HUMAN BIOLOGY

glucose → glucose-6-phosphate → 2 pyruvate

The first steps of glycolysis require energy. As

diagrammed in Figure A.5 on page A-4, they advance only

when two ATP molecules each transfer a phosphate group

to glucose and so donate energy to it. Such transfers, recall,

are phosphorylations. In this case, they raise the energy

content of glucose to a level that is high enough to allow the

energy-releasing steps of glycolysis to begin.

The first energy-releasing step breaks the activated glu-

cose into two molecules. Each of these molecules is called

PGAL (phosphoglyceraldehyde). Next, each PGAL is con-

verted to an unstable intermediate that allows ATP to form

by giving up a phosphate group to ADP. The next intermedi-

ate in the sequence does the same thing. Thus, a total of four

ATP form by substrate-level phosphorylation. This meta-

bolic event is the direct transfer of a phosphate group from a

substrate of a reaction to some other molecule—in this case,

ADP. Remember, though, two ATP were invested to jump-

start the reactions. So the net energy yield is only two ATP.

Meanwhile, the coenzyme NAD+ picks up electrons and

hydrogen atoms liberated from each PGAL, thus becoming

NADH. When the NADH gives up its cargo at a different

reaction site, it reverts to NAD+. Said another way, like

other coenzymes, NAD+ is reusable.

In sum, glycolysis converts energy stored in glucose

to a transportable form of energy, in ATP. NAD+ picks

up electrons and hydrogen that are removed from each

glucose molecule. The electrons and hydrogen have roles

in the next stage of reactions; so do the end products of

glycolysis—the two molecules of pyruvate.

With these changes, substrates fit precisely in the enzyme’s
active site. They now are in an activated state, in which they
will react spontaneously.

Glycolysis: the first stage

of the energy-releasing pathway

Energy that is converted into the chemical bond energy of
adenosine triphosphate—ATP—fuels cell activities. Cells
make ATP by breaking down carbohydrates (mainly glu-
cose), fats, and proteins. During the breakdown reactions,
electrons are stripped from intermediates, then energy
associated with the liberated electrons drives the formation
of ATP.
Recall that cells rely mainly on aerobic respiration, an
oxygen-dependent pathway of ATP formation. The main
energy-releasing pathways of aerobic respiration all start
with the same reactions in the cytoplasm. During this
initial stage of reactions, called glycolysis, enzymes break
apart and rearrange a glucose molecule into two molecules
of pyruvate, which has a backbone of three carbon atoms.
Following up on the discussion in Section 3.14, here you
can track in a bit more detail on what happens to a glucose
molecule in the first stage of aerobic respiration.
Glucose is one of the simple sugars. Each molecule of
glucose contains six carbon, twelve hydrogen, and six oxy-
gen atoms, all joined by covalent bonds (Figure A.4). The
carbons make up the backbone. With glycolysis, glucose
or some other carbohydrate in the cytoplasm is partially
broken down, the result being two molecules of the three-
carbon compound pyruvate:

Figure A.4 A glucose molecule has six carbon atoms,

which are colored black here. (© Cengage Learning)

GLUCOSE

Appendix i A-3

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