The cell membrane is a semipermeable lipid bilayer that surrounds the cytoplasm of all cells. In animal cells, it is the outermost layer of the cell. In plants, fungi and some bacteria, a cell wall surrounds the cell membrane to form the cell's outermost layer.
The cell membrane protects the integrity of the remainder of the cell by selectively allowing substances to pass in and out of the structure. Its primary components are lipids and proteins. Phospholipids automatically arrange themselves to shield the cell from its surrounding polar liquids. Proteins in the cell wall form gates that allow the cell to absorb fluids and other molecules as needed.
Cholesterol is one of the lipids found in the cell walls of animals but not in plants. Depending on the location of the cell in the body, the membrane contains from 20 to 80 percent cholesterol. While some cell membranes are smooth, others develop microvilli, fingerlike projections that increase their surface area and their ability to absorb nutrients from the outside. Other extensions of the cell wall allow the cell to sense its environment or aid in locomotion of the cell, especially in single-celled organisms. In multicellular organisms, the cell walls attach to other cells to form tissues.