The cell membrane is a thin, double layer consisting of fatty lipids and protein that separates the cell interior from its surroundings, and controls what goes in and out of the cell. The cell membrane is the outermost layer of animal cells, but an additional covering--called the cell wall--encapsulates organisms such as bacteria, yeasts and plants. Cell walls are made of sturdy materials that provide strength so cells can maintain their shape and live in harsh environments. Because maintaining an intact membrane is key to survival, scientists are looking closely at how cells repair outer membranes injured by wounds or infection and how membrane passageways work to balance the amount of vital substances inside and outside the cell.