Chemical Composition of the Body 47purposes as well. These include roles as energy carriers (ATP and
GTP); regulation of cellular events (cyclic AMP, or cAMP); and
coenzymes (nicotinamide adenine dinucleotide, or NAD; and
flavine adenine dinucleotide, or FAD). These are discussed in
chapters 4, 5, and 6. Purines (ATP and adenosine) are even used
as neurotransmitters by some neurons (chapter 7, section 7.6).
Summary
2.2 Carbohydrates and Lipids 33
A. Carbohydrates contain carbon, hydrogen, and oxygen, usu-
ally in a ratio of 1:2:1.
1. Carbohydrates consist of simple sugars
(monosaccharides), disaccharides, and polysaccharides
(such as glycogen).
2. Covalent bonds between monosaccharides are formed
by dehydration synthesis, or condensation. Bonds are
broken by hydrolysis reactions.
B. Lipids are organic molecules that are insoluble in polar sol-
vents such as water.
1. Triglycerides (fat and oil) consist of three fatty acid
molecules joined to a molecule of glycerol.
2. Ketone bodies are smaller derivatives of fatty acids.
3. Phospholipids (such as lecithin) are phosphate-
containing lipids that have a hydrophilic polar group.
The rest of the molecule is hydrophobic.
4. Steroids (including the hormones of the adrenal cortex and
gonads) are lipids with a characteristic four-ring structure.
5. Prostaglandins are a family of cyclic fatty acids that
serve a variety of regulatory functions.2.3 Proteins 41
A. Proteins are composed of long chains of amino acids bound
together by covalent peptide bonds.
1. Each amino acid contains an amino group, a carboxyl
group, and a functional group. Differences in the
functional groups give each of the more than 20
different amino acids an individual identity.
2. The polypeptide chain may be twisted into a helix
(secondary structure) and bent and folded to form the
tertiary structure of the protein.
3. Proteins that are composed of two or more polypeptide
chains are said to have a quaternary structure.2.1 Atoms, Ions, and Chemical Bonds 25
A. Covalent bonds are formed by atoms that share electrons.
They are the strongest type of chemical bond.
- Electrons are equally shared in nonpolar covalent bonds
and unequally shared in polar covalent bonds. - Atoms of oxygen, nitrogen, and phosphorus strongly
attract electrons and become electrically negative
compared to the other atoms sharing electrons with them.
B. Ionic bonds are formed by atoms that transfer electrons.
These weak bonds join atoms together in an ionic
compound. - If one atom in this compound takes an electron from
another atom, it gains a net negative charge and the
other atom becomes positively charged. - Ionic bonds easily break when the ionic compound is
dissolved in water. Dissociation of the ionic compound
yields charged atoms called ions.
C. When hydrogen bonds with an electronegative atom, it gains
a slight positive charge and is weakly attracted to another
electronegative atom. This weak attraction is a hydrogen
bond.
D. Acids donate hydrogen ions to solution, whereas bases lower
the hydrogen ion concentration of a solution. - The pH scale is a negative function of the logarithm of
the hydrogen ion concentration. - In a neutral solution, the concentration of H^1 is equal to
the concentration of OH^2 , and the pH is 7. - Acids raise the H^1 concentration and thus lower the
pH below 7; bases lower the H^1 concentration and thus
raise the pH above 7.
E. Organic molecules contain atoms of carbon and hydrogen
joined together by covalent bonds. Atoms of nitrogen,
oxygen, phosphorus, or sulfur may be present as specific
functional groups in the organic molecule.
| CHECKPOINTS
10a. What are nucleotides, and of what are they
composed?
10b. List the types of RNA, and explain how the structure
of RNA differs from the structure of DNA.- Describe the structure of DNA, and explain the law of
complementary base pairing.
Brian has multiple myeloma, a type of cancer affecting
cells of the immune system called plasma cells, which
secrete antibodies. This disease has different degrees
of severity and various forms of treatment, including
the use of thalidomide. Brian needn’t be concerned
about the teratogenic effects of one of the enantiomers
of thalidomide, because he obviously won’t become
pregnant. His ketonuria is related to his weight loss,
because ketone bodies are produced from fatty acids
released by adipose cells when they hydrolyze their
stored triglycerides.
See the additional chapter 2 Clinical Investigation on High
Cholesterol in the Connect site for this text.