Final answer:
Membrane proteins that penetrate cell membranes often have hydrophobic amino acids in their transmembrane domains.
Step-by-step explanation:
Membrane proteins that penetrate cell membranes often have hydrophobic amino acids in their transmembrane domains. These hydrophobic amino acids help anchor the proteins in the lipid bilayer of the membrane. Amino acids with charged or polar side chains are less likely to be found in the transmembrane domains, as they would interact with the hydrophobic environment of the membrane.