10
U N I T 1
Cell and Tissue Function
Cell metabolism is the process that converts dietary fuels from carbohydrates,
proteins, and fats into adenosine triphosphate (ATP), which provides energy
for the cell. Adenosine triphosphate is formed through three major pathways:
(1) the glycolytic pathway, (2) the citric acid cycle, and (3) the electron transport
chain. In fuel metabolism, which is an oxidation–reduction reaction, the fuel
donates electrons and is oxidized, and the coenzymes nicotinamide adenine
dinucleotide (NAD) and flavin adenine dinucleotide (FAD) accept electrons and
are reduced.
U N D E R S T A N D I N G
Cell Metabolism
Glycolytic Pathway.
Glycolysis,
which occurs in the cytoplasm of the
cell, involves the splitting of the six-
carbon glucose molecule into two
three-carbon molecules of pyruvic
acid. Because the reaction that splits
glucose requires two molecules of
ATP, there is a net gain of only two
molecules of ATP from each mol-
ecule of glucose that is metabolized.
The process is anaerobic and does
not require oxygen (O
2
) or produce
carbon dioxide (CO
2
). When O
2
is
present, pyruvic acid moves into
the mitochondria, where it enters
the aerobic citric acid cycle. Under
anaerobic conditions, pyruvate is
converted to lactic acid, allowing
glycolysis to continue as a means of
supplying cells with ATP when O
2
is
lacking.
1
NADH + 2H
+
C
C
C C C
C
C C
C
O
1 Glucose
2
2 NAD
+
2
2
ATP
ATP
ADP P
P
+
C C C P
P
4 ADP + 4
4
2
Pyruvic acid
(aerobic)
Lactic acid
(anaerobic)
NADH
+ H
+
NADH
+ H
+
NADH
+ H
+
FADH
2
ATP
Pyruvic acid
Acetyl-coenzyme A
To electron
transport
chain
To electron
transport
chain
Citric acid
cycle
CoA
CO
2
CO
2
Citric Acid Cycle.
Under aerobic
conditions, both of the pyruvic acid
molecules formed by the glycolytic
pathway enter the mitochondria,
where each combines with acetyl-
coenzyme to form acetyl-coenzyme
A (acetyl-CoA). The formation of
acetyl-CoA begins the reactions that
occur in the citric acid cycle. Some
reactions release CO
2
and some
transfer electrons from the hydrogen
atom to NADH or FADH. In addi-
tion to pyruvic acid from the glycol-
ysis of glucose, fatty acid and amino
acid breakdown products can also
enter the citric acid cycle. Fatty acids,
which are the major source of fuel in
the body, are oxidized by a process
called
β
-
oxidation
to acetyl-CoA for
entry into the citric acid cycle.
2
