Cellular Respiration

 



Introduction:

All the cellular activities can be grouped into two   categories:

(i) Anabolism  ( biosynthetic) which is endergonic  ( energy dependent).

(ii) Catabolism ( breaking _ up) which is exergonic ( energy producing).

The sun total of anabolic and catabolic reactions occurring in a cell at any any time is called metabolism.Main energonic biological processes are active transport, muscle _ contraction ,growth, development,  movement,  cell division,  nerve _ impulse conduction,  homeothermy etc. These activities  in all living organisms are dependent on main sources  of energy  .i.e. Cellular Respiration. 




Respiration: 

It is  an exergonic and catabolic physio_ chemical  process which involves  the exchange of environmental oxygen  and body carbon dioxide  through a liquid medium  and the oxidation  of glucose inside the mitochondria to produce energy  which is stored in the high energy bonds of ATP molecules. There are two types of respiration, aerobic and anaerobic according  to their dependence  on oxygen.



(1) Aerobic Respiration:

It occurs inside the mitochondria in the presenc of molecular oxygen  in most of plants and animals, called earobes. It is divided into 4 phases: 



( a) Glycolysis or EMP__ Pathways 

(b) Oxidative decarboxylation

(c) Krebs or TCA cycle

(d) Electron transport system 

Glycolysis:

One molecule of glucose ( 6C) breaks into two molecules of pyruvic acid  (3C) 

         1 Glucose ➡️2  Pyruvic acid
           (6C)                  (3)



Properties:

(i) Occurs in cytoplasm outside the mitochondria. 

(ii) It is anaerobic phase, common in both aerobic and anaerobic respiration. 

In glycolysis ( Embden_ Meyerhof Parnas pathway), the glucose molecule is phosphorylated to phosphoric esters which then undergo break down to two molecules of triose; the trioses  finally  form pyruvate. ATP being the donor of the phosphate  while glucose  undergoing phosphorylation.In the break down of  glucose 4 ATP molecules are formed, from which two molecules are used; this results in net balance  of two ATP molecules. The formation of pyruvate from glucose or glycogen is independent of the presence  of oxygen. Anaerobically, pyruvate is converted to lactate, whereas aerobically,  it forms " active acetate" which with co_ enzyme A, forms acetyl CoA enters the Krebs cycle and is completely oxidized to carbon_ dioxide ( CO2) and water  ( H2O). All the enzymes  of Embden__ Meyerhof Parnas Pathway are localized in the " soluble " cell fraction.


Embden_ Meyerhof of pathway (glycolysis)

Oxidative decarboxylation:

Pyruvic acid and NADH2 formed anaerobically in the cytoplasm  enter inside the mitochondria where  oxidative decarboxylation occur in which pyruvic acid undergoes oxidation ( by dehydrogenation) and decarboxylation ( loss of carbon _ dioxide). It occurs in the presence  of an H_ acceptor ( NAD+), pyruvic acid dehydrogenase complex ( 5 different  coenzymes  and 3 different  enzymes) and co_ enzyme A ( CoA ~ S~ H).



Kreb's Cycle ( TCA or Citric Acid Cycle) and Oxidative phosphorylation:

Aerobic respiration is the series of reactions  by which organic substances are broken down to carbon dioxide and water in the presence of molecular oxygen. This final phase takes place  in mitochondria and is intimately connected  with their molecular structure. The key compound is the break down of carbohydrates is pyruvate which in the presence of oxygen enters the Krebs cycle.Here the combustion of pyruvate takes place, through a series of intermediates and with simultanous formation of higher energy  bonds ATP.


 The first  step of the Krebs cycle is the condensation of acetate with oxaloacetate to form citrate. Oxaloacetate formed from citrate.Oxaloacetate formed from malate, condenses with acetate  from acetyl__ Co__ enzymes  A to reform a molecule of citrate and the cycle  begins again.


As two  molecules of acetyl__ CoA are formed from one oxidised  glucose molecule,  krebs cycle must rotate  twice for each  molecule  to respire.Therefore net gain of Krebs cycle is:

2AcetylCoA➕ 8H2O➕6NAD+➕2FAD➕2GDP➕2Pi➡️4CO2➕2H2O➕6NADH➕6H+➕2FADH2➕2GTP➕2CoA


The overall reaction of glycolysis acetyl_ CoA formation and krebs cycle is :

    C6 H12 O6 + 6H2O➡️ 6CO2 + 4ATP + 12H2( 34 ATP)

Only 4 ATP molecules have been produced from one  glucose molecule in cell respiration  upto krebs cycle. Twelve molecules of hydrogen  have been used to form 10 NADH2 and 2FADH2  molecules to generate 34ATP molecules






The oxidative steps synthesizes ATP from inorganic  phosphate.One of the hydrogen reduce NAD+,and the ionized second hydrogen  loses and electron, which cycling through a flycoprotein, coenzyme Q ( uniquinone), cytochromes b, c a and a3, finally combines with molecular oxygen to form a molecule of water. Three molecules of ATP are formed from ADP and inorganic phosphate at each link of respiratory chain. Oxidation  and phosphorylation take place simultaneously,  this process is known as oxidative phosphorylation. 




Anaerobic Respiration: 

It is also called fermentation, involves the production of energy from the food nutrients in the absence of oxygen. The six carbon chain of glucose can be degraded by glycolysis into different  smaller molecules. In muscles, each molecule of glucose  is converted into two lactic acid. In yeast degradation the main products are ethanol and carbondioxide, as in the following  reaction called alcoholic fermentation:

C6 H12 O6➡️ 2C2H5OH ➕2CO2➕2ATP
                          Ethyl alcohol 





In 1861, Pasteur demonstrated that yeast can produce alcohol  in complete absence of oxygen. 

In muscle the reaction of glycogsis is:

 C6 H12 O6➡️ 2C3H6O3 ➕ kcale
                          lactic acid 


Hexose Monophosphate shunt ( or Pentose phosphate Pathway):

There is another process through  which plants oxidatise carbohydrates to obtain energy.In this hexose sugars undergo oxidative degradation  through 5_ carbon sugar intermediates. The reaction were elucidated by M. Gibbs, B, Axelrod and H. Beavers ( 1950s). It was discovered by Horecker.


The main features of HMS pathway are:

1): All reaction  takes place in cytoplasm.

2): Oxygen is required  from the beginning. 

3): Break down of glycose into CO2 and H2O, without participation of glycolysis and krebs cycle.


4): The electron acceptor  is NADP.

5): The intermediate  products are 4, 5 and 7 carbon sugar.

6): Complete oxidation  of one molecule of glucose  yields  35ATP molecule.
















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