A common theme in biology is how large "macromolecules" from subunits, usually known as monomers. Really, I'm underselling this point. You will see it over and over for the next several months.
Two linked together is a "dimer," three is a trimer...you get the idea. We usually get lazy after "tetramer" and start calling them "oligomer" for groups that are in the range of 5-10 and then "polymer," when you get to many units…hundreds or thousands…even more. Hey…one common organic compound we will get to know is known as an "ester." What do you think we call a fiber made of many of these linked together?

One of the main things a membrane has to be able to do is control what flows through it. Sometime, that's just opening a channel and letting the dissolved thing flow "down gradient." But, sometime the cell has to do active work to pump ions to one side or the other of a membrane.
Once that gradient is established, it can be used to do work…and in fact used in myriad ways you can hardly imagine. Since it is so important, we will spend some time on it.
By the way, the figure is a cartoon of a protein called "aquaporin." Guess what it transports. It's a fascinating protein. Think about how you can make a protein that allows water…and only water…to pass through.