Glycos = sugar : lysis = splitting of sugar

  • It is a reductive process that occurs in cytosol of all living cells (prokaryotes and eukaryotes).
  • Common in both aerobic and anaerobic respiration.
  • Basically it is an anaerobic pathway.
  • It is an enzymatic, 10 stepped reductive process . Respiratory substrates are reduced to form Pyruvic Acid with the production of ATP and (NADH2 ).
  • Glycolysis yields only 5% of total ATP in respiration .
  • Anaerobic glycolysis was the first process to occur during the origin of life.
  • All the reaction of glycolysis are readily reversible except (step No. 1) for the one catalysed by hexokinase.
  • Nearly all glycolytic enzymes require Mg ++ as co-factor for their activity.
  • There is no decarboxylation in Glucolysis.

Mechanism of Glycolysis

  1. All steps of glycolysis occur in the cytoplasm of a cell.
  2. The first half of this pathway activates glucose.
  3. The second half actually extracts the energy.

Glucose activation

  • First step of glycolysis is phosphorylation of hexose sugar in which glucose / fructose is phosphorylated at 1 and 3 by using two ATP to form Fructose 1, 6 biphosphate.
  • This compound is split in step 4 by aldolase forming two compounds of 3-C
  • Dihydroxyacelone phosphate (DHAP) are tautomers and exist in dynamic equilibrium. DHAP is isomerised into PGAL.
  • d) This PGAL acts as connecting link between respiration and photosynthesis.

Energy Extraction

  • Each molecule of PGAL is oxidised into 1, 3-Biphosphoglyceric Acid by utilising h4PO4 and H2O .
  • In glycolysis oxidation of PGAL occurs by removal of 2 Hydrogen atoms .
  • No free oxygen is used.
  • These two hydrogen atoms dissociate into 2 protons and 2 electrons.
  • Of these 2 H atoms , one complete hydrogen atom and one extra electron of another H atom is picked up by NAD+ and get reduced to form NADH, remaining (H+) protons free in cytoplasm. 2H —–> 2H+ + 2e- NAD+ + 2H+ + 2e- —-> NADH + H+
  • This NADH is a high energy substance and called REDUCING POWER.
  • Dephosphorylation (step 7) to form 3-phosphoglyceric acid (PGA) in presence of phosphoglyceric kinase and cofactors Mg++
  • ATP is produced at step 7 by substrate phosphorylation i.e. direct synthesis of ATP without ETS.
  • In step No. 8 intramolecular arrangement of phosphate group takes place.
  • 2 PGA undergoes dehydration in presence of enolase and cofactor Mg++ .
  • Finally PEP forms pyruvic acid after donating its phosphate group to ADP to form ATP.

Significance of Glucolysis

It degrades glucose to generate ATP . b) It provides building blocks for the synthesis of cellular compounds.

Summary of Glycolysis

  1. In this process one molecule of hexose sugar is split to form two molecules of a 3c compound , pyruvic acid.
  2. 4 ATP molecules are produced and 2 molecules are consumed and therefore net gain is 2 ATP .
  3. 2 mol. of NADH+2H+ (reducing power) at step No. 6 when PGAL is oxidised to 1, 3-biphosphoglyceric acid.
  4. 4 mol. of H2O are formed.
  5. If oxygen is available, these 2 mol. of NADH+2H+ enter the mitochondria and oxidised through ETS to form 6 ATP . Thus aerobic glucolysis can produce, 2+6=8 ATP.

So acrobic glycolysis is four times more effecient than anaerobic glycolysis