In animal cells cholesterol is normally found dispersed in varying degrees throughout cell membranes, in the irregular spaces between the hydrophobic tails of the membrane lipids, where it confers a stiffening and strengthening effect on the membrane.  Additionally, the amount of cholesterol in biological membranes varies between organisms, cell types, and even in individual cells. Cholesterol, a major component of animal plasma membranes, regulates the fluidity of the overall membrane, meaning that cholesterol controls the amount of movement of the various cell membrane components based on its concentrations.  In high temperatures, cholesterol inhibits the movement of phospholipid fatty acid chains,causing a reduced permeability to small molecules and reduced membrane fluidity. The opposite is true for the role of cholesterol in cooler temperatures. Cholesterol production, and thus concentration, is up-regulated (increased) in response to cold temperature. At cold temperatures, cholesterol interferes with fatty acid chain interactions. Acting as antifreeze, cholesterol maintains the fluidity of the membrane. Cholesterol is more abundant in cold-weather animals than warm-weather animals. In plants, which lack cholesterol, related compounds called sterols perform the same function as cholesterol. 
The cell is highly organized with many functional units or organelles. Most of these units are limited by one or more membranes. To perform the function of the organelle, the membrane is specialized in that it contains specific proteins and lipid components that enable it to perform its unique roles for that cell or organelle. In essence membranes are essential for the integrity and function of the cell.
All plasma membranes share the characteristic of being selective permeable and containing various proteins. Some membranes, however, have components that are specialized for a specific purpose. Microvilli are finger-like projections on the surface of some cells. These projections increase surface are for absorption. Cells that line the small intestine contain microvilli. Junctions are another specialized group of proteins that connect to other cells. There are three main types of junctions: tight junctions, desmosomes, and gap junctions. Tight junctions are proteins that hold adjacent cells together very tightly so nothing can penetrate between them. Cells that line the digestive and urinary tract contains many tight junctions to ensure the contents within those hollow organs do not leak out into the outer layers or body cavity. Desmosomes are sometimes called anchoring junctions. These junctions hold cells together by fibers, which allows movement without separation. Cells that contain desmosomes are found within the muscle tissue and the skin. Gap junctions are open areas within the plasma membrane found between two adjacent cells. The proteins connect two cells while allowing chemicals to pass between the cells.