Tiêu đề Respiration in Plants 4.0.pdf ✅

Respiration in Plants 1 2 Respiration in Plants Introduction • All the energy required for ‘life’ processes is obtained by oxidation. • Cellular respiration is the process / mechanism of breakdown of food materials within the cell to release energy, and the trapping of this energy for synthesis of ATP. • The breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to release of considerable amount of energy is called respiration. • The compounds that are oxidised during this process are known as respiratory substrates. • Usually carbohydrates are oxidised to release energy, but proteins, fats and even organic acids can be used as respiratory substances in some plants, under certain conditions. • During oxidation within a cell, all the energy contained in respiratory substrates is not released free into the cell, or in a single step. • It is released in a series of slow step-wise reactions controlled by enzymes, and it is trapped as chemical energy in the form of ATP. • The energy released by oxidation in respiration is not (or rather cannot be) used directly but is used to synthesise ATP, which is broken down whenever (and wherever) energy needs to be utilised. • ATP acts as the energy currency of the cell. • This energy trapped in ATP is utilised in various energy-requiring processes of the organisms, and the carbon skeleton produced during respiration is used as precursors for biosynthesis of other molecules in the cell. • It is a exothermic process. • Respiration is represented by following general equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy • Respiration is oxidation of organic food, which release the energy, which is utilized in synthesis of ATP. • Various respiratory intermediates are used up in biosynthesis so amphibolic process. • All living cells participate in the respiration process. DO PLANTS BREATHE? • Well, the answer to this question is not quite so direct. Yes, plants require O2 for respiration to occur and they also give out CO2. • Plants, unlike animals, have no specialised organs for gaseous exchange but they have stomata and lenticels for this purpose. • There are several reasons why plants can get along without respiratory organs. • First, each plant part takes care of its own gas-exchange needs. • There is very little transport of gases from one plant part to another. • Second, plants do not present great demands for gas exchange. • Roots, stems and leaves respire at rates far lower than animals do. • Third, the distance that gases must diffuse even in large, bulky plants is not great. • Each living cell in a plant is located quite close to the surface of the plant.
2 Respiration in Plants • ‘This is true for leaves’, you may ask, ‘but what about thick, woody stems and roots?’ In stems, the ‘living’ cells are organised in thin layers inside and beneath the bark. • They also have openings called lenticels. • The cells in the interior are dead and provide only mechanical support. Thus, most cells of a plant have at least a part of their surface in contact with air. This is also facilitated by the loose packing of parenchyma cells in leaves, stems and roots, which provide an interconnected network of air spaces. • 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 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. • During the process of respiration, oxygen is utilised, and carbon dioxide, water and energy are released as products. • Some organisms are facultative anaerobes, while in others the requirement for anaerobic condition is obligate. • In any case, all living organisms retain the enzymatic machinery to partially oxidise glucose without the help of oxygen. • This breakdown of glucose to pyruvic acid is called glycolysis. Type of Respiration On the basis of availability of Oxygen: • Aerobic respiration - The complete oxidation of food with the use of oxygen and when entire carbon released as CO2 . C6H12O6 + 6O2 + Enzyme ⎯⎯⎯⎯⎯→ cyto. &mito 6CO2 + 6H2O + 686 Kcal E (38 ATP) • Anaerobic respiration – This is incomplete oxidation. • When food is oxidized into alcohol or organic acids without use of oxygen. It occurs in cytoplasm and only net 2ATP are produced. C6H12O6 Enzyme ⎯⎯⎯⎯⎯→ cytoplasm 2C2H5OH + 2CO2 + 2ATP C6H12O6 Enzyme ⎯⎯⎯⎯⎯→ cytoplasm 2C3H6O3 + 2ATP • Initial steps of aerobic and anaerobic respiration are same i.e. Glucose is converted to pyruvic acid. Glycolysis • The term glycolysis has originated from the Greek words, glycos for sugar, and lysis for splitting. • Glycolysis was discovered by Embden, Meyerhoff and Parnas and hence it is called as EMP pathway.
Respiration in Plants 3 • Glycolysis is independent of O2 . • Glycolysis is completed in cytosol (cytoplasm). • In anaerobic organisms, it is the only process in respiration. • In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid. • In plants, this glucose is derived from sucrose or from storage carbohydrates. • Sucrose is converted into glucose and fructose by the enzyme, invertase, and these two monosaccharides readily enter the glycolytic pathway. • Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate by the activity of the enzyme hexokinase. • This phosphorylated form (glucose-6- phosphate) of glucose then isomerises to produce fructose- 6- phosphate. • Subsequent steps of metabolism of glucose and fructose are same. • In glycolysis, a chain of ten reactions, under the control of different enzymes, takes place to produce pyruvate from glucose. • ATP is utilised at two steps: first in the conversion of glucose into glucose 6-phosphate and second in the conversion of fructose 6-phosphate to fructose 1, 6-bisphosphate. • The fructose 1, 6-bisphosphate is split into dihydroxyacetone phosphate and 3- phosphoglyceraldehyde (PGAL). • There is one step where NADH + H+ is formed from NAD+; this is when 3-phosphoglyceraldehyde (PGAL) is converted to 1, 3-bisphosphoglycerate (BPGA). • Two redox-equivalents are removed (in the form of two hydrogen atoms) from PGAL and transferred to a molecule of NAD+. PGAL is oxidised and with inorganic phosphate to get converted into BPGA. • The conversion of BPGA to 3-phosphoglyceric acid (PGA), is also an energy yielding process; this energy is trapped by the formation of ATP. • Another ATP is synthesised during the conversion of PEP to pyruvic acid. • Pyruvic acid is the key product of glycolysis. • Mainly Glucose is substrate of glycolysis. • The glycolysis is common Pathway for aerobic & anaerobic respirations both. • In glycolysis, neither consumption of oxygen nor liberation of CO2 take place. • Substrate level phosphorylation forms total 4 ATP and net 2 ATP - [When the substrate releases energy for phosphorylation of ADP or formation of ATP, without ETS then called as substrate level phosphorylation or direct synthesis of ATP. • 1 st & 3rd and last reaction of glycolysis are irreversible reactions. • Further oxidation of puruvic acid and NADH2 after glycolysis in mitochondria requires oxygen. • There are three major ways in which different cells handle pyruvic acid produced by glycolysis. These are lactic acid fermentation, alcoholic fermentation and aerobic respiration. • Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes.