[Campbell Biology P.126] An unknown amino acid has a side chain that is primarily composed o... | Practice Question

An unknown amino acid has a side chain that is primarily composed of carbon and hydrogen atoms, forming a long, unbranched aliphatic chain. Based on the classifications in Figure 5.14, which category would this amino acid most likely belong to, and why?

  • A: Polar, hydrophilic, because long chains are always water-soluble.
  • B: Electrically charged, acidic, because aliphatic chains can readily donate protons.
  • C: Nonpolar, hydrophobic, because the side chain contains only nonpolar C-H bonds, leading to poor water solubility.
  • D: Electrically charged, basic, because long aliphatic chains tend to gain protons.

Explanation

The description of the unknown amino acid's side chain primarily carbon and hydrogen, long, unbranched aliphatic chain perfectly matches the characteristics of nonpolar, hydrophobic side chains as shown in the first section of Figure 5.14 e.g., Leucine, Isoleucine . These chains are dominated by nonpolar C-H bonds, which do not interact favorably with water, making them poorly water-soluble and thus hydrophobic.