Proteins, represented as purple blobs in this close-up of an animal cell, are the most complicated molecules known. A cell contains thousands of kinds of proteins, which carry out a variety of functions. In most cases, a protein's function depends on its complex three-dimensional structure. Let’s see a demonstration of each of the protein molecules shown here and learn about its function. Observe how function depends on protein shape and changes in shape. Structural proteins have many functions. Like tent poles and ropes, they shape cells and anchor cell parts. They may serve as tracks along which cell parts can move. They bind cells together, making organized units such as muscles, ligaments, and the tendons that bind muscles to bones. The silk of spiders and the hair of mammals are also structural proteins. Signal proteins include hormonal proteins that help coordinate an organism's activities by acting as signals between cells. For example, insulin, a hormonal protein secreted by the pancreas, signals an animal's cells to take in sugar. The hormone receptor is also a protein. Signal molecules bind to receptor proteins that can then relay messages inside a cell. Receptors are thus important links in the system of communication among cells. Some signal molecules are also proteins. Transport proteins carry molecules from place to place. The example shown here allows certain solute molecules to enter the cell. Hemoglobin is the transport protein that carries oxygen in the blood. Sensory proteins detect environmental changes such as light, and respond by emitting or producing signals that call for a response. An enzyme is a protein that changes the rate of a chemical reaction without itself being changed into a different molecule in the process. Enzymes promote and regulate virtually all chemical reactions in cells. Storage proteins stockpile building components that cells can use to make other proteins. Storage proteins in seeds provide raw materials used by the developing plant – unless an animal eats them first! Ovalbumin the main substance in egg white, serves as a storage protein for developing chick embryos. Contractile proteins, like the motor protein shown here, can move parts of a cell. Other contractile proteins, working together in muscle cells, can move a whole animal. Gene regulatory proteins bind to DNA in particular locations and control whether or not certain genes will be read. This allows cells to become specialized for different functions, and respond to changes in their surroundings. The immune system makes defensive proteins called antibodies that bind to invaders (such as the virus shown here) and mark the foreign objects for destruction.