When viewed with an electron microscope, proteins appear to penetrate into the hydrophobic interior of the membrane. A membrane is a fluid mosaic of lipids, proteins and carbohydrates The Fluid Quality of Membranes Membranes are held together by hydrophobic interactions, which are weak attractions. Most membrane lipids and some proteins can drift laterally within the membrane. Molecules rarely flip transversely across the membrane, because hydrophilic parts would have to cross the membrane's hydrophobic core. Phospholipids move quickly along the membrane's plane, averaging 2 m m per second.
Membrane proteins drift more slowly than lipids.
The fact that proteins drift laterally was established experimentally by fusing a human and mouse cell. Membrane proteins of a human and mouse cell were labeled with different green and red fluorescent dyes. Cells were fused to form a hybrid cell with a continuous membrane. Hybrid cell membrane had initially distinct regions of green and red dye. In less than an hour, the two colors were intermixed.