Title Med-RM_Bot_SP-2_Ch-11-Respiration in Plants.pdf ✅

Chapter Contents Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Do Plants Breathe? Glycolysis Fermentation Aerobic Respiration The Respiratory Balance Sheet Amphibolic Pathway Respiratory Quotient When it comes to life, respiration is considered as one of the basic features which helps the organism to survive. Respiration provides energy for carrying out daily life activities, be it absorption, transport, movement, reproduction or even breathing. The gaseous exchange, i.e., intake of oxygen and release of carbon dioxide is called breathing while respiration involves biological oxidation of organic molecules i.e., breaking up of C—C bonds by using enzymes and results in the release of energy in the form of ATP. The oxidation of macromolecules that takes place inside the body is called as “Respiration”. The compounds subjected to biological oxidation are called respiratory substrate. These may be carbohydrates, fats, proteins or organic acids. Carbohydrates are the most preferred substrate for biological oxidation, other substrates are used under certain specific conditions. DO PLANTS BREATHE? Plants require oxygen for respiration and release carbon dioxide. Animals have special organs for breathing like lungs, gills, trachea etc. Plants unlike animals, have no specialised organs for gaseous exchange but they have stomata and lenticels for this purpose. The complete combustion of glucose, which produces CO2 and H2O as end products, yields energy most of which is given out as heat. C6H12O6 + 6O2  6CO2 + 6H2O + Energy If this energy is to be useful to the cell, it should be able to utilise it to synthesise other molecules that the cell requires. The strategy that the plant cell uses is to catabolise the glucose molecule in such a way that not all the liberated energy goes out as heat. The key is Chapter 11 Respiration in Plants
134 Respiration in Plants NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 to oxidise glucose not in one step but in several small steps enabling some steps to be just large enough such that the energy released can be coupled to ATP synthesis. This is done by the process of cellular respiration. Types of Respiration : 1. On the basis of respiratory substrate, Blackman classified two types of respiration: (i) Floating Respiration : When respiratory substrate is fat or carbohydrate. (ii) Protoplasmic Respiration : When respiratory substrate is protein. 2. On the basis of whether oxygen is present for oxidation of food or not, respiration is divided into two types : (i) Aerobic respiration : It is the process of complete oxidation of organic molecules in the presence of molecular oxygen into carbon dioxide and water. (ii) Anaerobic respiration : It is the process of incomplete oxidation of organic molecules in the absence of molecular oxygen. Some organisms are facultative anaerobes, while in others the requirement for anaerobic condition is obligate. (a) Facultative anaerobes : The aerobic organisms which can respire even in the absence of O2. (b) Obligate anaerobes : Organisms which respire anaerobically only. They lack enzymes necessary for carrying out aerobic respiration. In any case, all living organisms retain the enzymatic machinery to partially oxidise glucose without the help of O2. This breakdown of glucose to pyruvic acid is called glycolysis. GLYCOLYSIS It is a common step of both types of respiration which takes place in cytoplasm. Glycolysis (Greek glycon – for sugar and lysis – for splitting) means splitting up of sugar. This scheme of glycolysis was given by Gustav Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the EMP pathway. In plants, the glucose is derived from sucrose, which is 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. In animals, starch is digested into glucose and that is used in the glycolysis. The first half of this pathway activates glucose (glucose activation phase). The second half extracts the energy (energy extraction phase). Product of glycolysis : (1) 2 molecules of pyruvic acid. (2) Net gain to 2ATP molecules. (3) 2 molecules of NADH2.
NEET Respiration in Plants 135 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 (6C) Glucose-6-phosphate Glucose (6C) Hexokinase ADP ATP Mg++ (6C) Fructose-6-phosphate Phosphohexoisomerase (6C) Fructose-1,6-bisphosphate Aldolase (3C) Triose phosphate (Glyceraldehyde-3-phosphate) Triose phosphate (Dihydroxy acetone phosphate) (3C) 2NADH + 2H+ 2NAD Triose bisphosphate (1,3 bisphosphoglyceric acid) Triose phosphate ( ) 3-phosphoglyceric acid 2H PO 3 4 Glyceraldehyde 3-phosphate dehydrogenase (3C) (3C) 2ATP 2ADP Phosphoglycerate kinase, Mg++ 2-phosphoglycerate (3C) Phosphoglyceromutase Enolase, Mg++ Phosphoenolpyruvate (3C) Pyruvate kinase, Mg , K ++ + Pyruvic acid (3C) 2 ATP 2ADP Phosphofructokinase (PFK : Pacemaker enzyme) ADP ATP Mg++ Phosphotriose isomerase Activation phase Or Energy consuming phase Pay-off phase Or Energy producing phase 2 molecules 2 molecules 2 molecules 2 molecules 2 molecules 2H O2 Rate limiting step Fig. : Steps of Glycolysis
136 Respiration in Plants NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Fate of Pyruvic Acid Pyruvic acid is the key product of glycolysis. The fate of pyruvic acid depends upon the cellular needs. There are three major ways in which different cells handle pyruvic acid. These are lactic acid fermentation, alcoholic fermentation and aerobic respiration. Fermentation takes place under anaerobic conditions in many prokaryotes and unicelled eukaryotes. For the complete oxidation of glucose to CO2 and H2O, however, organisms adopt Krebs cycle, which is also called aerobic respiration. This requires O2 supply. Pfeffer-Kostychev scheme represents interrelationship between aerobic and anaerobic respiration. Carbohydrates Glucose Pyruvic acid This is common to both aerobic as well as anaerobic respiration Glycolysis or EMP Pathway In the absence of O 2 Alcoholic fermentation (CH OH + CO + Energy) 25 2 Lactic acid fermentation (CH O + Energy) 363 In the absence of O2 In the presence of O2 Aerobic respiration (CO + H O + Energy) 2 2 FERMENTATION Fermentation is a kind of anaerobic respiration, carried out primarily by fungi and bacteria. Although people had been using this process in the preparation of wines since prehistoric times but failed in their attempts to understand the alcoholic fermentation. Gay Lussac was the first to discover fermentation. Types of fermentation: 1. Alcoholic fermentation Pyruvic acid formed at the end of glycolysis is converted to alcohol by using two enzymes, pyruvic acid decarboxylase and alcohol dehydrogenase. (i) In the first step, pyruvic acid is decarboxylated resulting in the formation of acetaldehyde and CO2. 3 32     Pyruvic acid decarboxylase Mg (Pyruvic acid) (Acetaldehyde) 2CH COCOOH 2CH CHO 2CO (ii) In the second step, acetaldehyde is reduced to alcohol by 2NADH + H+.   3 25     Alcohol dehydrogenase (Acetaldehyde) (Ethanol) 2CH CHO 2NADH 2H 2C H OH 2NAD Example : This is commonly seen in yeast and certain bacteria. Ethyl alcohol does not stay inside micro-organisms but is excreted. Accumulation of alcohol beyond a certain limit can, however, kill the microorganisms.Yeasts poison themselves to death when the concentration of alcohol reaches about 13%. Therefore, maximum concentration of alcohol in beverages that are naturally fermented is 13%. A higher concentration of alcohol is a beverage is achieved through distillation.