The boundary of a living, metabolizing cell is surprisingly similar to that hedge. The cellular membrane, once thought to be an inert barrier, is one of the most dynamic parts of the cell. The membrane and the plethora of proteins that stud its surface direct the exchange of information between cells, tightly control the flow of materials from inside to outside the cell, and provide surfaces for some of life's most vital chemical reactions.

Illustration: Jillian Tamaki