Biology (Holt)

Stage One: Breakdown of Glucose

The primary fuel for cellular respiration is glucose, which is formed
when carbohydrates such as starch and sucrose are broken down. If
too few carbohydrates are available to meet an organism’s glucose
needs, other molecules, such as fats, can be broken down to make
ATP. In fact, one gram of fat contains more energy than two grams of
carbohydrates. Proteins and nucleic acids can also be used to make
ATP, but they are usually used for building important cell parts.

Glycolysis

In the first stage of cellular respiration, glucose is bro-
ken down in the cytoplasm during a process called
(glie KAHL uh sihs). Glycolysis is an
enzyme-assisted anaerobic process that breaks down
one six-carbon molecule of glucose to two three-
carbon pyruvate ions. Recall that a molecule that has
lost or gained one or more electrons is called an ion.
Pyruvate is the ion of a three-carbon organic acid
called pyruvic acid. The pyruvate produced during gly-
colysis still contains some of the energy that was
stored in the glucose molecule.
As glucose is broken down, some of its hydrogen
atoms are transferred to an electron acceptor called
NAD. This forms an electron carrier called.
For cellular respiration to continue, the electrons car-
ried by NADH are eventually donated to other organic
compounds. This recycles NAD, making it available
to accept more electrons. Glycolysis is summarized in
Figure 11.

Step In a series of three reactions, phosphate

groups from two ATP molecules are trans-

ferred to a glucose molecule.

Step In two reactions, the resulting six-carbon com-

pound is broken down to two three-carbon

compounds, each with a phosphate group.

Step Two NADH molecules are produced, and one

more phosphate group is transferred to each

three-carbon compound.

Step In a series of four reactions, each three-carbon

compound is converted to a three-carbon
pyruvate, producing four ATP molecules in
the process.
Glycolysis uses two ATP molecules but produces four
ATP molecules, yielding a net gain of two ATP mol-
ecules. Glycolysis is followed by another set of reactions
that use the energy temporarily stored in NADH to
make more ATP.

NADH

glycolysis

CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC

CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC

CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC

CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC

CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC

BIO

graphic

BIO

graphic

O

Glucose

6 - carbon

compound

Two 3 - carbon

pyruvates

Two 3 - carbon

compounds

Two 3 - carbon

compounds

2 ADP

2 NAD+

2 NADH + 2HNADH +

2

4 ADP

1

2

3

4

Glycolysis

SECTION 3 Cellular Respiration 105

Figure 11