PinkMonkey Online Study Guide-Biology
The initial steps in glycolysis involve the phosphorylation
of glucose into an unstable fructose 1, 6-diphosphate for easy breakdown.
This is called the preparatory phase of glycolysis during
which 2 ATP are used. The reactions are as follows.
(1) In the first step, 6-C glucose is converted
into glucose-6-phosphate. One ATP is used in the reaction.
(2) The next step involves isomerization of
glucose 6-phosphate into Fructose 6-phosphate.
(3) Fructose-6-phosphate is then phosphorylated
to fructose-1, 6-diphosphate. This reaction utilizes one ATP.
Thus, two ATP are used up in the preparatory phase.
(B) Cleavage of fructose-1, 6 - diphosphate
(4) The molecule of 6-C fructose-1, 6-diphosphate
undergoes cleavage to produce two molecules of 3-C triose phosphates.
One is 3-phosphoglyceraldehyde (3-PGAL) and the other is dihydroxy
acetone phosphate (DHAP).
The two triose phosphates can isomerize into
each other. However, further reactions in glycolysis utilize only
PGAL. Therefore, DHAP is first converted into PGAL.
Thus, in all, two molecules of 3-PGAL are formed
from the cleavage of one fructose-1, 6- diphosphate.
(C) Formation of 3-C pyruvic acid
Further reactions in glycolysis result in the conversion
of 3-C PGAL into 3-C pyruvic acid through the following intermediate
(5) 3-PGAL is converted into
1, 3 diphosphoglyceric acid (1, 3 diPGA). This is an oxidation and phosphorylation
reaction. It takes place in the presence of inorganic phosphate (P.)
and co-enzyme NAD (nicotinamide adenine dinucleotide). The reduced co-enzyme
NADH2 is formed in the process.
(6) In the next step, 1, 3-diPGA undergoes dephosphorylation
to form 3-phosphoglyceric acid (3-PGA). Phosphate removed from each diPGA
is transferred to by ADP to form ATP. Two ATP molecules are formed at
(7) 3-PGA is transformed into 2-PGA.
(8) In the next step, 2-PGA is converted into 2-phosphoenol
pyruvic acid (2-PEP) with the loss of water (dehydration).
(9) In the final step of glycolysis, 2-PEP is dephosphorylated
to form 3-C pyruvic acid, which is the end product of glycolysis. The
reaction produces 2 ATP.
Thus, when a molecule of 6-C glucose undergoes the reactions
in glycolysis (EMP pathway), the overall process may be represented as
However, 2#ATP are used up in the preparatory phase of
glycolysis. Therefore the net gains of glycolysis are:
Phase-II of Aerobic Respiration
Phase-II of aerobic respiration comprises a series of
enzymic reactions in which 3-c pyruvic acid (the end product of glycolysis)
is broken down step-wise to form the end products of aerobic respiration,
(i.e. CO2 , H2 O and ATP energy), inside the mitochondria.
Phase-II involves the following two main sets of reactions:
(1) The T.C.A. cycle
(2) Terminal oxidation and oxidative phosphorylation (E.T.S. and the redox reactions).