Final answer:
Integral or transmembrane proteins are those that span across the lipid bilayer of cell membranes, providing pathways for molecules and ions.
Step-by-step explanation:
Membrane proteins that extend through the lipid bilayer are called integral proteins or transmembrane proteins. These proteins are critical components of the lipid bilayer, providing pathways for various molecules to traverse the otherwise impermeable membrane, playing a role as channels, ports, or enzymes. They are characterized by having hydrophobic regions that interact with the non-polar core of the membrane, while often also presenting hydrophilic domains that interface with the aqueous environment on either side of the membrane.