So there are many types of transport proteins. For instance, we have pumps that use energy, we also have channels and so forth. And we'll discuss these in much more detail in a future lecture. Now, proteins are the molecules that basically act in signal transduction. So, for instance, some type of hormone can bind onto a protein and that's the protein that will create that signal inside the cell, that will lead to some type of process inside the cell. So proteins function as transporters.