And number three is there is asymmetry as a result of a difference in the enzymatic activities of the two sides. Basically, those reactions that take place on this side of the membrane don't take place on this side and vice versa. For instance, if a protein needs to use ATP molecules, the ATP molecules are only found in the cytoplasm. And so that protein component can only interact with the ATP from the cytoplasmic side of the membrane, as we'll discuss in more detail in just a moment. Now, why is asymmetry so prevalent? So every single plasma membrane basically is asymmetric.