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CBSE Biology Class XI

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  • The breaking of C-C bonds of complex compounds through oxidation within the cells, leading to release of considerable amount of energy is called respiration.
  • The compound that oxidized during this process is known as respiratory substrates.
  • In the process of respiration the energy is released in a series of slow step-wise reactions controlled by enzymes and is trapped in the form of ATP.
  • ATP acts as the energy currency of the cell.

Glycolysis : 

  • The term has originated from the Greek word, glycos =glucose, lysis = splitting or breakdown means breakdown of glucose molecule.
  • It is also called Embeden-Meyerhof-Paranus pathway. (EMP pathway)
  • It is common in both aerobic and anaerobic respiration.
  • It takes place outside the mitochondria, in the cytoplasm.
  • One molecule of glucose (Hexose sugar) ultimately produces two molecules of pyruvic acid through glycolysis.
  • Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate, catalyzed by hexokinase.
  • This phosphorylated form of glucose is then isomerizes to produce fructose-6-phosphate.
  • ATP utilized at two steps:
    • First in the conversion of glucose into glucose-6-phosphate
    • Second in fructose-6-phosphate→fructose 1, 6-diphosphate.

  • The fructose-1, 6-diphosphate is split into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (DPGA).
  • In one step where NADH + H+ is formed form NAD+; this is when 3-phosphogleceraldehyde (PGAL) is converted into 1, 3-bisphophoglyceric acid (DPGA).
  • The conversion of 1, 3-bisphophoglyceric acid into 3-phosphoglyceric acid is also an energy yielding process; this energy is trapped by the formation of ATP.
  • Another ATP synthesized when phosphoenolpyruvate is converted into pyruvic acid.
  • During this process 4 molecules of ATP are produced while 2 molecules of ATP are utilized. Thus net gain of ATP is of 2 molecules.


  • There are three major ways in which different cells handle pyruvic acid produced by glycolysis:
    • Lactic acid fermentation.
    • Alcoholic fermentation.
    • Aerobic respiration.

  • Alcoholic fermentation :
    • The incomplete oxidation of glucose to achieved under anaerobic conditions by sets of reactions where pyruvic acid is converted into CO2 and ethanol.
    • The enzyme pyruvic acid decarboxylase and alcohol dehydrogenase catalyze these reactions.
    • NADH + H+ is reoxidised into NAD+.

  • Lactic acid fermentation:
    • Pyruvic acid converted into lactic acid.
    • It takes place in the muscle in anaerobic conditions.
    • The reaction catalysed by lactate dehydrogenase.
    • NADH + H+ is reoxidised into NAD+.

  • Aerobic respiration:
    • Pyruvic acid enters into the mitochondria.
    • Complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of CO2.
    • The passing on the electrons removed as part of the hydrogen atoms to molecular oxygen (O2) with simultaneous synthesis of ATP.


  • The overall mechanism of aerobic respiration can be studied under the following steps :
  • Glycolysis (EMP pathway)
  • Oxidative Decarboxylation
  • Krebs’s cycle (TCA-cycle)
  • Oxidative phosphorylation


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CBSE Biology (Chapter Wise) Class XI ( By Mr. Hare Krushna Giri ) 
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