Modern Biology

Problem Set #1: Answer Key

1. Consult Table 3-1 on pages 52-53 of the text for structures of functional groups.

methyl: nonpolar group found in hydrophobic amino acids that make up proteins or as side chains of fatty acids that make-up phospholipids used to form plasma membranes.

amino: polar, basic group when not charged or polar, acidic when positively charged found primarily in proteins.

carboxyl: polar, acidic when not charged or polar, basic when negatively charged found primarily in proteins.

hydroxyl: polar, neutral group found in side chain of amino acids in proteins or on carbohydrates used to make cellulose.

aldehyde: polar group found in the open chain form of the simple carbohydrates

phosphate: polar, acidic when not charged or polar, basic when negatively charged group found in phospholipids that make-up membranes.

2. Carbohydrates are uniquely suited for the formation of the cellulose cell wall since the abundance of hydroxyl groups maximize the potential for hydrogen bonding used to stabilize the interchain interactions in the fiber of the cell wall.

Fatty acids are uniquely suited to be part of the plasma membrane in the formation of a semi-permeable membrane because of the regular length (average 20 carbons) of the hydrocarbon, nonpolar, hydrophobic, linear chains attached to the single carboxyl group that can form ester bonds with the glycerol backbone.

Amino acids are uniquely suited to the formation of the wide variety of proteins used in cells because of the great variety of functional groups and structures associated with the side chains of the 20 different amino acids.

3. The calculation is based on the structure illustrated in figure 5-3 of the text. The basic assumption used to develop this structure is that there are linear hydrocarbon chains of average 20 carbon lengths formed in a symmetrical bilayer arrangement. The calculation is based on multiplying the carbon-carbon bond length by the number of carbons in the chain (20) and then multiplying that by 2 to include the symmetry of the molecule.

4. Hydrophobic molecules: any of the hydrophobic amino acids given on page 67 of the text and fatty acids or phospholipids.

Hydrophilic amino acids are any of those list in the Figure 3-17 on page 66.

An amphipathic molecule is illustrated by any of the hydrophobic amino acids on page 67 or any of the phospholipids. A molecule that has a polar end and a hydrophobic end.

5. The bacterial cell wall (composed of a peptidoglycan) is more stable than a plant cell wall (cellulose) which is more stable than the plasma membrane of an animal cell. The reason is that the peptidoglycan is completely held together by covalent bonds, while the plant cell wall is fibers of cellulose held together by hydrogen bonding and the plasma membrane is held together by the hydrophobic effect of phospholipids. Covalent bonds are approximately 20 times stronger than hydrogen bonds.