For an introduction to the cell membrane, you need to watch one of the two following videos: 

Crash Course, In Da Club - Membranes & Transport, https://youtu.be/dPKvHrD1eS4 

If Hank Green is not to your liking you can watch the video from Bozeman Science, The Cell Membrane, https://youtu.be/S7CJ7xZOjm0