Which of the following best describes the structure of a biological membrane? two layers of phospholipids with proteins embedded between the two layers a mixture of covalently linked phospholipids and proteins that determines which solutes can cross the membrane and which cannot two layers of phospholipids with proteins either spanning the layers or on the surface of the layers a fluid structure in which phospholipids and proteins move freely between sides of the membrane two layers of phospholipids (with opposite orientations of the phospholipids in each layer) with each layer covered on the outside with proteins

The best description of the structure of a biological membrane is: two layers of phospholipids with proteins either spanning the layers or on the surface of the layers. Biological membranes are composed of a phospholipid bilayer with embedded proteins. The phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, which arrange themselves in a bilayer with the tails pointing inward and the heads pointing outward. The embedded proteins can span the membrane (integral proteins) or be attached to the surface (peripheral proteins). This structure allows the membrane to be selectively permeable, controlling the movement of substances in and out of the cell.