Final answer:
The structure that crosses the lipid bilayer in an integrin is a transmembrane helix. Integrins are integral membrane proteins with hydrophobic regions that interact with the membrane's hydrophobic core.
Step-by-step explanation:
In an integrin, the structure that crosses the lipid bilayer is known as a transmembrane helix. Integrins are a type of integral membrane protein which, as suggested by the name, integrates completely into the membrane structure. Their hydrophobic membrane-spanning regions interact with the phospholipid bilayer's hydrophobic region. These transmembrane regions typically consist of alpha-helices, composed of 20-25 hydrophobic amino acids, and are capable of spanning the entire membrane. These alpha-helices interact with the hydrophobic core of the lipid bilayer, allowing the integrin to maintain a stable position within the membrane.