Final answer:
Transmembrane proteins have hydrophobic transmembrane segments that allow them to integrate into the phospholipid bilayer, and they may expose regions to either the interior or exterior of the cell.
Step-by-step explanation:
The portions of transmembrane proteins that cross the lipid bilayer are predominantly hydrophobic transmembrane segments consisting of 20-25 amino acids. These proteins can either span part of the membrane or stretch from one side to the other, exposing regions on either the cytosol or extracellular fluid.
Such integral proteins are crucial for various functions, including acting as channels for ions and other molecules that otherwise cannot pass through the phospholipid bilayer. The structure of these proteins is adapted to the membrane environment, with hydrophobic regions interacting with the interior of the bilayer, and hydrophilic regions protruding to allow interaction with aqueous solutions inside or outside the cell.