The Song of the Cell

Every cell in a multicellular organism is surrounded by an oily membrane that separates it from other cells and from the extracellular fluid that bathes all cells. The cell surface membrane is permeable to certain substances, thereby allowing an exchange of nutrients and gases to take place between the interior of the cell and the fluid surrounding it. Inside the cell is the nucleus, which has a membrane of its own and is surrounded by an intracellular fluid called the cytoplasm. The nucleus contains the chromosomes, long thin structures made of DNA that carry genes like beads on a string. In addition to controlling the cell's ability to reproduce itself, genes tell the cell what proteins to make to carry out its activities. The actual machinery for making proteins is located in the cytoplasm. Seen from this shared perspective, the cell is the fundamental unit of life, the structural and functional basis of all tissues and organs in all animals and plants.

Besides their common biological features, all cells have specialized functions:

• Liver cells, for instance, carry out digestive activities
• Brain cells have particular ways of processing information and communicating with one another. These interactions allow nerve cells in the brains to form complete circuits that carry and transform information