is simultaneously phosphorylated by inorganic phosphate. Here 2NAD is changed to 2NADH. 7. 1,3, biphosphoglycerate is dephosphorylated in presence of enzyme phosphoglycerate kinase to 3 phosphoglycerate and ATP is changed to ADP. 8. Now the phosphate group changes its position from 3 to carbon 2 catalysed by enzyme phosphoglyceromutase. 9. 2, phosphoglycerate is changed to phosphoenol pyruvate (PEP) by enzyme enolase containing high energy enol phosphate. 10. Phosphoenol pyruvate is dephosphorylated to pyruvate in presence of enzyme pyruvate kinase. Here ADP is changed to ATP. In glycolysis there is a net formation of two ATP molecules and 2 NADH molecules which in ETC oxidative phosphorylation release 6 molecules of ATP. So total gain of ATP molecule is glycolysis is 8 i.e. one NADH molecule produces 3 ATP molecules in ETC. What happens to pyruvaye that is produced in glycolysis. The fate of pyruvate varies from cell to cell and its environment. In cells having mitochondria and adequate supply of oxygen, pyruvate is oxidized in TCA cycle. In the absence of oxygen pyruvate is reduced by NADH to either lactate or alcohol. Such anaerobic routes are called fermentation pathways. 2 Oxidative decarboxylation of Pyruvate Glycolysis occurs in the cytoplasm of the cell but pyruvate is metabolized aerobically in the mitochondria. The pyruvate is transported to the mitochondria either by simple diffusion or by the pyruvate – hydroxyl ion antiport system where pyruvate is exchanged for a hydroxyl ion. The pyruvate is oxidatively decarboxylated by the multienzyme complex called the pyruvate dehydrogenase consisting of multiple copies of three enzymes ( E1. E2, E3 ) each with specific binding site for the substrate and different cofactors. Since this reaction links glycolysis with TCA it is also termed as link reaction. Pyruvate in this reaction is changed to acetyle COA with the removal of CO2 and a pair of hydrogen atoms. The hydrogen atoms released combine with NAD forms NADH. Tricarboxylic Acid cycle Discovered by Hans Kreb therefore it is also called Krebs cycle. Krebs cycle is the most important metabolic pathway for the energy supply to the body. About 65 – 75 % of the ATP is synthesized in Krebs cycle. This cycle utilizes about two thirds of total oxygen consumed by the body. The name TCA cycle is used, since at the onset of cycle, tricarboxylic acids citrate and isocitrate participate Acetyl CoA now enters the Krebs cycle. Each step is catalysed by a specific enzyme. The various reactions occurring in the cycle are. 1. Acetyl CoA , condences with oxaloacetate (4–carbon compound )to form citric acid ( citrate) a 6-Carbon compound . CoA is liberated. 2. Citrate is converted into isocitrate by rearrangement of atom groups by the enzyme aconitase. 3. Isocitrate gives of a pair of H atoms in presence of enzyme isocitrate dehydrogenase to form oxalosuccinate, and NAD is changed to NADH. 4. Oxalosuccinat in presence of isocitrate dehydrogenase loses a molecule of CO2 and forms ∞ Ketoglutarate (5 C ). The pair of H atoms passes to NAD forming NADH. (the enzyme isocitrate dehydrogenase catalyses the reaction ) 5. ∞ ketoglutarate is transformed to succinyl CoA (4 C ) . This reaction involves CoA and NAD, the products being CO2, NADH in addition to succinyl CoA. (With this reaction all the C atoms that entered into citric acid cycle as pyruvic acid are released as CO2 ). 6. Succinyl CoA is acted upon by enzyme (Succinyl thiokinase ) to form succinate. This reaction releases sufficient energy to form ATP (in plants) or GTP (in animals). 7. Succinate undergoes dehydrogenation to form fumarate (by the enzyme succinate dehydrogenase ). In this reaction FADH2 (reduced flavin adenine dinucleotode ) is produced. 8. A molecule of 3 water gets added to fumarate to form malate in presence of enzyme Fumarase. 9. Malate is dehydrogenated (or oxidized) to produce oxaloacetate in presence of enzyme Malate dehydrogenase. The pair of H atoms passes to NAD forming NADH2. Oxaloacetate combines with another molecule of acetyl CoA to repeat the cycle. TCA cycle – the central metabolic pathway The citric acid cycle is the final common oxidative pathway for carbohydrates fats and amino acids. This cycle not only supplies energy but also provides many intermediates required for the synthesis of amino acid, glucose, heame etc. Krebs cycle is the most important central pathway connecting almost all the individual metabolic pathways. Energetics of Citric Acid Cycle During the process of oxidation of acetyl CoA via citric acid cycle 4 reducing equivalents (3 as NADH and one as FADH2) are produced. Oxidation of one NADH by electron transport chain coupled with oxidative phosphorylation results in the synthesis of 3ATP, where as FADH2 leads to the formation of 2 ATP. Total ATP molecules produced during respiration:In Glycolysis Direct = 2 2 molecules of NADH = 6 Total: = 8 In Link reaction: Pyruvic acid to Acetyle co-A 2 molecules of NADH = 6 Citric acid cycle 6NADH = 18 2FADH2 = 4 Direct = 2 Total = 24 Grand total = 38 Krebs cycle is both catabolic and anabolic in nature. Hence regard as amphibolic. Pentose phosphate pathway ( PPP ) Glycolysis is a major route of degradation of glucose to pyruvate. Many other routes of glucose metabolism have also been investigated in plant and animal tissue. One of the important pathways that operates in a wide variety of organisms including animals, plants and many organisms called pentose phosphate pathway, or hexose monophsphate shunt or phosphogluconate pathway given by Warburg-Dickens, so called Warburg-Dickens pathway. The pathway is divided into Oxidative phase and Non oxidative phase. 1. Oxidative phase :- The various reactions occurring in this phase are 1. Glucose 6 phosphate is oxidized to 6 phosphoglucono 1, 5 lactone, by removal of electrons which are accepted by NADP. 2. Hydrolysis of the 6 phosphoglucono 1, 5 lactone results in the formation of 6 phosphogluconate. 3. 6 phosphogluconate is decarboxylated to ribulose 5 phosphate and second molecule of NADPH is formed. Enzymes of these reactions are activated by MG2+ . 2. Non oxidation phase :-This pathway operates according to the specific requirements of the tissue.I tissue active in nucleotide synthesis ribulose 5 phosphate is isomerised to ribose 5 phosphate. The pathway terminates at this point. The tissue active in lipid synthesis has a great demand for NADPH. Under this situation the reactions occurring are as under. 1. Isomerisation of ribulose 5 phosphate occurs by two different enzymes forms ribose 5 phosphate and xylulose 5 phosphate. 4 2. These two 5 carbon sugars condense together by the enzyme transketolase to form 7 carbon sedoheptulose phosphate and three carbon glyceraldehydes 3 phosphate. 3. Now 3 & 7 carbon sugars condense by another enzyme called transaldolase to form 6 carbon fructose 6 phosphate and 4 carbon erythrose phosphate. 4. Another molecule of 5 carbon of xylulose 5 phosphate condenses with 4 carbon erythrose phosphate and forms fructose 6 phosphate and 3 carbon glyceraldehyde 3 phosphate. The end products of the pentose phosphate pathway can enter into the glycolytic pathway. Thus this pathway is basically a shunt as its branches out from the glycolytic pathway at the point of glucose 6 phosphate and rejoins at the point of fructose 6 phosphate and glyceraldehyde 3 phosphate. Fructose 6 phosphate may isomerise into glucose 6 phosphate and re-enter in the oxidative phase of cycle. This pathway does not occur in muscle cells.The pathway occurs in the cytoplasm of cell. Glucose-6-phosphate Glucose 6 phosphate dehydrogenase NADP NADPH+ H 6-phospho-glucono-1,5-lactone 6 phospho gluconase H 2O 6-phosphogluconate NADP Phosphogluconate dehydrogenase Co2 NADPH +H Robulose-5-phosphate Isomerase. Ribose-5-phosphate (Pathway terminates here when the cell is actively involved in DNA and RNA synthesis). 5 Reaction occurring in lipid synthesizing cells Ribulose-5-phosphate Isomerase Ribose-5-phosphate Xylose-5-phosphate Condensation. Transketolase Pseudo heptulose-7-phosphate Glyceraldehyde-3-phosphate Condensation. Transaldolase Frutose-6-phosphate Erythorase-4-phosphate + Xylose-5-phosphate Condensation Fructose-6-phosphate Glyceraldehyde-3-phoaphate Significance of PPP 1. NADPH not NADH is produced in PPP, which is required as reductant in fat synthesis. 2. This pathway produces several pentoses which are required for nucleic acid synthesis. 3. The oxidative pentose phosphate pathway is thought to be involved in generating calvin cycle intermediates before the leaves became fully photoautotrophic. Fermentation In the absence of O2 pyruvate is reduced by NADH to either lactate or alcohol. Such anaerobic routes are called fermentation pathways. The process of conversion of glucose to alcohol is known as alcoholic fermentation and the process of conversion of glucose to lactate is called lactate fermentation. Alcoholic fermentation:- During this process at first glucose is converted into pyruvate. Pyruvate in the presence of enzyme pyruvate decarboxylase is converted into acetaldehyde one molecule of CO2 is liberated in this reaction. Pyruvate decarboxylase 6 Pyruvate Acetaldehyde + CO2 In the 2 reaction acetaldehyde is reduced to ethyl alcohol in presence of enzyme alcohol dehydrogenase. At this step one molecule of NADH is oxidized to NAD. Alcoholdehydrogenase Acetaldehyde Ethyl alcohol It can occur in any sugar solution. The fruit juices show alcoholic fermentation when yeast powder is added or the juice is left as such open in air. Lactic acid fermentation:- It is carried out by lactic acid bacteria. Pyruvate is reduced in presence of enzyme pyruvate dehydrogenase in presence of Zn2+ & FMN ( Flavin mononucleotide ) to lactic acid. One glucose molecule gives two lacticacid molecules CO2 is not involved in this reation. In muscle cells,Pyruvate is also changed into lactic acid which is carried to liver through blood supply, where it is again converted into glucose and enters the blood. Energy yield in fermentation. Fermentation yields only about 5% of the energy obtained by aerobic respiration. This small amount of energy is sufficient to maintain the life of organisms. Such as yeasts, many bacteria and other anaerobes. Net gain of ATP in Glycolysis = 8 Loss of ATP in fermentation =6 Balance ATP =2 Importance of fermentation: 1. It supplements the energy provided by aerobic respiration during intense muscular activity. 2. Brewing industry produces beer, wines by fermenting sugar solution with yeasts. 3. Baking industry uses CO2 released by yeast cells in alcoholic fermentation in raising the dough and making bread spongy. 4. Dairy industry produces yogurt, cheese and butter by fermenting milk sugar lactose into lactic acid by strepto coccus lacti. Lactic acid coagulates the milk protein casein and fuses droplets of milk fat. 5. Tea and Tobacco leaves are cured (freed of bitterness and important pleasant flavour) by fermentation with certain bacteria ( Bacillus megatherium ). 6. Vinegar is produced by fermenting molasses with yeast to ethyl alcohol which is oxidized to acetic acid by aerobic bacteria Acetobacter aceti. 7. Butyl alcohol and acetone are manufactured from molasses by fermentation with bacteria Clostridium acetobutylicum 8. Fermentation is used for cleaning hides. 9. Retting of fibers by Pseudomonas. 10. Ensilage a nutrient fodder for cattle is prepared by fermentation with bacteria in air tight chambers. nd Respiratory quotient (RQ):- During aerobic respiration O2 is consumed and Co2 is released. The ratio of the volume of the CO2 evolved to the volume of O2 consumed in the respiration is called respiratory quotient or Repiratory ratio. Volume of CO2 evolved RQ = Volume of O2 consumed. The RQ depends upon the type of respiratory substrate used during respiration. This is different for different substrates. 7 1.Carbohydrates. C6H12 O6 + 6O2 6C O2 +6H2O +Energy 6C O2 RQ = = 1 6O2 So when carbohydrate is the substrate RQ is equall to one. Because equall amount of O2 and CO2 are consumed and evolved. 2. Fats. 2(C51 H98 O2) + 145 O2 Tripalmatin. 102CO2 + 98H2O +Energy. 102CO2 RQ = = 0.7 145 O2 So RQ is less than one when fat is the substrate . Fat contains less O2 than carbohydrates. So requires greater amount of O2 for oxidation. 3. Organic acids C4H6 O2 +3O2 Mallic acid. 4C O2 +3H2O + Energy 4C O2 RQ =1.3 3O2 RQ is more than one when the substrate used is organic acid. It is because organic acids contain more O2 than carbohydrates so relatively less O2 is required for their oxidation. In anaerobic respiration C O2 is evolved but O2 is not consumed .Therefore RQ in such cases will be infinite eg. Zymase C6H12O6 2C2H5OH + 2C O2+ energy RQ = ∞ Importance of RQ. 1. Knowledge of RQ helps in determining respiratory substrate. 2. It helps in knowing the type of respiration being performed. Compensation point:- Photosynthesis and respiration are the process which are exactly opposite to each other. Photosynthesis builds up carbohydrates with the absorption of O2 and release of CO2. The process of respiration goes on all the times and in all living cells. Where as photosynthesis proceeds only in sunlight and is confined to the green cells. Under normal conditions a green plant accumulates organic substances besides utilizing them in respiration The green cells must therefore built up more of organic food during the few hours of sunlight than is broken down in respiration. Photosynthesis is therefore a more rapid process than respiration (approximately10:1) Photosynthesis being a more rapid process utilizes all the CO2 liberated in respiration , However under a critical low light intensity (ie. in the morning and evening) both the processes are approximately equal. The O2 evolved in photosynthesis will be utilized in the respiration and CO2 evolved in respiration will be utilized in photosynthesis. The light intensity at which the rate of photosynthesis is just equal to rate of respiration is called compensation point. At the compensation point there is no increase in the dry matter of plant.