Glycos = sugar : lysis = splitting of sugar
- It is a reductive process that occurs in cytosol of all living cells (prokaryotes and
- 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
- All steps of glycolysis occur in the cytoplasm of a cell.
- The first half of this pathway activates glucose.
- The second half actually extracts the energy.
- 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
- 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.
- 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
Significance of Glucolysis
It degrades glucose to generate ATP .
b) It provides building blocks for the synthesis of cellular compounds.
Summary of Glycolysis
- In this process one molecule of hexose sugar is split to form two molecules of
a 3c compound , pyruvic acid.
- 4 ATP molecules are produced and 2 molecules are consumed and therefore net gain is 2 ATP .
- 2 mol. of NADH+2H+ (reducing power) at step No. 6 when PGAL is oxidised
to 1, 3-biphosphoglyceric acid.
- 4 mol. of H2O are formed.
- 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,
So acrobic glycolysis is four times more effecient than anaerobic glycolysis