Mastering Concepts
2.1
1. Which chemical elements do organisms require in large amounts?
Carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus are the chemical elements
that organisms require in large 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
mass?
An atom’s mass number is the sum of protons and neutrons in its nucleus. The atomic
mass is the average mass of all of the isotopes of that particular element.
5. How are different isotopes of the same element different from one another?
Isotopes of the same element differ in the number of the neutrons in their nuclei.
2.2
1. How are atoms, molecules, and compounds related?
Molecules and compounds are formed of atoms. Molecules are joined atoms, while
compounds are molecules formed of two or more different elements.
2. How does the number of valence electrons determine an atom’s tendency to form
bonds?
The number of valence electrons in the valence shell determines how many “vacancies”
an atom has to fill before it is stable. If its valence shell is filled, it is chemically inert; if
it has one or more vacancies it tends to be chemically reactive.
3. Explain how electronegativity differences between atoms result in nonpolar covalent
bonds, polar covalent bonds, and ionic bonds.
The difference in electronegativity between atoms results in different kinds of bonds:
- nonpolar covalent bonds form between atoms that have similar electronegativity
and share bond electrons equally;
- polar covalent bonds form between atoms that have different levels of
electronegativity and share bond electrons unequally;
- ionic bonds form between pairs of atoms that have extremely different levels of
electronegativity. Rather than sharing electrons, one atom takes an electron from
the other..
4. What is the relationship between polar covalent bonds and hydrogen bonds?
Hydrogen bonds form between molecules with polar covalent bonds. In water, for
example, hydrogen bonds form between the partial positive charge of one of a water
molecule’s hydrogen atoms and the partial negative charge of the oxygen atom of an
adjacent water molecule.
2.3
1. How are cohesion and adhesion important to life?
Cohesion decreases the rate of evaporative water loss and helps water move from roots to
leaves in plants; it also allows small insects to move on water. Adhesion helps with the
movement of water in plants as the water molecules adhere to the walls of the vessels.
2. What is the difference between hydrophilic and hydrophobic molecules?
Hydrophilic molecules will dissolve into and mix with water, while hydrophobic
molecules will not. Hydrophilic molecules are often polar, while hydrophobic molecules
are nonpolar.
3. How does water help an organism regulate its body temperature?
The hydrogen bonds in water keep it from changing temperatures quickly. Water also
evaporates and helps bring down high temperatures.
4. How does the density difference between ice and water affect life?
Because the density of ice is less than water, a layer of ice forms at the top of water
sources in cold temperatures. This locks in warmer temperatures below.
5. What happens in a chemical reaction?
In a chemical reaction, the chemicals involved (the reactants) swap atoms, forming
different chemicals (the 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 often is
a reactant and a product of the chemical reactions typical of life.
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+.
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 value of a completely neutral solution.
3. How do buffer systems 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 the solution.
2.5
1. What is the relationship between hydrolysis and dehydration synthesis?
Hydrolysis reactions break covalent bonds in polymers by adding water, while
dehydration synthesis forms covalent bonds between monomers by removing water; thus
the reactions are opposite.
2. Describe the monomers that form polysaccharides, proteins, and nucleic acids.
The monomers that form polysaccharides are simple sugars such as monosaccharides and
disaccharides. The monomers that form proteins are amino acids. The monomers that
form nucleic acids are nucleotides.
3. List examples of carbohydrates, lipids, proteins, and nucleic acids, and name the
function of each.
Carbohydrates include simple sugars such as glucose; disaccharides such as sucrose; and
polysaccharides such as cellulose, chitin, starch, and glycogen. Glucose and sucrose are
energy sources; cellulose and chitin make up plant and fungal cell walls; starch and
glycogen store energy.
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.
Proteins include actin and myosin, which participate in muscle contraction; antibodies,
which participate in immune function; 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 and may act as an enzyme.
4. What are the components of a triglyceride?
The components of a triglyceride are a glycerol molecule and three fatty acids.
5. 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 heat, strong
acids, or strong bases.
6. What are some differences between RNA and DNA?
Some of the differences between DNA and RNA include:
- RNA is a single strand of nucleotides; DNA is a double strand of nucleotides.
- RNA has ribose sugar; DNA has deoxyribose sugar.
- RNA has the nitrogenous base uracil; DNA has thymine.
- DNA’s information is transcribed to RNA molecules, but not vice versa (usually)
- RNA acts as an enzyme; DNA does not.
2.6
1. What question were these researchers trying to answer?
The researchers wanted to know whether the organic molecules in the meteorite were
contaminants or extraterrestrial.
2. Why are 15N and 13C called “heavy” isotopes? How are they different from 14N and
12C?
15
N and 13C each have one additional neutron compared to 14N and 12C. The extra neutron
makes them “heavy”. These two isotopes are slightly more abundant in space than on
Earth.
3. Both groups of researchers collected samples from the meteorite’s interior. Why does
the sample location matter?
The meteorite’s interior was not exposed to contamination by earth molecules.
4. How would the results have differed if the amino acids and bases were contaminants
acquired after the meteorite fell to Earth?
If the bases were contaminants then the numbers in the results table should be negative
rather than strongly positive.
Write It Out
1. Define the following terms: atom, element, molecule, compound, isotope, and ion.
atom - the smallest portion of an element that retains characteristics of the element
element – a pure substance that cannot be broken down by chemical means into other
substances.
molecule - two or more chemically joined atoms.
compound - a molecule composed of two or more different elements
isotope – one of two or more forms of an atom of the same element; different isotopes of
the same element have the same number of protons but different numbers of neutrons.
ion - an atom or molecule with a positive or negative charge.
2. Consider the following atomic numbers: oxygen (O) = 8; fluorine (F) = 9; neon (Ne) =
10; magnesium (Mg) = 12. Build a Bohr model of each atom, and then predict how many
bonds each atom should form.
(See Bohr models in text). Nitrogen: 3 covalent bonds. Oxygen: 2 covalent bonds.
Fluorine: 1 ionic bond. Neon: 0 bonds (its outermost shell is full). Magnesium: 2 ionic
bonds.
3. 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.
4. 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.
5. 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.
6. 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.
7. Explain why each of the following properties of water is essential to life: cohesion,
adhesion, ability to dissolve solutes, resistance to temperature change.
Without cohesion, water would evaporate instantly in the temperatures that prevail at
most locations on Earth. Cohesion also helps move water from the roots to the leaves of
plants (which form the base of most ecosystems on land). Adhesion helps move water
from the roots to the leaves of plants (which form the base of most ecosystems on land).
Water’s versatile ability to dissolve solutes means that most substances that are essential
to life can exist within a cell’s watery cytoplasm. Water’s ability to resist temperature
change helps keep ecosystem and body temperatures stable even when the environment
becomes warmer or cooler.
8. Using your knowledge of the properties of water, explain the quote “Hydrogen bonds
sank the Titanic.”
The quote, “Hydrogen bonds sank the Titanic” can be interpreted in multiple ways.
Hydrogen bonds mean that ice floats on water, so that icebergs formed in the cold ocean
waters. The Titanic hit an iceberg. In addition, because water molecules stick together
by hydrogen bonds, the water was able to flow into the ship, flood it, and finally sink the
Titanic.
9. Why are buffer systems important in organisms?
Buffer systems are important in organisms because they resist pH changes that could
otherwise damage biological molecules.
10. Compare and contrast the chemical structures and functions of carbohydrates, lipids,
proteins, and nucleic acids.
Carbohydrates are organic molecules that consist of carbon, hydrogen, and oxygen.
They include monosaccharides, disaccharides and polysaccharides. The function of
carbohydrates is to provide a ready source of energy and to support cells and organisms.
Lipids are organic compounds that do not dissolve in water and are made of nonpolar
carbon-carbon and carbon-hydrogen bonds. Unlike the other three organic molecules,
lipids are not polymers. The function of lipids is to form cell membranes, store energy,
stabilize animal membranes, act as sex hormones, and provide water-proofing. Proteins
control all of the activities of life. They are made up of chains of amino acids folded into
specific shapes. Nucleic acids are polymers consisting of nucleotides. The two types of
nucleic acids are DNA and RNA. Nucleic acids store and use genetic information and
transmit it to the next generation.
11. How is an amino acid’s R group analogous to a nucleotide’s nitrogenous base?
The R group is the variable portion of an amino acid, just as the nitrogenous base is the
variable portion of a nucleotide.
12. Pickles and several other foods are preserved in acids such as vinegar. Why is an acid
a good preservative? (Hint: Consider the effect of acids on protein shape.)
Acids are good preservatives because they denature the proteins in bacteria and other
organisms that would otherwise spoil the food.
13. Complete and explain the following analogy: a protein is to a knitted sweater as a
denatured protein is to a ____.
tangle of unraveled yarn. A protein is a highly structured chain made of amino acids, just
as a sweater is a structured garment made of yarn. When unraveled, a sweater is just a
ball of yarn; when denatured, a protein is just a mass of amino acids with no function.
14. A topping for ice cream contains fructose, hydrogenated soybean oil, salt, and
cellulose. What types of chemicals are in it?
The topping contains simple sugars, complex carbohydrates, NaCl, and triglycerides
(lipids).
15. Using information in “Sugar Substitutes and Fake Fats” on page 39 and the amino
acid structures in appendix E, draw the dipeptide called aspartame (NutraSweet).
(See comparable figure in text).
Pull It Together
1. How do ions and isotopes fit into this concept map?
“Ions” could be connected to “Atoms” with the phrase “that have gained or lost electrons
are called”. “Isotopes” could be connected to “Atoms” with the phrase “with a different
number of neutrons are called”.
2. Add covalent bonds, ionic bonds, and hydrogen bonds to this concept map.
“Hydrogen bonds” could be connected to “Water” with the phrase “molecules are
cohesive due to”. “Ionic bonds” and “Covalent bonds” could be connected to “Organic
molecules” with the phrase “are held together by both”.
3. Besides water, what are other examples of molecules that are essential to life?
Besides water, many other molecules are essential to life. Life requires lipids to make
cell membranes, amino acids to build proteins, sugars for energy, and oxygen and carbon
dioxide gasses to respire.
4. Add monomers, polymers, dehydration synthesis, and hydrolysis to this concept map.
“Monomers” could be connected to “Organic molecules” with the phrase “are made up
of subunits called”. “Dehydration synthesis” could be connected to “Monomers” with the
phrase “are joined together by”. “Polymers” could be connected to “Dehydration
synthesis” with the phrase “forms”. “Hydrolysis” could be connected to “Polymers” with
the phrase “are broken down by”.