Which of the following is true of integral membrane proteins? A. They lack tertiary structure
B. They serve only a structural role in membranes
C. They are loosely bound to the surface bilayer
D. They are usually transmembrane proteins
E. They are not mobile within the bilayer

D. They are usually transmembrane proteins

Explanation:

Membranes are made up of a phospholipid bilayer, cholesterol and various mimbrane proteins. The lipids consist of outward facing hydrophilic heads and inwards facing hydrophobic tails. Cholesterol molecules help maintain membrane fluidity.

Transmembrane proteins play an important role. These help to transport various molecules and ions like glucose, and Na+ across the selectively permeable bilayer. These are permanently embedded within the membrane as integral membrane proteins, and do not move around.