All living organisms are comprised of one or more cells. Most animal cells range in size between 10 and 100 micrometers and have several key elements.

The outer layer of a cell, the cell membrane, consists of a phospholipid bilayer, which serves primarily as a barrier from the external environment, and integral proteins which function mainly to regulate transport. The cell membrane creates an enclosed space in which the chemical processes, or metabolism, of the cell can occur. It also serves as a gatekeeper for the cell, carefully regulating what passes in and out. For example, nutrients pass in, and metabolic waste passes out.

The inside of an animal cell contains several organelles, specialized structures that perform specific functions for the cell. These organelles are suspended in a thick aqueous solution, the cytoplasm. The coordinated activity of the organelles is required for the survival of the cell. Many organelles are...