Some living things, such as bacteria, have only one cell. Larger organisms, like people, are made of billions of cells. You can think of a cell as the basic unit of life. All cells come from other cells. Life is passed on from one living thing to another by reproduction. If you cut an orange in half, the fibers that you see are cells. When the orange was cut, the cell walls were broken, and so the juice ran out. You probably didn't even realize that you were dealing with cells!.
Your cells are covered by a thin membrane, or layer of tissue. This cell membrane holds all the cell material inside it. The membrane also lets food, oxygen, water, and wastes flow in and out of the cell. Inside of each cell is a jellylike substance called cytoplasm. Cytoplasm contains chemicals that break up food taken into the cell. It produces building materials and energy for the cell to carry on its life activities. In the middle of the cytoplasm lies a sphere, or ball-shaped structure, surrounded by another membrane. This sphere is called the cell's nucleus. It controls how the cell gets rid of wastes. It controls how the cell reproduces. Without the nucleus, the cell would die. The nucleus also contains the materials of heredity called chromosomes. Chromosomes determine the sex of an individual. Chromosomes contain genes that determine the color of the eyes and hair, the physical appearance, and even some talents. Through heredity, chromosomes pass on to an individual some of the traits of parents.
The cell membrane, the cytoplasm, and the nucleus make up the protoplasm of the cell. Protoplasm is a chemical substance that regulates all the cell's activities. Protoplasm takes in food and oxygen. It changes food into substances that the cell can use. It gets rid of cells wastes and causes the cell to produce itself.
Cells work together. They are organized into groups. Each group consists of special kinds of cells that do a particular job.