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Chapter 3: The Chemistry of Life
3.1 Atoms
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An atom is the smallest particle that retains the chemical properties of its substance. Atoms
contain a core nucleus of protons and neutrons with electrons orbiting around the nucleus
(figure 3.1) (58.0K) .
Protons are positively charged particles, neutron particles carry no charge, and electrons are
negatively charged particles. Electrons also carry energy and determine the chemical behavior of
the atom (figure 3.2) (96.0K) .
Most electron shells hold up to eight electrons and atoms will undergo chemical reactions in
order to fill the outer electron shell, either by gaining, losing, or sharing electrons (figure 3.3)
(68.0K) .
3.2 Ions and Isotopes
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Ions are atoms that have gained one or more electrons (negative ions) or lost one or more
electrons (positive ions) (figure 3.4) (83.0K) .
Isotopes are atoms that have the same number of protons but differing numbers of neutrons.
Some are unstable and have applications in medicine and dating fossils (figure 3.5) (60.0K)
and (figure 3.7) (100.0K) .
3.3 Molecules
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Molecules form when atoms are held together with energy. The force holding atoms together is
called a chemical bond. There are three main types of chemical bonds.
Ionic bonds form when ions of opposite charge are attracted to each other (figure 3.8)
(105.0K) .Covalent bonds form when two atoms share electrons (figure 3.9) (99.0K) .
Hydrogen bonds form between polar molecules. Polar molecules are held together by covalent
bonds in which the shared electrons are unevenly distributed around their nuclei, giving them a
slightly positive end and a slightly negative end. Hydrogen bonds form when the positive end of
one molecule is attracted to the negative end of another (figure 3.10) (103.0K) .
Water: Cradle of Life
3.4 Hydrogen Bonds Give Water Unique Properties
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Water molecules are polar molecules that form hydrogen bonds with each other and with other
polar molecules. Many of the physical properties of water, such as heat storage, ice formation,
high heat of vaporization, cohesion, and being a highly polar solvent, are attributed to this
hydrogen bonding (figure 3.11) (159.0K) (figure 3.12 (101.0K) (figure 3.13) (123.0K) .
3.5 Water Ionizes
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Water molecules dissociate forming negatively charged hydroxide ions and positively charged
hydrogen ions. This property of water is significant because the concentration of hydrogen ions
in a solution determines its pH.
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A solution with a higher hydrogen ion concentration is an acid with specific chemical properties
and a solution with a lower hydrogen ion concentration is a base with different chemical
properties (figure 3.14) (83.0K) . Buffers control changes in pH.
Macromolecules
3.6 Forming Macromolecules
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Large molecules called macromolecules are formed by dehydration reactions in which molecular
subunits called monomers are linked together through covalent bonding that produces a water
molecule as a product of the reaction (figure 3.18) (75.0K) .
Amino acids are the subunits that link together to form proteins. Nucleotide subunits link
together to form nucleic acids. Monosaccharide subunits link together to form carbohydrates.
Fatty acids link together to form lipids (table 3.3a) (103.0K) (table 3.3b) (113.0K) .
3.7 Proteins
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Proteins are macromolecules that carry out many functions in the cell (table 3.4a) (76.0K)
(table 3.4b) (63.0K) . They are produced by the linking together of amino acids with covalent
bonds referred to as peptide bonds. The sequence of amino acids is the primary structure of the
protein. The chain of amino acids then folds into its final shape, exhibiting different levels of
complexity, from secondary to tertiary to quaternary structures (figure 3.21) (76.0K) .
Chaperone proteins aid in the folding of the amino acid chain into the correct shape. If
misfolded, the protein will not function properly (figure 3.25) (70.0K) .
3.8 Nucleic Acids
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Nucleic acids, such as DNA and RNA, are long chains of nucleotides. They function as
information storage molecules in the cell, storing the information needed to build proteins. The
sequence of A, T, G, and C nucleotides in DNA determines the sequence of amino acids in a
protein (figure 3.27) (101.0K) and (figure 3.28) (190.0K) .
3.9 Carbohydrates
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Carbohydrates, also referred to as sugars, are macromolecules that serve two primary functions
in the cell: structural framework and energy storage. Carbohydrates such as cellulose and chitin
provide structural integrity to cells and bodies of organisms. Carbohydrates such as starch and
glycogen provide a means of storing energy in the cell. Starch and glycogen are broken down in
the cells when energy is needed (table 3.5).
3.10 Lipids
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Lipids are large nonpolar molecules that are insoluble in water. They make up the lipid bilayer of
the plasma membrane of cells and function in long-term energy storage (figures 3.32 and 3.33).
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