Describe how the plasma membrane controls what goes into and comes out of a cell.

The plasma membrane envelops a cell and provides a kind of barrier for substances to go into and out of the cell. The plasma membrane is composed of a lipid bilayer (that means two layers). This bilayer has many proteins and carbohydrates attached to one of the two sides (intracellular or extracellular). The main physicochemical properties of the plasma membrane are dictated by the large amount of lipids present. Small lipophilic substances (those that are soluble in lipids) will generally pass through the membrane and diffuse in or out of the cell with some ease. On the other hand, hydrophilic substances (charged substances that are soluble in water, but generally not soluble in lipids, such as ions) will generally not be able to go through the cell membrane. Some of the proteins that are in the membrane also form a kind of pore that allows specific ions and other hydrophilic substances to diffuse in or out of the cells. These proteins are generally called channels or transporters and, depending on their nature, allow for the movement of specific ions (such as K+, Ca+2, etc) or other more complex molecules, including certain drugs or glucose. The movement of these compounds in and out of the cell occurs in a highly regulated fashion.