Final answer:
Porin proteins, which form large, water-filled pores, fold into beta barrel structures. The amino acids on the outside of the barrel have hydrophilic side chains.
Step-by-step explanation:
Porin proteins, which form large, water-filled pores in mitochondrial and bacterial membranes, fold into beta barrel structures. The amino acids that face the outside of the barrel have hydrophilic side chains. These hydrophilic side chains are exposed to the fluid environments of the extracellular fluid and cytosol.