Mastering Concepts
2.1
1. Which four chemical elements do organisms require in the largest amounts?
Carbon, oxygen, hydrogen, and nitrogen are the chemical elements that organisms require
in the largest amounts.
2. Where in an atom are protons, neutrons, and electrons located?
An atom’s protons and neutrons are in its nucleus. A cloud of electrons surrounds the
nucleus.
3. What does an element’s atomic number indicate?
An atom’s atomic number indicates the number of protons in its nucleus.
4. What is the relationship between an atom’s mass number and an element’s atomic
weight?
An atom’s mass number is the sum of the numbers of protons and neutrons in its nucleus.
The atomic weight is the average mass of all of the isotopes of that element.
5. How are different isotopes of the same element different from one another?
Isotopes of the same element have the same number of protons but differ in the number
of neutrons.
2.2
1. How are atoms, molecules, and compounds related?
Two or more atoms bonded together form a molecule. A compound is a molecule made
of atoms of two or more different elements.
2. How does the number of valence electrons determine an atom’s tendency to form
bonds?
An atom is most stable if its valence (outermost) shell is full. The number of electrons
and number of “vacancies” in the valence shell determine how many electrons an atom
will gain, lose, or share to achieve a full valence shell. Some atoms, such as neon and
argon, do not form bonds because they already have a full valence shell.
3. Explain how electronegativity differences between atoms result in each type of
chemical bond.
Ionic bonds form between pairs of atoms that have extremely different
electronegativities; in that case, one atom takes one or more electrons from the other.
Polar covalent bonds form between atoms that have moderately different
electronegativities; such atoms share electrons unequally and thus have partial positive or
partial negative charges. Nonpolar covalent bonds form between atoms that have similar
electronegativity and share electrons approximately equally. The partial charges on
molecules with polar covalent bonds may lead to hydrogen bonds between adjacent
molecules.
2.3
1. How are cohesion and adhesion important to life?
Cohesion is water’s tendency to stick to itself; adhesion is water’s tendency to stick to
other substances. Cohesion means water is liquid at most temperatures on Earth’s
surface; it also allows small insects to “skate” on the water surface. Both cohesion and
adhesion help water move from roots to leaves in plants.
2. Distinguish between a solute and a solvent and between a hydrophilic and a
hydrophobic molecule.
A solute (such as salt) is a substance that dissolves into a liquid solvent (such as water).
Hydrophilic molecules will dissolve in water, whereas hydrophobic molecules will not.
3. How does water help an organism regulate its body temperature?
The hydrogen bonds in water keep it from changing temperatures quickly, even if the
external temperature rises or drops. Sweat is an adaptation that cools the body as water
evaporates from the skin.
4. How does the density difference between ice and water affect life?
Because the density of ice is less than that of liquid water, a layer of ice forms at the top
of a lake in cold temperatures. Insulated from the cold air above, the liquid water below
the ice layer is less likely to freeze. Aquatic organisms that cannot tolerate freezing can
therefore survive the cold season.
5. What happens in a chemical reaction?
In a chemical reaction, reactants swap atoms to form products. As atoms are swapped,
chemical bonds are broken, energy is released or used in the reaction, and new chemical
bonds are formed.
6. How does water participate in the chemistry of life?
Because cells are mostly made of water and because most cells are surrounded by water,
water is the medium in which nearly all of life’s chemical reactions occur. Water is either
a reactant in or a product of many of the chemical reactions that sustain life, including
respiration and photosynthesis.
2.4
1. How do acids and bases affect a solution’s H+ concentration?
An acid adds H+ to the solution, whereas a base absorbs H+ or releases OH-.
2. How do the values of 0, 7, and 14 relate to the pH scale?
0 and 14 on the pH scale represent the strongest acidic and basic solutions respectively,
while 7 is the pH value of a neutral solution.
3. How do buffers regulate the pH of a fluid?
In buffer systems, pairs of weak acids and bases consume or release H+. Adding an acid
or base therefore does not affect the pH of a buffered solution.
2.5
1. Distinguish between hydrolysis and dehydration synthesis.
Hydrolysis reactions break covalent bonds in polymers, releasing individual monomers.
The opposite reaction, dehydration synthesis, forms covalent bonds between monomers,
forming a polymer.
2. Compare and contrast the structures of polysaccharides, proteins, and nucleic acids.
All are long polymers built of monomers. The monomers that form polysaccharides are
monosaccharides; the monomers that form proteins are amino acids; and the monomers
that form nucleic acids are nucleotides.
3. List examples of carbohydrates, proteins, nucleic acids, and lipids, and name the
function of each.
Carbohydrates include simple sugars such as glucose and polysaccharides such as
cellulose, chitin, starch, and glycogen. Glucose is an energy source; cellulose and chitin
make up plant and fungal cell walls; starch and glycogen store energy.
Proteins have more functions than any other type of organic molecule. They include actin
and myosin, which participate in muscle contraction; enzymes, which speed chemical
reactions; and insulin, a hormone that controls the level of glucose in blood.
Nucleic acids include DNA, which encodes proteins, and RNA, which participates in
gene expression.
Lipids include saturated and unsaturated fats, which store energy. Sterols include
cholesterol, which adds fluidity to cell membranes, and the sex hormones. Waxes are
lipids that form waterproof coverings.
4. What is the significance of a protein’s shape, and how can that shape be destroyed?
A protein’s shape determines its function. That shape can be destroyed by exposure to
heat, strong acids, or strong bases; each of these can denature a protein. Also, genetic
mutations can change the amino acid sequence of a protein, perhaps altering its overall
shape.
5. What are some differences between RNA and DNA?
RNA is typically a single strand of nucleotides; DNA is a double strand of nucleotides.
RNA nucleotides include ribose; DNA nucleotides include deoxyribose. RNA has the
nitrogenous base uracil; DNA has thymine.
6. What are the components of a triglyceride?
The components of a triglyceride are a glycerol molecule and three fatty acids.
2.6
1. How did the researchers determine that bacteria increased green pigment production in
aphids?
They knew Rickettsiella bacteria were involved by selectively killing other bacteria with
antibiotics; the aphids still turned green as they aged. Injecting Rickettsiella bacteria into
aphids also induced the color change. They verified that the bacteria increased green
pigment production by comparing bands of pigments extracted from Rickettsiellainfected and uninfected aphids.
2. How might the color change be adaptive to both aphids and the bacteria that infect
them?
The color change may promote reproductive success in aphids and the bacteria. Green
aphids may be protected against predation by ladybird beetles. Surviving aphids are
likely to reproduce and pass the Rickettsiella bacteria to their offspring.
Write It Out
1. Describe how the number of protons, neutrons, and electrons in an atom affects its
atomic number, mass number, and charge.
The atomic number equals the number of protons. The mass number is the number of
protons and neutrons added together. If an atom has more electrons than protons, it has a
negative charge. If it has more protons than electrons, it has a positive charge.
2. Distinguish between nonpolar covalent bonds, polar covalent bonds, and ionic bonds.
Nonpolar covalent bonds are bonds in which both atoms exert approximately equal pull
on the shared electrons. In a polar covalent bond, one nucleus exerts a stronger pull on
the shared electrons than does the other nucleus. An ionic bond results from the electrical
attraction between two ions with opposite charges.
3. If oxygen is highly electronegative, why is a covalent bond between two oxygen atoms
considered nonpolar?
The nucleus of each oxygen atom exerts an equal pull on the shared electrons, so the
bond is nonpolar.
4. Can nonpolar molecules such as CH4 participate in hydrogen bonds? Why or why not?
Nonpolar molecules cannot form hydrogen bonds because the atoms do not have partial
charges.
5. Define solute, solvent, and solution.
A solute is a substance that is dissolved in a solvent. Solutions consist of one or more
solutes dissolved in a liquid solvent.
6. Give an example from everyday life of each of the following properties of water:
cohesion, adhesion, ability to dissolve solutes, resistance to temperature change.
A water droplet combining with another droplet on a windshield shows cohesion. A
splash of water sticks to the wall with adhesion. Mixing instant coffee into hot water
illustrates water’s ability to dissolve solutes. Coastal climates are relatively mild because
the ocean is vast and therefore extremely resistant to temperature change.
7. Draw from memory a diagram showing the interactions among a few water molecules.
The diagram should resemble components of figure 2.9. Hydrogen atoms should have
partial positive charges that are attracted to the partial negative charges on the oxygen
atoms of adjacent water molecules.
8. How do hydrogen ions relate to the pH scale?
The concentration of hydrogen ions determines a solution’s pH. The higher the
concentration, the lower the pH.
9. Sketch a monosaccharide, amino acid, nucleotide, glycerol molecule, and fatty acid.
Then show how those smaller molecules form carbohydrates, proteins, nucleic acids, or
fats.
Answers will be visual. Consult the following figures to check your work: 2.17, 2.18,
2.22, and 2.24.
10. Refer to figure 2.21 and explain why regulating body temperature is essential to
survival.
Excessive heat causes proteins to become denatured, which ruins their shapes. If a
protein’s shape is destroyed, it can no longer function. Since proteins do nearly all the
work in living cells, excessive heat can be deadly.
11. You eat a sandwich made of starchy bread, ham, and cheese. What types of chemicals
are in it?
The bread is mostly starch, although it may also contain some fiber (cellulose) if it is
made with whole wheat; both starch and cellulose are polysaccharides. The ham and
cheese are composed mainly of protein and fat.
Pull It Together
1. How do ions and isotopes fit into this concept map?
“Atoms” could connect with the phrase “that have gained or lost electrons are called” to
“Ions.” “Atoms” could connect with the phrase “with different numbers of neutrons are
called” to “Isotopes.”
2. Besides water, what other molecules are essential to life?
Besides water, life requires lipids to make cell membranes, amino acids to build proteins,
sugars for energy, nucleic acids to store and express genetic information, and oxygen and
carbon dioxide gases for respiration and photosynthesis.
3. Describe how hydrogen bonds form.
Polar covalent bonds create areas of partial and negative charges within a molecule. A
hydrogen bond forms if a partially positive part of one molecule attracts a partially
negative part of another molecule.
4. Add monomers, polymers, dehydration synthesis, and hydrolysis to this concept map.
“Carbohydrates,” “Proteins,” and “Nucleic acids” could connect with the phrase “are” to
“Polymers,” which in turn could connect with the phrase “are made of subunits called” to
“Monomers.” “Monomers” could connect with the phrase “are joined together by” to
“Dehydration synthesis.” “Polymers” could connect with the phrase “are broken down
by” to “Hydrolysis.”