Biology (Holt)

SECTION 3 Chemistry of Cells 37

P

P P P

P

P

P

P

P

P

P

P

P

P

P

P

P

P P

P

P

P

Phosphate

group

P

Base

Sugar

Nucleotide

DNA is made of two strands of multiple, linked nucleotides.

Figure 11 Structure of nucleic acids

Nucleic Acids
All of your cells contain nucleic acids. A is a long chain
of smaller molecules called nucleotides. A has three
parts: a sugar, a base, and a phosphate group, which contains phos-
phorus and oxygen atoms. There are two types of nucleic acids—
DNA and RNA—and each type contains four kinds of nucleotides.
, or deoxyribonucleic acid, consists of two strands of
nucleotides that spiral around each other, as shown in Figure 11.
Chromosomes contain long strands of DNA, which stores heredi-
tary information.
,or ribonucleic acid, consists of a single strand of nucleo-
tides. RNA plays many key roles in the manufacture of proteins.
RNA can also act as an enzyme, promoting the chemical reactions
that link amino acids to form proteins.

ATP
Another important biological molecule is ATP. ,or adenosine (uh
DEHN uh seen) triphosphate, is a single nucleotide with two extra
energy-storing phosphate groups. When food molecules are broken
down inside cells, some of the energy in the molecules is stored
temporarily in ATP. Cells need a steady supply of ATP to function.

ATP

RNA

DNA

nucleotide

nucleic acid

Section 3 Review

Identifywhat all organic compounds have in

common, and list the four principal classes of

organic compounds.

Comparethe structures of saturated and

unsaturated lipids. 9A

Describethe three parts of a nucleotide and

how they are attached to one another. 6A

Critical ThinkingInferring Relationships

Compare the role of ATP in cells with the roles

of RNA. 9A

Molecule Xcontains a sugar

and a phosphate group. What is molecule X? 9A

Aa carbohydrate Ca fatty acid

Ba nucleotide Dan amino acid

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