In addition to forming a barrier keeping desired materials inside and unwanted materials outside, they must also permit the controlled import and export of specific chemicals. Moreover, membranes must be able to fuse with transport vesicles, or to create vesicles. Thus, membranes must fulfill a variety of criteria. Living organisms construct membranes capable of performing these functions from a mixture of lipids and proteins, where the lipids form a bilayer in which the proteins are embedded.
Substances move in and out of cells either by diffusion, or by active transport.
Diffusion is the net movement of chemicals to regions where their concentration is lower as a result of random motions. The first part of this week's lab will demonstrate this effect. In particular, osmosis is the diffusion of water across a membrane which is freely permeable to water but not to some other chemicals. If certain molecules are trapped inside a membrane-bound vesicle which is placed in water, the concentration of water inside the vesicle will be lower than that of the surroundings. In this situation we say that the vesicle is hyperosmotic to its surroundings, and water will flow into the vesicle by osmosis along its concentration gradient.