Download 3.5 What are the chemical structures and functions of nucleic acids?

Chapter 3 – Macromolecules and the Origin of Life
3.1 What kinds of molecules characterize living things?
See Web/CD Tutorial 3.1
Macromolecules are polymers constructed by the formation of covalent bonds between smaller
molecules called monomers. Macromolecules in living organisms include polysaccharides,
proteins, and nucleic acids.
Functional groups are small groups of atoms that are consistently found together in a variety of
different macromolecules. Functional groups have particular chemical properties that they confer
on any larger molecule of which they are a part. Review Figure 3.1, Web/CD Activity 3.1
Structural and optical isomers have the same kinds and numbers of atoms, but differ in their
structures and properties. Review Figure 3.2
The many functions of macromolecules are directly related to their three-dimensional shapes,
which in turn is the result of the sequences and chemical properties of their monomers.
Monomers are joined by condensation reactions, which release a molecule of water for each
bond formed. Hydrolysis reactions use water to break polymers into monomers. Review Figure
3.4
3.2 What are the chemical structures and functions of
proteins?
Web/CD Activity 3.2
The functions of proteins include support, protection, catalysis, transport, defense, regulation,
and movement.
Amino acids are the monomers from which proteins are constructed. The properties of the
amino acids depend on their side chains, or R groups, which may be charged, polar, or
hydrophobic. Review Table 3.2
Peptide bonds covalently link amino acids into polypeptide chains. These bonds form by
condensation reactions between the carboxyl and amino groups. Review Figure 3.6
The primary structure of a protein is the order of amino acids in the chain. This chain is folded
into a secondary structure, which in different parts of the protein may be an α helix or a β
pleated sheet. Review Figure 3.7A-C
Disulfide bonds and noncovalent interactions between amino acids cause the polypeptide chain
to fold into a three-dimensional tertiary structure and allow multiple chains to interact in a
quaternary structure. Review Figure 3.7D, 3.7E
The specific shape and structure of a protein allows it to bind non- covalently to other molecules,
often called ligands.
Heat, alterations in pH, or certain chemicals can all result in protein denaturation, which
involves the loss of tertiary and/or secondary structure as well as biological function. Review
Figure 3.11
Chaperonins assist protein folding by preventing binding to inappropriate ligands. Review
Figure 3.12
3.3 What are the chemical structures and functions of
carbohydrates?
Carbohydrates contain carbon bonded to hydrogen and oxygen atoms in a ratio of 1:2:1, or
(CH2O) n.
Monosaccharides are the monomers that make up carbohydrates. Hexoses such as glucose are
six-carbon monosaccharides; pentoses have five carbons. Review Figure 3.14, Web/CD
Activity 3.3
Glycosidic linkages , which have either an α or a β orientation in space, covalently link
monosaccharides into larger units such as disaccharides, oligosaccharides, and
polysaccharides. Review Figure 3.15
Starch stores energy in plants. Starch and glycogen are formed by α-glycosidic linkages
between glucose monomers and are distinguished by the amount of branching they exhibit. They
can be easily broken down to release stored energy.
Cellulose, a very stable glucose polymer, is the principal component of the cell walls of plants.
3.4 What are the chemical structures and functions of lipids?
Fats and oils are triglycerides, composed of three fatty acids covalently bonded to a molecule
of glycerol by ester linkages. Review Figure 3.18
Saturated fatty acids have a hydrocarbon chain with no double bonds. The hydrocarbon chains
of unsaturated fatty acids have one or more double bonds that bend the chain, making close
packing less possible. Review Figure 3.19
Phospholipids have a hydrophobic hydrocarbon "tail" and a hydrophilic phosphate "head." In
water, the interactions of the hydrophobic tails and hydrophilic heads of phospholipids generate a
phospholipid bilayer that is two molecules thick. The heads are directed outward, where they
interact with the surrounding water. The tails are packed together in the interior of the bilayer.
Review Figure 3.20
3.5 What are the chemical structures and functions of
nucleic acids?
The unique function of the nucleic acids—DNA and RNA—is information storage; they form
the hereditary material that passes genetic information to the next generation.
Nucleic acids are polymers of nucleotides. A nucleotide consists of a phosphate group, a pentose
sugar (ribose in RNA and deoxyribose in DNA), and a nitrogen-containing base. Review
Figure 3.23, Web/CD Activity 3.4
In DNA the nucleotide bases are adenine, guanine, cytosine, and thymine. Uracil substitutes
for thymine in RNA. The nucleotides are joined by phosphodiester linkages between the sugar
of one nucleotide and the phosphate of the next.
RNA is single-stranded. DNA is a double helix in which there is complementary base pairing
based on hydrogen bonds between adenine and thymine (A-T) and between guanine and cytosine
(G-C). The two strands of the DNA double helix run in opposite directions. Review Figure 3.24
and 3.26, Web/CD Activity 3.5
The information content of DNA and RNA resides in their base sequences.
3.6 How did life on Earth begin?
Chemical evolution proposes that conditions on early Earth could have produced the
macromolecules that distinguish living things. Review Figure 3.28, Web/CD Tutorial 3.2
Because it can form a three-dimensional shape, RNA could act as a ribozyme, an RNA surface
on which chemical reactions proceed at a faster rate. Review Figure 3.29
Experiments have ruled out the continuous spontaneous generation of life. Review Figure 3.30,
Web/CD Tutorial 3.3