Many proteins are embedded in the phospholipid bilayer.Channel proteins span the bilayer and form hydrophilic pores which are water-filled tubes to allow the diffusion of ions across the membrane.Carrier proteins also span the bilayer. They bind to ions such as sodium and potassium ions and molecules such as glucose and amino-acids and change shape to allow these substances to diffuse across the membrane.The change in shape may be caused by the binding and hydrolysis of ATP to the carrier protein. When this occurs it is known as active transport because it requires energy in the form of ATP.Proteins on the cell surface give the cell additional support by interacting with the aqueous layer.Some proteins may be bound to carbohydrate chains to act as receptors for hormones, antigens or may have a role in cell-cell adhesion.Some cell surface proteins are enzymes, such as dipeptidases in the ileum.This makes sure that the enzyme catalysed reaction occurs close to the site of absorption.Membrane proteins are either extrinsic or intrinsic.Extrinsic membrane proteins are on the surface of the membrane, but are bound to it by weak molecular attractions (ionic, hydrogen, or Van der Waals bonds).Intrinsic membrane proteins are embedded in the membrane.