Formation of biological membranes. Illustration of phospholipids forming biological membranes. At top left, a phospholipid molecule is shown with a hydrophilic (water-soluble) head (round) and a hydrophobic (water-insoluble) tail. Molecules with these properties are termed amphipathic. These molecules can form a layer at a water-air interface (upper left). When dispersed in water, such molecules naturally form spherical aggregations called micelles (centre left). Another possible formation is the phospholipid bilayer (bottom), which is the basis of cell membranes. For this artwork with labels, see image C023/8709.