Human Physiology, 14th edition (2016)

44 Chapter 2

Nucleotides are the subunits of nucleic acids, bonded together

in dehydration synthesis reactions to form long polynucleo-

tide chains. Each nucleotide, however, is itself composed of

three smaller subunits: a five-carbon ( pentose ) sugar, a phos-

phate group attached to one end of the sugar, and a nitrog-

enous base attached to the other end of the sugar ( fig. 2.31 ).

The nitrogenous bases are nitrogen-containing molecules of

two kinds: pyrimidines and purines. The pyrimidines contain

a single ring of carbon and nitrogen, whereas the purines have

two such rings.

Deoxyribonucleic Acid

The structure of DNA (deoxyribonucleic acid) serves as the

basis for the genetic code. For this reason, it might seem logi-

cal that DNA should have an extremely complex structure.

DNA is indeed larger than any other molecule in the cell, but

its structure is actually simpler than that of most proteins.

This simplicity of structure deceived some early investiga-

tors into believing that the protein content of chromosomes,

rather than their DNA content, provided the basis for the

genetic code.

Sugar molecules in the nucleotides of DNA are a type

of pentose (five-carbon) sugar called deoxyribose. Each

Many proteins in the body are normally found combined,
or conjugated, with other types of molecules. Glycoproteins
are proteins conjugated with carbohydrates. Examples of such
molecules include certain hormones and some proteins found
in the cell membrane. Lipoproteins are proteins conjugated
with lipids. These are found in cell membranes and in the
plasma (the fluid portion of the blood). Proteins may also be
conjugated with pigment molecules. These include hemoglo-
bin, which transports oxygen in red blood cells, and the cyto-
chromes, which are needed for oxygen utilization and energy
production within cells.

Functions of Proteins

Because of their tremendous structural diversity, proteins
can serve a wider variety of functions than any other type
of molecule in the body. Many proteins, for example, con-
tribute significantly to the structure of different tissues and
in this way play a passive role in the functions of these tis-
sues. Examples of such structural proteins include collagen
( fig. 2.30 ) and keratin. Collagen is a fibrous protein that pro-
vides tensile strength to connective tissues, such as tendons
and ligaments. Keratin is found in the outer layer of dead
cells in the epidermis where it prevents water loss through
the skin.
Many proteins play a more active role in the body where
specificity of structure and function is required. Enzymes and
antibodies, for example, are proteins—no other type of mole-
cule could provide the vast array of different structures needed
for their tremendously varied functions. As another example,
proteins in cell membranes may serve as receptors for specific
regulatory molecules (such as hormones) and as carriers for
transport of specific molecules across the membrane. Proteins
provide the diversity of shape and chemical properties required
by these functions.

2.4 Nucleic Acids

Nucleic acids include the macromolecules DNA and RNA,

which are critically important in genetic regulation, and the

subunits from which these molecules are formed. These

subunits are known as nucleotides.

Figure 2.30 A photomicrograph of collagenous
fibers within connective tissue. Collagen proteins strengthen
the connective tissues.

Elastic fibers

Collagenous

fibers

| CHECKPOINTS

8a. Write the general formula for an amino acid, and

describe how amino acids differ from one another.

8b. Describe and account for the different levels of

protein structure.

9. Describe the different categories of protein function
   in the body, and explain why proteins can serve
   functions that are so diverse.

LEARNING OUTCOMES

After studying this section, you should be able to:

10. Describe the structure of nucleotides and distinguish
    between the structure of DNA and RNA.


11. Explain the law of complementary base pairing, and
    describe how that occurs between the two strands
    of DNA.