125
CHAPTER FOUR
Cellular Metabolism
genetic information for making a particular protein is called
a
gene
(je
¯n). Enzymes control synthesis reactions, so all four
groups of organic molecules—proteins, carbohydrates, lip-
ids, and nucleic acids—depend on proteins, and thus require
genetic instructions.
RECONNECT
To Chapter 3, Cell Nucleus, page 90.
The complete set of genetic instructions in a cell con-
stitutes the
genome.
The “F rst draft” of the human genome
sequence was announced in June 2000, following nearly F f-
teen years of discussion and work by thousands of research-
ers worldwide. Only a small part of the human genome
encodes protein. The rest includes many controls over which
proteins are produced in a particular cell under particular
circumstances, called
gene expression
. Chapter 24 (p. 917)
discusses the human genome.
Recall from chapter 2 (p. 68) that nucleotides are the
building blocks of nucleic acids. A nucleotide consists of a
5-carbon sugar (ribose or deoxyribose), a phosphate group,
and one of several nitrogenous bases
(f g. 4.16)
. DNA and
RNA nucleotides form long strands (polynucleotide chains)
by alternately joining their sugar and phosphate portions by
dehydration synthesis, which provides a “backbone” struc-
ture
(f
g. 4.17)
.
A DNA molecule consists of two polynucleotide chains,
making it double-stranded. The nitrogenous bases project
from the sugar-phosphate backbone of one strand and bind,
or pair, by hydrogen bonds to the nitrogenous bases of the
second strand
(f
g. 4.18)
. The resulting structure is some-
what like a ladder, in which the rails represent the sugar and
phosphate backbones of the two strands and the rungs rep-
resent the paired nitrogenous bases. The sugars forming the
two backbones point in opposite directions. ±or this reason,
the two strands are called
antiparallel.
Genetic Information
Children resemble their parents because of inherited traits,
but what passes from parents to a child is genetic informa-
tion, in the form of DNA molecules from the parents’ sex
cells. Chromosomes are long molecules of DNA and associ-
ated proteins. As an offspring develops, mitosis passes the
information in the DNA sequences of the chromosomes to
new cells. Genetic information “tells” cells how to construct
a great variety of protein molecules, each with a specific
function. The portion of a DNA molecule that contains the
ATP
2
ATP
2
Glucose
Pyruvic acid
Pyruvic acid
Acetyl CoA
CO
2
2 CO
2
Citric acid
Oxaloacetic
acid
1
/
2
O
2
H
2
O
2e
+ 2H
+
High energy
electrons (e
) and
hydrogen ions (H
+
)
High energy
electrons (e
) and
hydrogen ions (H
+
)
High energy
electrons (e
) and
hydrogen ions (H
+
)
Electron transport chain
ATP
32-34
Cytosol
Mitochondrion
FIGURE 4.13
An overview of aerobic respiration, including the net
yield of ATP at each step per molecule of glucose.
Hydrolysis
Carbohydrates
from foods
Monosaccharides
Catabolic
pathways
Energy + CO
2
+ H
2
O
Glycogen or Fat
Anabolic
pathways
Amino acids
FIGURE 4.14
Hydrolysis breaks down carbohydrates from foods
into monosaccharides. The resulting molecules may enter catabolic
pathways and be used as energy sources, or they may enter anabolic
pathways and be stored as glycogen or fat, or react to yield amino acids.
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