RESPIRATION

 

 During the electron transport, FAD and the iron atom of different cytochromes get successively reduced (FE2+) and oxidized (FE3+) and enough energy is released at some places which are utilized in the phosphorylation of ADP molecules to generate energy-rich ATP molecules. Because this oxidation accompanies phosphorylation (production of high energy phosphate bonds of ATP and Pi), it is called Oxidative Phosphorylation . It takes place on stalked particles situated on cristae in mitochondria. It is inhibited by 2,4-Dinitrophenol. During the system, one ATP molecule (contains 7.6K.Cal. energy) is synthesized at each place when electrons are transferred from : i)reduced NADH2 NADPH2 to FAD, ii) reduced Cytochrome b to Cytochromec, and (iii) reduced Cytochrome a to Cytochrome a3. Thus, oxidation of one molecule of reducing NADH2 NADPH2 will result in the formation of 3 ATP molecules while oxidation of FADH2 will lead to the synthesis of 2ATP molecules. Complete oxidation of a glucose mol

 The last step in aerobic respiration is the oxidation of reduced co-enzymes produced during glycolysis and Kreb’s cycle by molecular O2 through FAD, cyt. b, cyt. c, cyt. a and cyt.a3 (cytochrome oxidase). Two H-atoms or electrons from the reduced coenzyme (NADH2 or NADPH2) travel through FAD and the cytochromes each with a more positive oxidation-reduction potential and ultimately combine with1/2 O2 molecule to produce one molecule of H2O. This is called Termination Oxidation. 

 This cycle is named after H.A.Kreb. The Kreb’s cycle is also known as Tricarboxylic acid, (TCA) cycle, Organic acid cycle, Mitochondrial respiration, Oxidation of pyruvate, and Citric Acid cycle. It takes place in mitochondria where all the necessary enzymes required for it are found on cristae. In this cycle, at first, the carbon atom 3or 4, then 2 or 5, and lastly carbon number 1 or 6 of the glucose molecules are released in the form of CO2 molecules. Various reactions of the Kreb’s (TCA) cycle is schematically Significance of Kreb’s cycle. Kreb’s cycle occupies a central and very important place in the various metabolism of plants. i. It provides energy in the form of ATP molecules for various metabolic activities. ii. It is directly related to nitrogen metabolism iii. It is also intimately related to the fat metabolism iv. It is closely associated with the glyoxalate cycle v. It is related to other metabolic processes through its intermediates in one way or another.

 Respiratory Substrate The substrates, which are broken down in respiration for the release of energy, maybe carbohydrates, fats, or proteins. Proteins are used up in respiration only when carbohydrates and fats are not available. As regards carbohydrates, not only simple hexose sugars like glucose and fructose but complex disaccharides particularly sucrose and polysaccharides such as lignin, inulin, and hemicellulose are also used as respiratory substrates. Fats are used as respiratory substrates after their hydrolysis to fatty acids and glycerol by lipase and their subsequent conversion to hexose sugars. Proteins serve as respiratory substrates after their breakdown into aminoacids by proteolytic enzymes. During respiration, the complex substrates are broken down into simpler ones and finally, CO2 is liberated and water is formed. During the oxidation of the respiratory substrate, some energy is released. Part of this energy is trapped in the form of energy-rich compounds such as ATP

 On the basis of the availability of O2, respiration has been divided into two categories. a. Aerobic respiration It is of common occurrence found in all plants and takes place in the presence of O2 and the stored food (respiratory substrate) gets completely oxidized into CO2 and water as,                                      C6 H12O6 + 6O2 → 6CO2 + 6H2O + 686 K.Cal. b. Anaerobic respiration It takes place in the absence of O2 or when O2 conc. is less than 1%. The stored food is completely oxidized and instead of CO2 and water, certain other compounds are also formed. This is of rare occurrence but common among micro-organisms like yeasts and can be represented by,                                        C6H12O6 + → 2C2H5OH + 2CO2 + 56 KCal.

 1. Absorption of O2. 2. Conversion of carbohydrate (complex) to CO2 and H2O and simpler substances (i.e. oxidation of food). 3. Release of energy -a part of which is utilized in various vital processes and the rest may be lost in the form of heat. 4. Formation of intermediate products playing different roles in metabolism. 5. Liberation of CO2 and H2O 6. Loss in weight in plants as a result of oxidation. Therefore, respiration is a reverse process of photosynthesis.

 Important plant life processes such as proteins, fats, and carbohydrates, require a certain expenditure of energy. This energy is mainly derived from the breakdown of complex macromolecules. During photosynthesis, light energy is converted into chemical energy and stored in the bonds of complex organic molecules. The major portion of stored energy in plants is found in the form of carbohydrates like glucose and starch. The energy stored in carbohydrate molecules during photosynthesis is released during cellular oxidation of carbohydrates into CO2 and H2O. This is called Respiration. During this process, O2 is consumed and CO2 released out. In respiration, oxidation of various organic food substances like carbohydrates, fats, proteins, etc., may take place. Among these, glucose is the commonest . Its oxidation proceeds a shown below in the simplest equation :                                     C6 H12O6 + 6O2 → 6CO2 + 6H2O + Energy (686 K.Cal.) Breakdown of glucose involves many steps