So we know that the plasma membrane is a fluidlike structure. And what determines the fluidity is the relative movement of all the molecules, the phospholipids and proteins that exist within that membrane. So the more movement we have, the more fluid that membrane is. The less movement we have, the more rigid that membrane is. Now, if the fluidity is determined by the relative movement, what determines the relative movement itself? Well, the relative movement of the molecules within the membrane is determined by the strength of the attractions, the non covalent intermolecular bonds that exist between the phospholipids, the molecules in the membrane.