Many types of proteins are associated with a cell membrane, and each type adds a specific function to it, different cell membranes can have different characteristics depending on which proteins are associated with them. For example, a plasma membrane has certain proteins that no internal cell membrane has. Many plasma membrane proteins are enzymes. Others are adhesion proteins, which fasten cells together in animal tissues. Recognition proteins function as identity tags for a cell type, individual, or species. Being able to recognize “self” means that foreign cells (harmful ones, in particular) can also be recognized. Receptor proteins bind to a particular substance outside of the cell, such as a hormone or toxin. Binding triggers a change in the cell’s activities that may involve metabolism, movement, division, or even cell death. Receptors for different types of substances occur on different cells, but all are critical for homeostasis. Additional proteins occur on all cell membranes. Transport proteins move specific substances across a membrane, typically by forming a channel through it. These proteins are important because lipid bilayers are impermeable to most substances, including ions and polar molecules. Some transport proteins are open channels through which a substance moves on its own across a membrane.