Final answer:
The amino acid most likely to be found in the domains of membrane proteins that penetrate cell membranes is Leucine. It is hydrophobic and suits the non-polar environment of a lipid bilayer's interior.
Step-by-step explanation:
The amino acid that would most likely reside in the membrane protein domains that penetrate cell membranes is B. Leucine. This is because the interior of a lipid bilayer is hydrophobic, and hence proteins embedded within the membrane tend to have hydrophobic amino acids in their transmembrane regions. Leucine is a hydrophobic, non-polar amino acid, making it well-suited for this environment. In contrast, polar and charged amino acids such as glutamine and aspartic acid are typically found on the surface of soluble proteins or in regions of membrane proteins that interface with the aqueous environment inside or outside the cell. Glycine, while being small and flexible, is not as hydrophobic as leucine.
Thus, in the hydrophobic core of membrane-spanning domains, one would expect a high proportion of hydrophobic amino acids such as leucine, valine, isoleucine, and phenylalanine. Integral membrane proteins might also show polar amino acids, but they are usually positioned such that they interact with other portions of the protein, shielding them from the fatty acid environment of the membrane, or reside in regions that do not span the membrane.