Biology 12

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.

C