 Now, notice that these blue molecules are diffusing directly across the cell membrane, and there are some substances that can do this. Let's make a list, and here's how I'm going to title my list. Who, who can diffuse across the cell membrane? Who can do it? All right, I'm going to give you a list of characteristics. Number one, little guys, the smaller the molecule, the more likely it is going to be able to directly diffuse across the cell membrane. I'm going to give you an example of a couple of small molecules. These guys can diffuse directly across the cell membrane in either direction, like just down their concentration gradient. Can you visualize that? Down the concentration gradient. I'm going to write that down because I don't want anybody to not understand that. Down the concentration gradient, that means from a high concentration to a, oops, low concentration. That works, right? If you go from a high concentration to a low concentration, you're going down the concentration gradient. If you go against the concentration gradient, then that's going to be the opposite. You're cool with that. So if I go from a low concentration to a high concentration, molecules aren't going to do that very well. I mean, some molecules do, but they're countered by many more coming the other direction. So the net movement is always by itself going to be down a concentration gradient. Small molecules can diffuse down their concentration gradients directly across the cell membrane. However, they can't go against their concentration gradient. So if there's a whole bunch of oxygen outside the cell and not very much inside, the oxygen from inside can't diffuse out. Did you follow that? Because that's going from low concentration to high concentration outside. Outside oxygen is going to go into the cell down its concentration gradient, even though it can cross the membrane on its own. Another example of a small molecule is water. And water can diffuse across the cell membrane, even though it is polar. And I'm going to tell you why the polarness matters because the next characteristic is lipophilic or hydrophobic molecules can diffuse across the cell membrane. So fats can diffuse across the cell membrane. Hydrophobic and lipophilic are the same general terms. Hydrophobic substances tend to be lipophilic because they like fats. Substances that like fats don't tend to like water. An example of this would be like a steroid hormone. A steroid hormone, it's one of the reasons why taking steroids is bad news. A very small amount of steroid hormone can have a huge impact on your body because steroid hormones can just go right through the cell membrane. Your cell can't regulate whether or not they feel like being affected by steroid hormones. If there's steroid hormones in your mix, they're going to diffuse directly through the cell membrane into your body and cause the effect that they are meant to cause, no matter what, because they are hydrophobic and they can diffuse across the cell membrane, essentially regardless of size. So small guys and lipophilic guys. Now, if you look at water, you would think, okay, it is small, but it's also not hydrophobic. And so it's kind of in that in-between stage. We do put water channels into our cells to make water go back and forth easier. Sometimes we want a cell to be waterproof, but not very often. Okay, so those are your rules that you will use to evaluate whether or not a substance can just diffuse across the cell membrane on its own. If it can't diffuse across the cell membrane all by itself, then we might need a transporter to help make it happen, and that is facilitated diffusion. So let's look at that next.