AP Biology Essential ChemistryName:

Refresh your understanding by refering to pages 37–41 of Biology, 7th edition, which describe types of chemical bonds: nonpolar and polar covalent bonds, ionic bonds, hydrogen bonds. The molecule of acetic acid diagrammed here can also be represented by the formula CH3COOH.

The best way to determine the types of bond is to determine each atom’s electronegativity, or its attraction for electrons. As a general rule, the more filled the outer electron shell of an atom is, the higher is its electronegativity. In addition, the fewer electron shells, the greater the electron negativity. As a result, an atom’s attraction for electrons increases as you go from left to right in the periodic table. Electronegativity values tend to decrease as you go from top to bottom of the periodic table. To determine whether bonds are ionic, polar covalent, or nonpolar covalent, you need to determine the difference in electronegativity between the atoms that make up a molecule. If the difference in electronegativity is small, the bond is likely to be nonpolar covalent. If the difference is very large, the bond is likely to be ionic. Intermediate differences produce polar covalent bonds. The table lists specific electronegativity values for selected elements.

Using specific electronegativity values, you can determine the type of bond:

  • If the difference in electronegativity between two atoms in a compound is less than 0.5, the bond is nonpolar covalent.
  • If the difference is between 0.5 and 1.6, the bond is polar covalent.
  • If the difference is greater than 1.6, the bond is ionic.

Understanding Acids

In addition to being polar, water molecules can dissociate into hydronium ions (H3O+, often described simply as H+) and hydroxide ions (OH−). In a neutral solution at room temp the (H+) concentration is 10-7 and the (OH-) is 10-7. Thus -log 10-7 and –log 10-7 gives us 10-14 and why our pH scale is from a 0-14 range. Substance within a 0-6.9 range are considered Acidic and those that are 7.1 to 14 are basic.

The negative 10 to a negative number means the number of (H+) in solution. This also implies the concentration of –OH. Confusing. . . right? Not really. Here is why. . . pH stands for the power of H. Look at the first example below and fill in the blanks.

So say:

(H+) / (OH-) / (PH) / Acid, Base or Neutral
10-3 / 10-11 / Is 3 because pH stands for the power of H+ thus if the scale is out of 14 the OH- must be 11 because -3+-11 gives us 14! / Acid
10-8
10-2
10-9
10-13
10-7

2. O2 and NH3 are both small covalent molecules found in cells. NH3 is extremely soluble in the aqueous environment of the cell, while O2 is relatively insoluble. What is the basis for this difference in solubility between the two molecules? In reaching your answer, refer to the structures of the molecules as valence shell diagrams. Given these diagrams, consider the types of interactions each molecule could have with water.

3. Life as we know it could not exist without water. All the chemical reactions of life occur in aqueous solution. Water molecules are polar and are capable of forming hydrogen bonds with other polar or charged molecules. As a result, water has the following properties:

A. H2O molecules are cohesive; they form hydrogen bonds with each other.

B. H2O molecules are adhesive; they form hydrogen bonds with polar surfaces.

C. Water is a liquid at normal physiological (or body) temperatures.

D. Water has a high specific heat.

E. Water has a high heat of vaporization.

F. Water’s greatest density occurs at 4°C.

Explain how these properties of water are related to the phenomena described in the statements below. More than one property may be used to explain a given phenomenon.

  1. During the winter, air temperatures in the northern United States can remain below 0°C for months; however, the fish and other animals living in the lakes survive.
  1. Many substances—for example, salt (NaCl) and sucrose—dissolve quickly in water.
  1. When you pour water into a 25-ml graduated cylinder, a meniscus forms at the top of the water column.
  1. Sweating and the evaporation of sweat from the body surface help reduce a human’s body temperature.
  1. Water drops that fall on a surface tend to form rounded drops or beads.
  1. Water drops that fall on your car tend to bead or round up more after you polish (or wax) the car than before you polished it.
  1. If you touch the edge of a paper towel to a drop of colored water, the water will move up into (or be absorbed by) the towel.

AP Biology Essential ChemistryName:

Refer to the figure (Some Simple Chemistry) on the next page when doing this activity.

Part A. Answer the questions. Then use your answers to develop simple rules for identifying carbohydrates, lipids, proteins, and nucleic acids.

4. What is the approximate C:H:O ratio in each of the following types of

macromolecules?

Carbohydrates / Lipids / Proteins / Nucleic Acids

5. What other elements are commonly associated with each of these four types of

macromolecules?

Carbohydrates / Lipids / Proteins / Nucleic Acids
Always contains P
Usually contains no P
Always Contains N
Usually contains no N
Often contains S
Rarely contains S

6. Functional groups can modify the properties of organic molecules. In the table below, indicate whether each functional group is polar or nonpolar and hydrophobic o rhydrophilic. Which of these functional groups are found in proteins and lipids? Use the functional group information in chapter 4 of your text p.64-65).

Functional Group / Name of Functional Group / Function of the group / Polar or Non-Polar / Hydrophobic or Hydrophilic / Found in all Proteins / Found in many Proteins / Found in lipids
-OH
-C=O
-- H
-COOH
-NH2
-SH
-PO4

AP Biology Essential ChemistryName:

7. Twenty amino acids are commonly utilized in the synthesis of proteins. These aminoacids differ in the chemical properties of their side chains (also called R groups). Whatproperties does each of the following R groups have? (use the electronegativity table on page 1) (Note: A side chain may displaymore than one of these properties.)

8. Hershey and Chase became famous for figuring out that DNA was the hereditary material.

The Hershey-Chase Experiment – Bacteriophage

1. Hershey and Chase forced one population of phages (viruses that invade bacteria) to synthesize DNA using radioactive phosphorous.

2. The radioactive phosphorous "labeled" the DNA.

3. They forced another group of phages to synthesize protein using radioactive sulfur.

4. The radioactive sulfur "labeled" the protein.

5. Bacteria infected by phages containing radioactive protein did not show any radioactivity.

6. Bacteria infected by phages containing radioactive DNA became radioactive.

7. This showed that it was the DNA, not the protein, that was the molecule of heredity.

Experiment A
Proteins contain sulfur but not phosphorous
/ Experiment B
DNA contains phosphorous but not sulfur.

You want to use a radioactive tracer that will label only the protein in an RNA virus.

Assume the virus is composed of only a protein coat and an RNA core. Which of the following would you use? Be sure to explain your answer referring to the Hershey Chase Experiment.

a) Radioactive P

b) Radioactive N

c) Radioactive S

d) Radioactive C

Create simple rule(s) can you use to identifythe following macromolecules?

Carbohydrates
Lipids
Proteins
Nucleic acids
DNA vs. RNA