The cells are the basic building blocks of all living things. The human body is composed of trillions of cells. As the cell diagram illustration suggests, a cell provides a structure for the body, takes in nutrients from food, converts those nutrients into energy, and carries out specialized functions. The cell diagram illustration shows a cell membrane, which physically separates the intracellular space from the extracellular environment. The cell membrane is composed of a double layer of special lipids called phospholipids that in return, consist of a hydrophilic head and hydrophobic tail. In the cell diagram illustration, a cell membrane is strong enough to provide the cell with mechanical support and flexible enough to allow cells to grow and move.