Some proteins may move within the plane of the membrane while others are anchored to structures in or near the membrane. The result is that the membrane is actually fluid and has the consistency of a light grade oil. It has been termed a fluid mosaic, mosaic because there is a definite pattern to it, fluid because the lipids are free to move about on each side of the membrane. Lipids do not genrally switch sides, moving from outside to inside or inside to outside. This arrangements confers a number of properties on the membrane which allow it to perform many functions.
Functions Retains the cytoplasm.
The concentration of solutes, sugars, ions etc. are much higher within the cell than outside. A fundamental principle of nature, however is that solute concentrations will tend to equilibrate. The cell membrane prevents free flow of material and thus serves as an osmotic barrier Selective barrier Since the cell is separated from its environment and needs to get nutrients in and waste out, the membrane must be able to accommodate this. It acts as a selective barrier. Some molecules can cross the membrane without assistance, most cannot.