Transport proteins are proteins that move materials within an organism. They are also known as transporter proteins or carrier molecules, among other names. There are multiple types of transport proteins that move molecules with different functions in the cell, including water transporter proteins, channel proteins and ATP-powered pumps.
Hemoglobin, for example, transports oxygen molecules to tissue. Myoglobin, another transport protein, then takes oxygen from the hemoglobin and stores it until it is needed in muscle tissue.
Transport proteins are essential to the function of living beings, and very few molecules are able to cross between membranes without the aid of a protein.