When a membrane is freeze-fractured, the bilayer splits down the middle between the two layers of phospholipids. in an electron micrograph of a freeze-fractured membrane, the bumps seen on the fractured surface of the membrane are

integral proteins.

Explanation:

An integral protein is a protein molecule (or set of proteins) that is permanently attached to the plasma membrane. These proteins cross the membrane and are surrounded by "annular" lipids that allow these proteins to be visualized in an electron micrograph of a membrane. Such proteins may leave the biological membrane through the use of detergents, non-polar solvents, or even denaturing agents.