Final answer:
The amino acids of GPCRs crossing the plasma membrane are hypothesized to be hydrophobic to interface with the lipid bilayer, while hydrophilic amino acids would be prevalent in hydrophilic regions exposed to an aqueous environment, such as channel linings for ion transport.
Step-by-step explanation:
In the context of amino acids of G protein-coupled receptors (GPCRs) that cross the plasma membrane, one can hypothesize about their hydrophobic or hydrophilic nature. The amino acids within the spans of the membrane are typically hydrophobic to interact effectively with the fatty acid tails of the phospholipid bilayer. In contrast, the amino acids that are exposed to the aqueous environment outside the cell or within the cytosol are hydrophilic to enable interaction with water and other polar molecules.
For a soluble protein, we would expect the surface-exposed amino acids to be hydrophilic as well, which helps to stabilize the protein in its aqueous environment. On the other hand, hydrophobic amino acids are commonly found in the interior of these proteins, away from the aqueous environment, maintaining the protein's structural integrity. When considering proteins embedded in a lipid bilayer, the membrane-spanning regions would predominantly contain hydrophobic amino acids, while hydrophilic amino acids would line the aqueous channels and are involved in the transport of ions or water across the membrane.