Topic Test : Molecules of Living Systems            [answers are given below]

1.      Covalent bonds are formed by sharing electrons in the outer shell of atoms.
2.      Polar molecules do not carry a net charge.
3.      The concentration of H+ in pure H 2O is 10 -7 M. 

Multiple Choice
4.   The ability of water to remain a liquid at room temperature (25 o C) is due to
     a)      hydrogen bonding between oxygen atoms of two H 2O molecules
     b)      hydrogen bonding between the hydrogen atoms of two H 2O molecules
     c)      hydrogen bonds between the hydrogen and oxygen atoms of two H 2O molecules
     d)      the nonpolar nature of the covalent bonds in a molecule of H 2O

5.   Small organic molecules such as sugars, amino acids, fatty acids and nucleotides make up
    approximately  ________ of the mass of a cell.
a)      10%        b)      25%        c)    50%            d) 90% 

6.      The chemistry of life is based on the chemistry of carbon because
a)      carbon has the ability to form ionic bonds with other atoms
b)      carbon can form up to four covalent bonds with other atoms
c)      carbon-carbon bonds are highly unstable
d)      the covalent bonds between carbon and other atoms are highly reactive

Short Answer/Essay

7.  Most biological macromolecules are made up of only a few elements, mostly carbon, hydrogen, oxygen, nitrogen, phosphorous and sulfur. What is there about the chemistry of carbon molecules that allows for the tremendous diversity of macromolecules in the cell?

8.  Small organic molecules in the cell are used as monomers to synthesize more complex polymeric macromolecules, which are eventually broken down to recycle the monomers. How might this ability allow cells to respond more rapidly to changes in their surroundings?

9.  Small excursions in intracellular pH may have dramatic effects on cell structure and function. How does the cell prevent changes in intracellular pH?
















Topic Test : Answers    Molecules of Life

1.      True.

2.      False. Polar molecules carry either a positive (+) or negative (-) charge.

3.      True. 

4.      c. The H2O molecule is a partial dipole, with the H atoms carrying a partial positive charge and
    the O atom a partial negative charge. Hydrogen bonding between the H and O of two H2O
    molecules is responsible for the life supporting properties of H2O

5.      a. The small organic molecules account for only about 10% of the mass of a cell. They are
    recycled to build complex macromolecules.

6.      b. Carbon can form up to four stable covalent bonds with other atoms 

7.      Carbon has four unpaired electrons in its outer energy shell and can form up to four covalent
    bonds with other atoms. One carbon atom can bind to another carbon atom forming
    carbon-containing backbones, which may be linear, branched or cyclic, generating large and
    complex molecules. This allows the versatility to synthesize the different molecules needed by

8.      Cell survival depends on the ability to respond to changes in the cellular environment by
    synthesizing new macromolecules. By maintaining a pool of small organic molecules through
    recycling, the monomers are readily available for rapid synthesis of complex molecules. 

9.      Many of the metabolic reactions that occur in cells produce or take up H+ ions. Certain cellular
    molecules (proteins, HCO3-) act as buffers to maintain a relatively constant cellular pH. Changes
    in cell pH can affect cell function by disrupting protein conformation or slowing the rates of
    metabolic reactions.