Although all cell membranes have the same general structure, the membrane composition is different for each species. Depending on their function, membranes vary in the amount of protein or the type of membrane receptors they contain. For example, plants that survive frigid temperatures, such as winter wheat, are able to increase their concentration of unsaturated phospholipids during the winter to prevent the membrane from freezing. Another example is muscle cells that have plasma membrane receptors for the neurotransmitter called acetylcholine—which “tells” the muscle when to contract.