65
CHAPTER TWO
Chemical Basis of Life
Proteins have complex three-dimensional shapes, called
conformations,
yet they are assembled from simple chains
of amino acids connected by peptide bonds. These are cova-
lent bonds that link the amino end of one amino acid with
the carboxyl end of another.
Figure 2.18
shows two amino
acids connected by a peptide bond. The resulting molecule is
a dipeptide. Adding a third amino acid creates a tripeptide.
Many amino acids connected in this way constitute a poly-
peptide
(f
g. 2.19
a
)
.
Proteins have four levels of structure: primary, second-
ary, tertiary and quaternary. The
primary structure
is the
amino acid sequence of the polypeptide chain. The primary
Like carbohydrates and lipids, proteins consist of atoms
of carbon, hydrogen, and oxygen. In addition, proteins always
include nitrogen atoms and sometimes sulfur atoms. The build-
ing blocks of proteins are
amino acids
(ah-me
no as
idz).
Twenty types of amino acids comprise proteins in organ-
isms. Amino acid molecules have an amino group (—NH
2
)
at one end and a carboxyl group (—COOH) at the other end.
Between these groups is a single carbon atom. This central
carbon is bonded to a hydrogen atom and to another group of
atoms called a
side chain
or
R group
(“R” may be thought of as
the “Rest of the molecule”). The composition of the R group
distinguishes one type of amino acid from another
(f g. 2.17)
.
C
H
C
O
H
C
H
H
H
H
C
H
H
N
C
H
H
Glycerol portion
(a)
A fat molecule
O
O
Fatty acid
Fatty acid
O
O
Fatty acid
Fatty acid
Fatty acid
H
O
P
O
C
H
O
Phosphate portion
(b)
A phospholipid molecule
(the unshaded portion may vary)
H
C
H
C
H
H
O
(c)
Schematic representation
of a phospholipid molecule
Water-insoluble
(hydrophobic)
“tail”
Water-soluble
(hydrophilic)
“head”
C
C
C
H
2
C
H
2
CC
H
(a)
General structure of a steroid
(b)
Cholesterol
C
CH
CH
2
CH
2
CH
CH
3
CH
2
H
C
HC
H
2
H
2
CH
2
CH
CH
2
CH
3
CH
3
C
H
2
C
H
CH
3
HO
C
CH
3
CH
2
CH
C
FIGURE 2.15
Fats and phospholipids. (
a
) A fat molecule (triglyceride) consists of a glycerol and three fatty acids. (
b
) In a phospholipid molecule, a
phosphate-containing group replaces one fatty acid. (
c
) Schematic representation of a phospholipid.
FIGURE 2.16
Steroid structure. (
a
) The general structure of a steroid. (
b
) The structural formula for cholesterol, a steroid widely distributed in the
body and a component of cell membranes.
TABLE
2.7
|
Important Groups of Lipids
Group
Basic Molecular Structure
Characteristics
Triglycerides
Three fatty acid molecules bound to a glycerol molecule
Most common lipid in the body; stored in fat tissue as an energy supply; fat
tissue also provides insulation beneath the skin
Phospholipids
Two fatty acid molecules and a phosphate group bound to
a glycerol molecule (may also include a nitrogen-containing
molecule attached to the phosphate group)
Used as structural components in cell membranes; large amounts are in the
liver and parts of the nervous system
Steroids
Four connected rings of carbon atoms
Widely distributed in the body with a variety of functions; includes
cholesterol, sex hormones, and certain hormones of the adrenal glands
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