124
UNIT ONE
Glucose can also react to form fat molecules, later depos-
ited in adipose tissue. This happens when a person takes in
more carbohydrates than can be stored as glycogen or are
required for normal activities. The body has an almost unlim-
ited capacity to perform this type of anabolism, so overeating
carbohydrates can cause accumulation of body fat.
This section has considered the metabolism of glucose,
although lipids and proteins can also be broken down to
release energy for ATP synthesis. In all three cases, the
F
nal process is aerobic respiration, and the most common
entry point is into the citric acid cycle as acetyl CoA
(f
g.
4.15)
. These pathways are described in detail in chapter 18
(pp. 702–704).
PRACTICE
17
State the products of the aerobic reactions.
18
List the products of the citric acid cycle.
19
Explain the function of the electron transport chain.
20
Discuss fates of glucose other than cellular respiration.
4.6
NUCLEIC ACIDS AND
PROTEIN SYNTHESIS
Enzymes control the metabolic pathways that enable cells to
survive, so cells must have information for producing these
specialized proteins. Many other proteins are important in
physiology as well, such as blood proteins, the proteins that
form muscle and connective tissues, and the antibodies that
protect against infection. The information that instructs a cell
to synthesize a particular protein is held in the sequence of
building blocks of
deoxyribonucleic acid (DNA),
the genetic
material. As we will see later in this chapter, the correspon-
dence between a unit of DNA information and a particular
amino acid constitutes the
genetic code
(je
˘-net
ik ko
¯d).
Figure 4.13
summarizes the steps in glucose metabo-
lism. More detailed descriptions of the reactions of cellular
respiration are in Appendix C, pages 944–947.
Cyanide is a deadly poison that halts ATP production in cells. It
binds to an iron atom that is part of the enzyme that enables NADH
from the citric acid cycle to transfer electrons to oxygen. Cyanide is
absorbed through the skin, gastrointestinal tract, and respiratory
tract, and exposure can kill in minutes. One source of cyanide is bit-
ter almonds (not the sweet type that people prefer), which produce
a compound called amygdalin that an enzyme in the human small
intestine breaks down, releasing the poison. Cyanide is encountered
in certain industrial processes, including metal plating, gold extrac-
tion, and in the raw materials for plastics. Rat poison and fumigants
also contain cyanide.
Carbohydrate Storage
Metabolic pathways are usually interconnected in ways that
enable certain molecules to enter more than one pathway.
±or example, carbohydrate molecules from foods may enter
catabolic pathways and be used to supply energy, or they
may enter anabolic pathways and be stored or react to form
some of the twenty different amino acids
(f
g. 4.14)
.
Excess glucose in cells may enter anabolic carbohy-
drate pathways and be linked into storage forms such as
glycogen. Most cells can produce glycogen; liver and mus-
cle cells store the greatest amounts. ±ollowing a meal,
when blood glucose concentration is relatively high, liver
cells obtain glucose from the blood and synthesize glyco-
gen. Between meals, when blood glucose concentration is
lower, the reaction reverses, and glucose is released into
the blood. This mechanism ensures that cells throughout
the body have a continual supply of glucose to support cel-
lular respiration.
ATP
ADP +
ATP synthase
Electron transport chain
Energy
P
2H
+
+ 2e
2e
2H
+
NADH + H
+
NAD
+
2H
+
+ 2e
FADH
2
FAD
1
/
2
O
2
H
2
O
Energy
Energy
FIGURE 4.12
A summary of ATP synthesis by oxidative phosphorylation.
previous page 154 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 156 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off