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  Introduction.                                                                                                                                                          

All organisms require continuous input of energy to carry on life process. These energy comes from cellular activities. All the cellular activities can be grouped into two categories : anabolism (biosynthetic activities of the cell) and catabolism (breaking- up process of the cell). The anabolic activities are endergonic (utilizes energy in cellular activities), while the catabolic activities are usually exergonic (energy releasing process by oxidation of food material). The sum of total catabolic and anabolic reactions occurring at any time in a cell is called metabolism.

Respiration is a vital process, includes the intake of oxygen. Chemically it is catabolic and brings about the oxidation and decomposition of organic compounds like carbohydrate, fat, protein in the cells of plants and animals with the release of energy. Oxidation of organic compounds by respiration, resulting in the release of chemical energies water and carbon dioxide. The overall process may be states according to the following general equation:

C6 H12O6 + 6CO2  ¾¾enz¾ym¾es ®

6CO2

• 6H₂O+ energy

glucose

carbondioxide

Water

(ATP)

In this reaction, six molecules of oxygen taken up and six molecules each of CO₂ and H₂O are formed with energy derived from respiration of each molecule of sugar oxidation. The plant cell is able to do chemical work in synthesizing energy- rich materials such as fat and hydrocarbon, osmotic work such as uptake and accumulation of salt and mechanical work such as involved in growth.

Respiration

Definition of respiration : Cellular respiration is an enzyme controlled process of biological oxidation of food materials in a living cell, using molecular O₂, producing CO₂ and H₂O, and releasing energy in small steps and storing it in biologically useful forms, generally ATP.

1. Use of energy : Cellular activities like active transport, muscle-contraction, bioluminescenes, homothermy locomotion, nerve impulse conduction, cell division, growth, development, seed germination require energy. Main source of energy for these endergonic activities in all living organisms including plants, comes from the

oxidation of organic molecules.

Reactions releasing energy

Inorganic

ATP

Reactions consuming energy

Synthesis of proteins, lipids, carbohydrates

Glucose Lipids Proteins

phosphate

P

ADP

Osmotic work

Growth, differentiation and development Active absorption

Cyclosis Translocation

Fig. ATP cycle : ATP is an intermediate energy-transfer compound between energy-releasing and energy consuming reactions

The energy released by oxidation of organic molecules is actually transferred to the high energy terminal bonds of ATP, a form that can be readily utilized by the cell to do work. Once ATP is formed, its energy may be utilized at various places in the cell to drive energy- requiring reactions. In these processes, one of the three phosphate groups is removed from the ATP molecule. Thus the role of ATP as an intermediate energy transforming compound between energy releasing and energy consuming reactions.

2. Significance of respiration : Respiration plays a significant role in the life of plants. The important ones are given below :

1. It releases energy, which is consumed in various metabolic process necessary for life of plant.
2. Energy produced can be regulated according to requirement of all activities.
3. It convert in soluble foods into soluble form.
4. Intermediate products of cell respiration can be used in different metabolic pathways e.g.

Acetyl- CoA (in the formation of fatty acid, cutin and isoprenoids) ; a - ketoglutaric acid (in the formation of glutamic acid) ; Oxaloacetic acid (in the formation of aspartic acid, pyrimidines and alkaloids); Succinyl- CoA (synthesis of pyrrole compounds of chlorophyll).

5. It liberates carbon dioxide, which is used in photosynthesis.
6. Krebs cycle is a common pathway of oxidative breakdown of carbohydrates, fatty acids and amino acids.
7. It activates the different meristematic tissue of the plant.

3. Comparison between respiration and photosynthesis : Photosynthesis associated with manufacturing of food, while respiration associated with releasing of energy from this food. Comparison between respiration and photosynthesis is given below :

4. Exchange of gases in photosynthesis and respiration : Respiration is continually going on in all living cells and oxygen is being continually absorbed and carbon dioxide liberate. The intake of oxygen (Liberated by photosynthesis) and liberation of carbon dioxide (evolved in respiration) takes place through the stomata and lenticels. The real process of respiration consists in the oxidation of organic substances which takes place in the protoplasm of the living cells and the gaseous exchange is an outward manifestation and an accompaniment of respiration. The intensity of gaseous exchange depends upon the intensity of respiration. It is comparatively rapid in meristematic and growing tissues where the formation of new cells and cell wall material requires a large supply of energy and is comparatively slow in mature cells due to the slowness of metabolic activities.

Fig. Showing gas exchange due to photosynthesis and respiration

Compensation  point  : It is that value or point in light intensity and atmospheric CO₂ concentration when rate of photosynthesis is just equivalent to the rate of respiration in photosynthetic organs so that there is no net

gaseous exchange. The value is 2.5- 100 ft candles/ 26.91-1076.4 lux in shade plants and 100-400 ft candles/ 1076.4-4305.6 lux in case of  sun plants. It is called light compensation point. There is, similarly, a

CO₂ compensation point. Its value is 25-100 ppm (25-100 ml.l^-1 ) in C₃ plants and 0-5 ppm (0-5 ml.l ^-1 ) in C₄

plants. A plant cannot survive for a long at compensation point because the nonphotosynthetic parts and dark respiration will deplete organic reserve of the plant.

CO₂ intake in photosynthesis balanced with CO₂ release in respiration = Compensation point.

5. Comparison between respiration and combustion : According Lavosier cell respiration resembles the combustion (e.g., burning of coal, wood, oil etc.) in the breakdown of complex organic compounds in the presence of oxygen and production of carbon dioxide and energy, but there are certain fundamental differences between the two processes:

Differences between cell respiration and combustion

 Phases of respiration.                                                                                                                                         

There are three phases of respiration :

1. External respiration : It is the exchange of respiratory gases (O₂ and CO₂) between an organism and its environment.
2. Internal or Tissue respiration : Exchange of respiratory gases between tissue and extra cellular environment .

Both the exchange of gases occur on the principle of diffusion.

3. Cellular respiration : It is an enzymatically-controlled stepped chemical process in which glucose is oxidised inside the mitochondria to produce energy-rich ATP molecules with high-energy bonds.

So, respiration is a biochemical process.

 Respiratory substrate or Fuel.                                                                                                                          

In respiration many types of high energy compounds are oxidised. These are called respiratory substrate or respiratory fuel and may include carbohydrates, fats and protein.

1. Carbohydrate : Carbohydrates such as glucose, fructose (hexoses), sucrose (disaccharide) or starch, insulin, hemicellulose (polysaccharide) etc; are the main substrates. Glucose are the first energy rich compounds to be oxidised during respiration. Brain cells of mammals utilized only glucose as respiratory substrate. Complex carbohydrates are hydrolysed into hexose sugars before being utilized as respiratory substrates. The energy present in one gram carbohydrate is – 4.4 Kcal or 18.4 kJ.
2. Fats : Under certain conditions (mainly when carbohydrate reserves have been exhausted) fats are also oxidised. Fat are used as respiratory substrate after their hydrolysis to fatty acids and glycerol by lipase and their subsequent conversion to hexose sugars. The energy present in one gram of fats is 9.8 Kcal or 41kJ, which is maximum as compared to another substrate.

The respiration using carbohydrate and fat as respiratory substrate, called floating respiration (Blackmann).

3. Protein : In the absence of carbohydrate and fats , protein also serves as respiratory substrate. The energy present in one gram of protein is : 4.8 Kcal or 20 kJ. when protein are used as respiratory substrate respiration is called protoplasmic respiration.

 Types of respiratory organism.                                                                                                                        

Organism can be grouped into following four classes on the basis of their respiratory habit -

1. Obligate aerobes : These organisms can respire only in the presence of oxygen. Thus oxygen is essential for their survival.
2. Facultative anaerobes : Such organisms usually respire aerobically (i.e., in the presence of oxygen) but under certain condition may also respire anaerobically (e.g., Yeast, parasites of the alimentary canal).
3. Obligate anaerobes : These organism normally respire anaerobically which is their major ATP- yielding process. Such organisms are in fact killed in the presence of substantial amounts of oxygen (e.g., Clostridium botulinum and C. tetani).
4. Facultative aerobes : These are primarily anaerobic organisms but under certain condition may also respire aerobically.

 Types of respiration.                                                                                                                                           

On the basis of the availability of oxygen and the complete or incomplete oxidation of respiratory substrate, the respiration may be either of the following two types : Aerobic respiration and Anaerobic respiration

Aerobic respiration

It uses oxygen and completely oxidises the organic food mainly carbohydrate (Sugars) to carbon dioxide and water. It therefore, releases the entire energy available in glucose.

C6 H12O6 + 6O2  ¾¾^enz¾^ym¾^es ® 6CO2 + 6H2O + energy (686 Kcal)

It is divided into two phases : Glycolysis, Aerobic oxidation of pyruvic acid

Glycolysis / EMP pathway

1. Discovery : It is given by Embden, Meyerhoff and Parnas in 1930. It is the first stage of breakdown of glucose in the cell.
2. Definition : Glycolysis ( Gr. glykys= sweet, sugar; lysis= breaking) is a stepped process by which one molecule of glucose (6c) breaks into two molecules of pyruvic acid (3c).
3. Site of occurrence : Glycolysis takes place in the cytoplasm and does not use oxygen. Thus, it is an anaerobic pathway. In fact, it occurs in both aerobic and anaerobic respiration.
4. Inter conversions of sugars : Different forms of carbohydrate before entering in glycolysis converted into simplest form like glucose, glucose 6-phosphate or fructose 6-phosphate. Then these sugars are metabolized into the glycolysis. The flow chart that showing inter conversion of sugar are given below :

Starch      UDPG                    Sucrose

+ UDP

Mannose

Glucose

Fructose

Starch                   Galactose

+H₃PO₄

+ATP

+ATP

hexokinase

+ATP

hexokinase

+ATP

hexokinase

Phosphorylase

Glucose

hexokinase

Galactose

1-phosphate

6-phosphate

Mannose 6-phosphate

Glucose

6-phosphate

Isomerase

Fructose

6-phosphate

Isomerase

+ATP

Phosphoglucomutase

Glucose

1. phosphate

Phosphohexokinase

To glycolysis

Fig : Schematic conversion of complex carbohydrates before entering into glycolysis

(5)

(– 1 ATP)                                                             1. Phosphorylation

Phosphoglucoisomerase

2. Isomerisation

First phase : Phosphorylation of glucose and its conversion into glyceraldehyde

3-phosphate

Phosphofructokinase

(– 1 ATP)

3. Phosphorylation

Fructose diphosphate aldolase

4. Cleavage

2NAD

2NADH+2H⁺

Glyceraldehyde phosphate dehydrogenase

5. Phosphorylation and Dehydrogenation

Second phase : Conversion of glyceraldehyde

3-phosphate into pyruvate and couple formation of ATP

2ADP

2ATP

Phosphoglycerate kinase

Enolase

(+2ATP)

6. Dephosph- orylation

7. Rearrangement

8. Dehydration

2ADP

2ATP

Pyruvate kinase

(+2 ATP)

9. Dephosphorylation Net gain = 2 ATP

(6)Enzymes of glycolysis and their co-factors

7. Steps of glycolysis : Glycolysis consists of 9 steps. Each step is catalysed by a specific enzyme. Most of the reaction are reversible.

1. First phosphorylation : The third phosphate group separates from adenosine triphospate (ATP) molecule, converting the latter into adenosine diphophate (ADP) and releasing energy. With this energy, the phosphate group combines with glucose to form glucose 6-phosphate, The reaction is catalysed by the enzyme, hexokinase or glucokinase in the presence of Mg²⁺. Thus, a molecule of ATP is consumed in this step. This glucose 6-phosphate (phosphoglucose) is called active glucose.

Glucose + ATP ¾¾^Hex¾^okin¾^a¾^se ®Glucose 6 - phosphate + ADP

Mg ++

2. Isomerisation : Glucose 6-phophate is changed into its isomer fructose 6-phophate by rearrangement. The rearrangement is catalysed by an enzyme, phophoglucose-isomerase or phosphohexose isomerase.

Glucose 6-phosphate

Phosphogluco isomerase

Fructose 6-phosphate

Fructose 6-phosphate may be formed directly from free fructose by its phosphorylation in the presence of an enzyme fructokinase, Mg ²⁺ and ATP

Fructose + ATP ¾¾^Fruc¾^tok¾^ina¾^se ®Fructose 6 - phosphate + ADP

Mg 2+

3. Second phosphorylation : Fructose 6-phosphate combines with another phosphate group from another ATP molecule, yielding fructose 1, 6-diphosphate and ADP , The combination is catalysed by an enzyme

phosphofructokinase in the presence of Mg²⁺ and appears to be irreversible. This phosphorylation, thus, consume another molecule of ATP. Excess of ATP inhibits phosphofructokinase.

Fructose 6 - phosphate + ATP ¾¾^Pho¾^sph¾^ofru¾^cto¾^- ®Fructose 1,6 - diphosphate + ADP

kinase, Mg ²⁺

phosphorylation reaction activate the sugar and prevent its excape from the cell. They go uphill, increasing the energy content of the products.

4. Cleavage : Fructose 1,6-diphosphate now splits into two 3-carbon, phosphorylated sugars : dihydroxyacetone phosphate (DHAP) and 3-phosphoglyceraldehyde (3-PGAL), or glyceraldehyde 3-phosphate (GAP). The reaction is catalyzed by an enzyme aldolase. DHAP is converted into PGAL with the aid of an enzyme phosphotriose isomerase.

Aldolase

Fructose 1,6-diphosphate                      3-phophoglyceraldehyde+Dihydroxyacetone phosphate

Dihydroxyacetone phosphate

Phosphotriose isomerase

3- phosphoglyceraldehyde

5. Phosphorylation and Oxidative dehydrogenation: In phosphorylation, 3-phosphoglyceraldehyde combines with a phosphate group derived from inorganic phosphoric acid (H₃PO₄) found in cytosol, not form ATP, forming1, 3-diposphoglycerate, or diphosphoglyceric acid. The reaction occurs with the aid of a specific enzyme.

1. In dehydrogenation, a pair of hydrogen atom separate from a molecule of 3-phosphoglyceraldehyde. Their separation releases a large amount of energy. A part of this energy is stored in newly formed phosphate bond of 1,3-diphosphoglycerate, making it a high energy bond. Separation of hydrogen is catalysed by an enzyme, 3- phosphoglyceraldehyde dehydrogenase.
2. As stated above, two hydrogen (H) atoms (2 proton and 2 electrons) separate from 3- phosphoglyceraldehyde. Of these, one complete hydrogen atom (proton and electron) and one additional electron are picked up by NAD⁺ which gets reduced to NADH. The remaining one hydrogen proton or ion (H⁺) remains free in the cytosol.

2H ⁺ + 2e ^- + NAD ⁺ ® NADH + H ⁺

NADH is a high-energy substance, carrying the rest of the energy released by separation of hydrogen atoms from 3- PGAL. Energy is actually released by transfer of electrons from 3-PGAL to NAD. The NADH provides energy to convert ADP to ATP by passing its electrons over the electron transmitter system if oxygen is available.

The overall reaction is as under –

3 - PGAL + NAD⁺ + Pi 2-  ¾¾^3-P¾^hos¾^pho¾^glyc¾^e¾^r- ®1,3 - diphosphoglycerate + NADH + H ⁺

aldehyde dehydrogenase

6. Dephosphorylation or ATP generation (First) : High-energy phosphate group on carbon 1 of 1,3 diphosphoglycerate is transferred to a molecule of ADP, converting it into an ATP molecule. 1, 3- diphosphoglycerate changes to 3-phosphoglycerate due to loss of a phosphate group. The reaction is catalysed by an enzyme diphosphoglycerokinase. Formation of ATP directly from metabolites is known as substrate level phophorylation.

Diphosphoglycero-

1, 3-diphosphoglycerate +ADP                        3-phosphoglycerate + ATP

kinase + Mg ²⁺

7. Isomerisation/  Rearrangement  :  The phosphate group on the third carbon of 3-phosphoglycerate shifts to the second carbon, producin