When the aortic semilunar valve closes and blood bounces off of it, it produces the heart sound that is ultimately related to when the

The heart sounds are primarily produced when the valves (i.e. semilunar valves, tricuspid valve, mitral valve) close rather than the sound of the actual contraction. In the closure of the aortic semilunar valve, the second heart sound is produced or the S2; in contrast to the closure of the mitral and tricuspid valves which produce the first heart sound or S1. The S2 is ultimately related to the diastole which corresponds to the filling phase of the ventricles.

The cell membrane is selectively permeable, meaning it only lets certain things in and out of the cell. If you think about our building analogy, this can be a very important thing. You wouldn't want just anyone running in and out of the White House, for example. It's important to have guards who determine who's allowed in and who's not. A cell is a living thing and needs just the right balance of nutrients and water, called homeostasis. The selective permeability of the membrane allows the cell to stay in homeosis.