Lipid bilayer model was proposed for its structure but it was modified and new structure is according to fluid mosaic model. Plasma membrane is selectively permible as it selects small molecules like lipid etc (composition is also of lipid and as solubility principle like dissolve like) so large and charged molecules like plasma and ion can't pass through it. Lipid are in form of layer so at the chain when ever other phosphate group attach the place conjugate molecule (phospholipid ) formed
