Chapter 2. Chapter 2: Chemistry

The mass of an electron is so small, less than one-twentieth of one percent of the weight of a proton, that its mass can be ignored when calculating the mass of an atom.

Atoms are most stable when their outermost shell, called the valence shell, is full. Bonding with another atom will not necessarily create a more stable atom than currently exists and could potentially produce a more unstable atom with a partially full valence shell.

Atoms are most stable when their outermost shell, called the valence shell, is full. Since the first shell holds a maximum of two electrons, a hydrogen bonded covalently to another hydrogen will fill its valence shell as each atom shares its single valence electron, forming more stable atoms within the H₂ molecule.

Covalent and ionic bonding involve the sharing or transfer of electrons, respectively. Hydrogen bonding involves the attraction between hydrogen and another molecule as a result of weak opposite charges, which is a much weaker attraction.

Water is a polar molecule; the region of the hydrogen atoms is slightly positive and the region of the oxygen is slightly negative. Other polar molecules and ionic substances will readily dissolve in water because charged regions or ions are attracted to the oppositely charged regions of the water molecules.

Vomit tastes sour because the stomach produces hydrochloric acid, which causes the stomach contents to have a pH of 1–3.

The chemical bonds of carbohydrates are readily broken, releasing energy that the cell can use to perform cellular activities. Excess carbohydrates can be stored for future use.

The three possible fates of glucose are: (a) breakdown for immediate energy needs, (b) storage as glycogen, or (c) conversion to fats.

An enzyme in saliva breaks the bonds of starch to produce sugar, which tastes sweet. Since it takes time for the enzyme to break these bonds, the degree of sweetness will increase over time as the sugar concentration increases.

Humans, as well as other mammals, do not produce the enzymes capable of breaking the chemical bonds of cellulose, preventing it from being digested and absorbed.

Lipids contain many more carbon–hydrogen bonds than carbohydrates. These bonds are rich in energy.

In animals, including humans, a strong taste preference for fats has evolved because fats store so much energy.

Phospholipids form the lipid bilayer of the plasma membrane.

Any two of the following functions are acceptable answers. Structural: Proteins are used to build structures such as hair and nails, among others. Protective: Proteins are used to form antibodies in an immune response, as well as forming the components of blood clots. Regulatory: Proteins form some types of hormones, which are chemical messengers, and almost all enzymes, the biological catalysts that speed up chemical reactions. Contractile: Proteins form contractile fibers used in the contraction of muscle cells and movement of cilia and flagella. Transport: Proteins assist with the transport of substances around the body, such as transport of oxygen by hemoglobin in the blood.

Essential amino acids cannot be synthesized by the body, but must be consumed in the diet.

The amino acid sequence determines the three-dimensional shape of a protein, and the shape of a protein determines the function of the protein. As the shape of a protein changes, so does its function.

If the shape of an enzyme changes, the active site may no longer be able to bind its specific substrate, preventing a chemical reaction from occurring.

Nucleic acids store information by varying which base is attached at each position in the backbone of the DNA molecule. The specific sequence of nucleotides determines the particular protein produced.

The base sequence determines the genetic information contained in a segment of DNA; if the base sequence changes, so does the genetic information it contains. Researchers working on the Human Genome Project presented the sequence of only one DNA strand because the complementary base sequence of the other DNA strand can be inferred from this strand.

(a) The sugar in DNA is deoxyribose, whereas the sugar in RNA is ribose. (b) DNA forms double strands, whereas RNA generally forms single strands. (c) The base thymine is found only in DNA, and the base uracil only in RNA.