Final answer:
Integral membrane proteins enter the lipid bilayer by being permanently embedded within the plasma membrane. They can act as channels or carrier proteins, facilitating the passage of certain substances across the membrane.
Step-by-step explanation:
Integral membrane proteins enter the lipid bilayer by being permanently embedded within the plasma membrane. The hydrophobic regions of these proteins interact with the hydrophobic regions of the lipid bilayer. Some proteins act as channels, allowing small ions and molecules soluble in water to enter and leave the cell. Others act as carrier proteins, facilitating the passage of certain molecules across the membrane through specific interactions with the protein.