66 MHR • Unit 1 Metabolic Processes
Glycolysis I
Glycolysis I involves a series of endothermic
reactions. In order for glycolysis to begin, activation
energy, from an ATP molecule, must be provided.
This is accomplished in the first reaction of
glycolysis by substrate-level phosphorylation, the
transfer of an inorganic phosphate group (Pi) from
one substrate to another by way of an enzyme, as
shown in Figure 3.4. This process can remove
(dephosphorylate) a phosphate from ATP or it can
add (phosphorylate) a phosphate to ADP. As shown
in Figure 3.5, one ATP is used to phosphorylate
glucose to form glucose-6-phosphate. This molecule
is then rearranged to form fructose-6-phosphate.
At this point, another ATP molecule must
phosphorylate the fructose-6-phosphate, producing
fructose-1,6-diphosphate. In turn, this molecule is
split into two PGAL(glyceraldehyde-3-phosphate).
These PGALs act as the reactants for glycolysis II.
Glycolysis II
Glycolysis II is a sequence of exothermic reactions
that provides energy for the cell. Following
glycolysis I, each PGAL is oxidized. When PGAL is
oxidized, energetic electrons move to NAD+, which
is reduced. A hydrogen ion attaches to the reduced
NAD−to form NADH. The oxidized form of PGAL
is now able to attract a free phosphate ion in the
cytosol, forming PGAP(1,3-biphosphoglycerate), as
shown in Figure 3.6. Two PGAP are produced for
each glucose molecule that enters glycolysis.
Following the formation of PGAP, two ADP
molecules (with the help of enzymes) each remove
one phosphate group from each PGAP to form PGA
(3-phosphoglycerate), as shown in Figure 3.6. Here,
substrate-level phosphorylation produces two ATP,
one for each PGAP. At this point, because two
molecules of ATP have been made, and two
molecules of ATP were used to start glycolysis, the
net change in the number of ATP molecules is zero.
Figure 3.5Glycolysis involves the formation of two PGAL
molecules.
Next, the two PGA molecules are each oxidized,
forming two water molecules and two PEP
(phosphoenolpyruvate) molecules. Finally, another
substrate-level phosphorylation occurs — two ADP
molecules each remove the remaining phosphate
group from each PEP molecule. The result is the
production of two ATP molecules and two
pyruvate molecules.
ATP
glucose-6-
phosphate
fructose-6-
phosphate
fructose-1,
6-diphosphate
glyceraldehyde-3-
phosphate (PGAL)
glyceraldehyde-3-
phosphate (PGAL)
glucose
ADP
ATP
ADP
substrate
adenosine
enzyme
P
P
P ADP
product
adenosine
P
P
ATP
P
A B
Figure 3.4Substrate-level
phosphorylation. Here, substrate
1 donates one phosphate group to
substrate 2 (ADP), making ATP. The
reverse reaction dephosphorylates
substrate 1, producing ADP.
Phosphorylation
of glucose by ATP
A
Rearrangement,
followed by a second
phosphorylation by ATP
B
The six-carbon
molecule is split
into two three-
carbon molecules.