40
Chapter 2
Polar Molecules
I. In polar covalent bonds, one atom attracts the bonding
electrons more than the other atom of the pair.
II. The electrical attraction between hydrogen and an oxygen or
nitrogen atom in a separate molecule or different region of the
same molecule forms a hydrogen bond.
III. Water, a polar molecule, is attracted to other water molecules
by hydrogen bonds.
Solutions
I. Substances dissolved in a liquid are solutes, and the liquid
in which they are dissolved is the solvent. Water is the most
abundant solvent in the body.
II. Substances that have polar or ionized groups dissolve in water
by being electrically attracted to the polar water molecules.
III. In water, amphipathic molecules form clusters with the polar
regions at the surface and the nonpolar regions in the interior
of the cluster.
IV. The molecular weight of a molecule is the sum of the atomic
weights of all its atoms. One mole of any substance is its
molecular weight in grams and contains 6
×
10
23
molecules.
V. Substances that release a hydrogen ion in solution are called
acids. Those that accept a hydrogen ion are bases.
a. The acidity of a solution is determined by its free
hydrogen ion concentration; the greater the hydrogen ion
concentration, the greater the acidity.
b. The pH of a solution is the negative logarithm of the
hydrogen ion concentration. As the acidity of a solution
increases, the pH decreases. Acid solutions have a pH less than
7.0, whereas alkaline solutions have a pH greater than 7.0.
Classes of Organic Molecules
I. Carbohydrates are composed of carbon, hydrogen, and oxygen
in the proportions C
n
(H
2
O)
n
.
a. The presence of the polar hydroxyl groups makes
carbohydrates soluble in water.
b. The most abundant monosaccharide in the body is glucose
(C
6
H
12
O
6
), which is stored in cells in the form of the
polysaccharide glycogen.
II. Most lipids have many fewer polar and ionized groups than
carbohydrates, a characteristic that makes them insoluble in water.
a. Triglycerides (fats) form when fatty acids are bound to each
of the three hydroxyl groups in glycerol.
b. Phospholipids contain two fatty acids bound to two of the
hydroxyl groups in glycerol, with the third hydroxyl bound
to phosphate, which in turn is linked to a small charged or
polar compound. The polar and ionized groups at one end
of phospholipids make these molecules amphipathic.
c. Steroids are composed of four interconnected rings, often
containing a few hydroxyl and other groups.
d. One fatty acid (arachidonic acid) can be converted to a class
of signaling substances called eicosanoids.
III. Proteins, macromolecules composed primarily of carbon,
hydrogen, oxygen, and nitrogen, are polymers of 20 different
amino acids.
a. Amino acids have an amino (—NH
2
) and a carboxyl
(—COOH) group bound to their terminal carbon atom.
b. Amino acids are bound together by peptide bonds between
the carboxyl group of one amino acid and the amino group
of the next.
c. The primary structure of a polypeptide chain is determined
by (1) the number of amino acids in sequence, and (2) the
type of amino acid at each position.
d. Hydrogen bonds between peptide bonds along a
polypeptide force much of the chain into an alpha helix
(secondary structure).
e. Covalent disulfi de bonds can form between the sulfhydryl
groups of cysteine side chains to hold regions of a
polypeptide chain close to each other (tertiary structure).
f. Multimeric proteins have multiple polypeptide chains
(quaternary structure).
IV. Nucleic acids are responsible for the storage, expression, and
transmission of genetic information.
a. Deoxyribonucleic acid (DNA) stores genetic information.
b. Ribonucleic acid (RNA) is involved in decoding the
information in DNA into instructions for linking amino
acids together to form proteins.
c. Both types of nucleic acids are polymers of nucleotides,
each containing a phosphate group, a sugar, and a base of
carbon, hydrogen, oxygen, and nitrogen atoms.
d. DNA contains the sugar deoxyribose and consists of two
chains of nucleotides coiled around each other in a double
helix. The chains are held together by hydrogen bonds
between purine and pyrimidine bases in the two chains.
e. Base pairings in DNA always occur between guanine and
cytosine and between adenine and thymine.
f. RNA consists of a single chain of nucleotides, containing
the sugar ribose and three of the four bases found in DNA.
The fourth base in RNA is the pyrimidine uracil rather
than thymine. Uracil base-pairs with adenine.
g. In all cells, energy from the catabolism of fuel molecules is
transferred to ATP. Hydrolysis of ATP to ADP + P
i
then
transfers this energy to power cell functions. ATP consists
of the purine adenine coupled by high-energy bonds to
three phosphate groups.
KEY TERMS
acid
26
acidic solution
26
acidity
26
adenine
36
adenosine diphosphate
(ADP)
39
adenosine triphosphate
(ATP)
38
alkaline solution
26
alpha helix
34
amino acid
31
amino acid side chain
31
amino group
21
amphipathic
25
anion
21
atom
19
atomic nucleus
19
atomic number
19
atomic weight
19
base
26
beta sheet
34
carbohydrate
27
carboxyl group
21
cation
21
chemical element
19
concentration
25
conformation
33
covalent bond
20
cytosine
36
dehydration
24
deoxyribonucleic acid
(DNA)
35
deoxyribose
36
disaccharide
28
disulfi de bond
34
electrolyte
21
electron
19
fatty acid
29
free radical
21
glucose
28
glycerol
30
glycogen
28
glycoprotein
33
gram atomic mass
20
guanine
36
hexose
28
hydrogen bond
23
hydrolysis
24
hydrophilic
24
hydrophobic
24
hydroxyl group
23
ion
20
ionic bond
24
isotope
20
lipid
29
macromolecule
27
mole
26
molecular weight
26
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