Upper cell membrane, illustration - stock illustration

Illustration of the outer surface of a cell membrane. Transmembrane proteins (purple) and glycolipids (yellow) protrude from the membranes surface. At upper left is an LDL (low-density lipoprotein) receptor bound to an LDL particle (large, orange and purple). To the right of the LDL receptor is a histamine channel, binding a histamine molecule. At right centre is a potassium channel with potassium (K+) ions. At front left is an acetylcholine receptor and acetylcholine molecule and at front right is a delta opioid receptor with a bound endorphin molecule.