 Hello, everybody. So today we get to start on lecture number two in the course. We're going to talk about how things get in and out of the cell. In the last lecture, we talked about homeostasis, and really in order to deal with homeostasis, we had to think about defining fluid compartments in the human body, because our definition of homeostasis was the maintenance of a relatively constant internal environment, and we defined that internal environment as the extracellular fluid in your body. Today, we're going to look at the single most critical structure that mediates the movement or communication or action between fluid compartments, between the extracellular fluid and the intracellular fluid. So just in case you don't know what the topic of today's lecture is, think about that for a second. What structure mediates interaction between extracellular fluid and intracellular fluid? There's one structure that you should know that you've rocked the general bio already. The one thing that separates intracellular fluid the inside of the cell from extracellular fluid, the outside of the cell is the cell membrane. And today, we're going to look at many different ways that the cell membrane carries out this task. Keeping some stuff out, allowing some stuff in, like investing in getting stuff in, or investing in creating concentration gradients that let you get other things in. It's really no surprise. Human physiology is just so freaking cool. So think about the cell for a minute. This video, if you have not seen this video, it's put together by this ex vivo group and they did this animation of actual cellular processes that is mind blowing. The three minute version has beautiful music, makes me cry every time. I try not to get too excited. I'm not actually going to show it to you here because you really should go watch it so that you can cry in peace and I won't cry in front of you. But I found an eight minute version. What? Where they actually narrate the entire process. Which by the end of the semester, you guys are actually going to understand some of the processes that are happening here, which is really cool. Okay, so let's get started. Let's do a quick review on the cell membrane. Like what? What is the cell membrane? What's its structure? And how does it carry out its task of keeping the intracellular fluid separate from the extracellular fluid so that the cell can function?