And although proteins can also move relatively quickly along the lateral direction, some proteins are actually immobilized because of attachments to other structures. Now, although these phospholipids can actually move transversely, they can flip flop. The rate at which they flip flop actually takes place at a much lower rate than lateral diffusion. And proteins cannot diffuse altogether because of that high energy barrier that must be overcome. The polar region, the extensive polar region of the protein cannot interact in a favorable way with the hydrophobic core of that lipid bilayer membrane. Now, in fact, inside our membranes, we have these special proteins known as flipases that actually allow the movement of these phospholipids across that membrane.