Answers to Mastering Concepts Questions
2.1
1. Which chemical elements do organisms require in large amounts?
Carbon, oxygen, hydrogen, and nitrogen 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 element’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 do all isotopes of the same element differ 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 atoms.
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 quite different levels of electronegativity and share bond electrons unequally;
- ionic bonds form between pairs of atoms that have extremely different electronegativities. 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.
5. What happens in a chemical reaction?
In a chemical reaction, molecules swap electrons and through the process form different molecules. This takes place by breaking or forming bonds between atoms.
2.3
1. How is cohesion 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.
2. Differentiate between hydrophilic and hydrophobic molecules?
Hydrophilic molecules dissolve into and mix with water, whereas hydrophobic molecules will not. Hydrophilic molecules are often polar, whereas 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, so an organism can experience high and low temperatures for an extended period before the body’s temperature will rise. Water also evaporates from the surface of the skin and helps bring down high body temperatures.
4. How do the different densities of 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, and this locks in warmer temperatures below. This means that aquatic life can survive underneath the frozen layer of a water body.
5. How does water participate in the chemistry of life?
Because cells are mostly made of water and because most cells are immersed in a film of water, water is the medium in which nearly all of life’s chemical reactions occur. Water often is a reactant or 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+. If the pH is too high a buffer would remove H+, and if the pH is too low a buffer would add H+ to maintain a constant pH in 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.
BIOLOGICAL EXAMPLES FUNCTIONS
MOLECULE
Carbohydrates Simple sugars energy source for cells
glucose, galactose,
fructose
Disaccharides
sucrose table sugar
lactose milk sugar
Polysaccharides
cellulose structure of plants
chitin hard structures of exoskeletons
starch energy storage in plants
glycogen energy storage in animals, fungi
Lipids Triglycerides saturated and unsaturated fats
Fatty acids
Sterols cholesterol, sex hormones
Waxes waterproof coverings
Proteins Actin, Myosin muscle contraction
Antibodies immunity
Enzymes speed chemical reactions in the body
Insulin controls level of glucose in blood
Keratin hair, fingernail structure
Collagen connective tissue
Nucleic Acids DNA provides the genetic code
RNA communicates DNA information,
acts as 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 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?
15N 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’s 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.
Answers to Write It Out Questions
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. The vitamin biotin contains 10 atoms of carbon, 16 of hydrogen, 3 of oxygen, 2 of nitrogen, and 1 of sulfur. What is its molecular formula?
C10H16O3N2S
3. Consider the following atomic numbers: oxygen (O) = 8; fluorine (F) = 9; neon (Ne) = 10; magnesium (Mg) = 12. Draw the electron shells of each atom, and then predict how many bonds each atom should form.
See models in text. Oxygen: 2 covalent bonds. Fluorine: 1 ionic bond. Neon: 0 bonds (its outermost shell is full). Magnesium: 2 ionic bonds.
4. 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.
5. If oxygen strongly attracts electrons, 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.
6. 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.
7. Define solute, solvent, and solution.
A solute is a substance that is dissolved in a solvent. The solvent is a liquid. Solutions consist of one or more solutes dissolved in a liquid solvent.
8. Explain why each of the following properties of water is essential to life: cohesion, ability to dissolve solutes, resistance to temperature change.
Cohesion contributes to water’s ability to resist temperature change and evaporation; it also 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.
9. 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.
10. Why would the formation of ice crystals inside a cell cause the cell to rupture?
Water expands as it freezes. Since most of the internal cytoplasm is water, when it freezes the cell would swell causing it to rupture.
11. Why are buffer systems important in organisms?
Buffer systems are important in organisms because they resist pH changes that could otherwise damage biological molecules.
12. 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.
13. 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.
14. 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.
15. 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.
16. 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).
17. Three very different proteins are silk, hair, and collagen. Chemically how are they similar, and how are they different?
All proteins are composed of amino acids. The proteins are different in the arrangement of the amino acids in the sequence, and the shape of the protein.
18. Why are proteins extremely varied in organisms, but carbohydrates and lipids are not?
A protein’s shape determines its function. Different functions would require different proteins. Also, DNA encodes proteins, so amino acid sequences are indirectly subject to mutation and natural selection.
19. Amyotrophic lateral sclerosis (also known as ALS or Lou Gehrig’s disease) paralyzes muscles. An inherited form of the illness is caused by a gene (sequence of DNA) encoding an abnormal enzyme that contains zinc and copper. The abnormal enzyme fails to rid the body of a toxic form of oxygen. Which of the molecules mentioned in this description is a: (a) protein, (b) nucleic acid, (c) bulk element, (d) trace element?
(a) The protein is enzyme.
(b) The nucleac acid is DNA.
(c) The bulk element is oxygen.
(d) The trace element is zinc or copper.
20. A man on a very low fat diet proclaims to his friend, “I’m going to get my cholesterol down to zero!” Why is achieving this goal impossible (and undesirable)?
Cholesterols are synthesized in the liver from other raw materials, so they could be created even if the friend is on a low fat diet. Cholesterols are part of cell membranes and help the body synthesize estrogen, testosterone, and Vitamin D.