Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 3 (p. 58)

1. Organic macromolecules are formed through dehydration synthesis. This involves splitting out water molecules, which join individual saccharide, amino acid, fatty acid, or nucleotide units to form carbohydrates, proteins, fats, and nucleic acids, respectively. Hydrolysis is the reverse, inserting water molecules between macromolecules units, converting them into their components.

2. Amino acids aggregate to form proteins through peptide bonds.

3. The primary structure of a protein is its amino acid sequence. The primary sequence then usually forms sheets or coils, forming the secondary structure. These sheets and coils subsequently fold back on themselves forming the tertiary structure of a protein. Several folded domains of a protein can interact with each other to form a quaternary structure.

4. Nucleotides are composed of a phosphate group, a sugar, and a nitrogenous base. Nitrogenous bases are covalently bound to the sugars, and the sugars and phosphates alternate to form a sugar-phosphate backbone, from which the nitrogenous bases project. The sugars and phosphates are joined by covalent phosphodiester bonds.

5. Purines are double-ring nitrogenous bases: adenine and guanine. Pyrimidines are single-ring nitrogenous bases: thymine and cytosine (uracil, in RNA). Adenine always pairs with thymine (or uracil, in RNA), and cytosine always pairs with guanine.

6. Fats are composed of long chains of fatty acids (three chains in triglycerides bound to a molecule of glycerol).

7. Fat molecules that have as much hydrogen as possible bound to their fatty acid chains (by virtue of the fact that they have no double bonds) are said to be saturated (e.g., with hydrogen). Unsaturated fats have one or more double or triple bonds in at least one of their fatty acids chains.

8. Glucose, fructose, and galactose are considered isomers because while they have the same empirical formula (C₆H₁₂O₆), their structures are slightly different. Glucose and fructose are structural isomers, containing a double-bonded oxygen in different place. Glucose and galactose are stereoisomers because their structural formula is the same except for the orientation of a single hydroxyl group.