Essentials of Ecology

S42 SUPPLEMENT 6

Figure 7 Straight-chain and ring structural formulas of glucose, a simple sugar that can be
used to build long chains of complex carbohydrates such as starch and cellulose.

Glucose (C 6 H 12 O 6 )

General structure Chain of glucose units Starch

CH 2 OH

CH 2 OH

C

H

OH H

HOH

HO

HH

OH

HO

H OH

H C OH

HO C H

H C OH

C

O

Amino

group

Carboxyl

group

R side group

(20 kinds, each with

distinct properties)

Amino acid

Valine

General structure

of amino acid

Chain of amino acids Protein

O–

R O

+H N C C

H

H

H

O–

HC

+H N C CO

CH 3

CH 3

H

H

H

Figure 8 General structural formula of amino acids and a specific structural formula of one of
the 20 different amino acid molecules that can be linked together in chains to form proteins
that fold up into more complex shapes.

They account for many of water’s unique prop-
erties (Science Focus, p. 67). Hydrogen bonds
also form between other covalent molecules or
portions of such molecules containing hydrogen
and nonmetallic atoms with a strong ability to
attract electrons.

Four Types of Large Organic
Compounds Are the Molecular
Building Blocks of Life

Larger and more complex organic compounds,
called polymers, consist of a number of basic
structural or molecular units (monomers) linked
by chemical bonds, somewhat like rail cars
linked in a freight train. Four types of macro-

Phosphate 5-Carbon sugar Nucleotide base

Deoxyribose in DNA

Ribose in RNA

Figure 9 Generalized structures of the nucleotide

molecules linked in various numbers and sequences

to form large nucleic acid molecules such as various

types of DNA (deoxyribonucleic acid) and RNA (ri-

bonucleic acid). In DNA, the 5-carbon sugar in each

nucleotide is deoxyribose; in RNA it is ribose. The

four basic nucleotides used to make various forms

of DNA molecules differ in the types of nucleotide

bases they contain—guanine (G), cytosine (C), ad-

enine (A), and thymine (T). (Uracil, labeled U, occurs

instead of thymine in RNA.)

molecules—complex carbohydrates, proteins,

nucleic acids, and lipids—are molecular building

blocks of life.

Complex carbohydrates consist of two

or more monomers of simple sugars (such as

glucose, Figure 7) linked together. One example

is the starches that plants use to store energy

and also to provide energy for animals that feed

on plants. Another is cellulose, the earth’s most

abundant organic compound, which is found in

the cell walls of bark, leaves, stems, and roots.

Proteins, are large polymer molecules

formed by linking together long chains of

monomers called amino acids (Figure 8). Living

organisms use about 20 different amino acid

molecules to build a variety of proteins, which

play different roles. Some help to store energy.

Some are components of the immune system that

protects the body against diseases and harm-

ful substances by forming antibodies that make

invading agents harmless. Others are hormones

that are used as chemical messengers in the

bloodstreams of animals to turn various bodily

functions on or off. In animals, proteins are also

components of hair, skin, muscle, and tendons.

In addition, some proteins act as enzymes that

catalyze or speed up certain chemical reactions.

Nucleic acids are large polymer molecules

made by linking hundreds to thousands of four

types of monomers called nucleotides. Two nucleic

acids—DNA (deoxyribonucleic acid) and RNA

(ribonucleic acid)—participate in the building

of proteins and carry hereditary information

used to pass traits from parent to offspring. Each

nucleotide consists of a phosphate group, a sugar

molecule containing fi ve carbon atoms (deoxy-

ribose in DNA molecules and ribose in RNA

molecules), and one of four different nucleotide

bases (represented by A, G, C, and T, the fi rst let-

ter in each of their names, or A, G, C, and U in

RNA; see Figure 9). In the cells of living organ-

isms, these nucleotide units combine in different

numbers and sequences to form nucleic acids such

as various types of RNA and DNA (Figure 10).

Hydrogen bonds formed between parts of the

four nucleotides in DNA hold two DNA strands

together like a spiral staircase, forming a double

helix (Figure 10). DNA molecules can unwind

and replicate themselves.

The total weight of the DNA needed to

reproduce all of the world’s people is only about

50 milligrams—the weight of a small match. If

the DNA coiled in your body were unwound,

it would stretch about 960 million kilometers

(600 million miles)—more than six times the

distance between the sun and the earth.

The different molecules of DNA that make

up the millions of species found on the earth are

like a vast and diverse genetic library. Each spe-

cies is a unique book in that library. The genome

of a species is made up of the entire sequence

of DNA “letters” or base pairs that combine to

“spell out” the chromosomes in typical members

of each species. In 2002, scientists were able to

map out the genome for the human species by