Nov. 16, 2019

Glycolysis & fermentation:



               The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the EMP pathway. In anaerobic organisms, it is the only process in respiration.

              Glycolysis occurs in the cytoplasm of the cell and is present in all living organisms. In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid.

              In plants, this glucose is derived from sucrose, which is the 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.

               Glycolysis literally means "Splitting sugars". Glucose, a six carbon sugar, is split into two molecules of a three carbon sugar. In the process of glycolysis two molecules of ATP and two "high energy" electron carrying molecules are produced. Common phase in both aerobic and anaerobic respiration is glycolysis.

              Glyceraldehyde-3 phosphate is phosphorylated and also biologically oxidized to 1, 3-diphosphoglyceric acid. This first biological oxidation reaction is catalyzed by glyceraldehydes-3-phosphate NAD oxidoreductase. NAD+ is the universal hydrogen acceptor. Mg2+ ions act as cofactors for phosphotransferases and enolase. In all there are four phosphorylations during glycolysis. Two phosphate groups are supplied by ATP and two phosphoric acid. No. of ATP used for phosphorylation of glucose in glycolysis is 2 (1st and 3rd reactions). No. of ATP formed during glycolysis is 4. Net gain of ATP during glycolysis is 2(4-2=2). IF fructose-6-phosphate is oxidized through glycolysis net gain of ATP is 3(4-1=3). Summary equation of glycolysis is :

   C6H12O6 + 2ADP + 2iP + 2NAD+                  2CH3COCOOH + 2ATP + 2NADH + 2H+

  (Glucose)                                                         (Pyruvic acid)

               Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate by the activity of the enzyme hexokinase. This phosphorylated form of glucose then isomerizes to produce fructose-6-phosphate. Subsequent steps of metabolism of glucose and fructose are same. In glycolysis, a chain of ten reactions, under control of different enzymes, takes place to produce pyruvate from glucose.


               Fermentation of sugars takes place by some micro-organisms. This was first stated by Louis Pasteur. In micro-organisms, the term anaerobic respiration is replaced by fermentation. This term was coined by Cruickshank (1897). Depending upon the end product the fermentation is of various typesvizalcoholic fermentation, lactic acid fermentation, citric aced fermentation etc. Alcoholic fermentation is brought about by yeast cells Saccharomyces sp. Enzymes useful for alcoholic fermentation is zymase. Saccharomyces cerevisiae is Brewer's yeast. Wing yeast is Saccharomyces ellipsodiens which perform alcoholic fermentation.

   C6H12O62C O2 +                 2C2H5OH + 17.8 Kcal + 56/22Kcal

               In fermentation, say by yeast, the incomplete oxidation of glucose is achieved under anaerobic conditions by sets of reactions where pyruvic acid is converted to CO2 and ethanol. The enzymes, pyruvic acid decarboxylase and alcohol dehydrogenase catalyse these reactions.

               Other organisms like some bacteria produce lactic acid from pyruvic acid. In animal cells also, like muscles during exercise, when oxygen is inadequate for cellular respiration pyruvic acid is reduced  to lactic acid by lactate dehydrogenase. The reducing agent is NADH + H+ which is reoxidised to NAD+ in both the processes.