The right answer is protein.
The plasma membrane constitutes a dynamic structure. The main model describing this dynamic is the fluid mosaic (proposed by Singer and Nicholson in 1972). According to this model, phospholipids and membrane proteins are free to move within the membrane.
Between different cell types, the amounts and types of proteins in the plasma membrane are extremely variable. Structural and functional differences also exist between the plasma membrane and the intracellular membranes of organelles (mitochondria, nucleus, etc.).
