Question 1 · 1.1 Structure of Water and Hydrogen Bonding
A water strider stands on a pond without sinking. Which property of water mainly makes this possible?
- A Adhesion of water to the insect's legs
- B High specific heat of the surface water
- C Surface tension produced by cohesion
- D Lower density of water at the surface
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C — Hydrogen bonds pull surface water molecules toward each other and toward the water below, forming a surface film strong enough to support a light insect. Adhesion is the tempting choice, but it describes water sticking to another substance, and a strider's legs actually repel water.
Question 2 · 1.2 Elements of Life
Three amino acids are linked into a tripeptide by dehydration synthesis, each peptide bond joining the carboxyl group of one amino acid to the amino group of the next. Before linking, every amino acid carried a heavy-oxygen (¹⁸O) label on both oxygen atoms of its carboxyl group. How many of the six labeled oxygen atoms end up in the tripeptide?
- A 2 of the 6
- B 3 of the 6
- C 4 of the 6
- D 6 of the 6
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C — A chain of three amino acids has two peptide bonds, so dehydration synthesis releases two water molecules. In each bond, the carboxyl group gives up its –OH and the amino group gives up a hydrogen, so every water takes its oxygen from a carboxyl group. Amino acids 1 and 2 each lose one labeled oxygen this way and keep the other, which stays in the peptide bond. Amino acid 3 sits at the end of the chain; its carboxyl group forms no bond, so it keeps both labeled oxygens. That leaves 1 + 1 + 2 = 4 labeled oxygens in the tripeptide, and the other 2 leave in the water. In general, n monomers form n − 1 bonds and release n − 1 water molecules, and the amino acid at the carboxyl end of the chain keeps its carboxyl group whole.
Question 3 · 1.3 Introduction to Macromolecules
Four molecules purified from sunflower seeds are each treated so that every linkage between their building blocks is hydrolyzed, and the products are identified. Based on the products, which molecules are polymers?
- A Molecule X alone
- B Molecules X and Z
- C Molecules W, X, and Z
- D Molecules X, Y, and Z
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B — A polymer is a long chain built from many repeating monomer units of the same class. X is a chain of 450 glucose units, a polysaccharide. Z is a chain of 96 nucleotides, a nucleic acid; its monomers come in four kinds, but they all belong to one class, just as a protein's 20 amino acids do. W is a fat: one glycerol joined to three fatty acids by dehydration reactions, but it is a small fixed assembly with no repeating chain, so it is not a polymer. Y is a disaccharide, sucrose: two monosaccharides joined once, far too short to count as a polymer.