LEARNING OBJECTIVE: Identify the parts of the cell and their functions.

The cell, the smallest unit of life, is the basic structural unit of all living things and a functional unit all by itself. Cells are composed of a viscid, jellylike substance, called protoplasm, upon which depend all the vital functions of nutrition, secretion, growth, circulation, reproduction, excitability, and movement. Protoplasm, thus, has often been called “the secret of life.”

A typical cell is made up of the plasma membrane, the nucleus, and the cytoplasm.

The plasma membrane is a selectively permeable membrane surrounding the cell. In addition to holding the cell together, the membrane selectively controls the exchange of materials between the cell and its environment by physical and chemical means. Gases (such as oxygen) and solids (such as proteins, carbohydrates, and mineral salts) pass through the plasma membrane by a process known as diffusion.

The nucleus is a small, dense, usually spherical body that controls the chemical reactions occurring in the cell. The substance contained in the nucleus is called nucleoplasm. The nucleus is also important in the cell's reproduction, since genetic information for the cell is stored there. Every human cell contains 46 chromosomes, and each chromosome has thousands of genes that determine the cell's function.

The cytoplasm is a gelatinous substance surrounding the nucleus and is contained by the plasma membrane. The cytoplasm is composed of all of the cell protoplasm except the nucleus.

The simplest living organism consists of a single cell. The amoeba is a unicellular animal. The single cell of such a one-celled organism must be able to carry on all processes necessary for life. This cell is called a simple or undifferentiated cell, one that has not acquired distinguishing characteristics.

In multicellular organisms, cells vary in size, shape, and number ofnuclei. When stained, the various cell structures can be more readily recognized under a microscope. Other differences such as the number and type of cells can be seen with the aid of a microscope. Many cells are highly specialized. Specialized cells perform special functions (e.g., muscle cells, which contract, and epithelial cells, which protect the skin).