Cell

Almost all living things are made of cells. Some tiny froms of life consist of only one cell. Larger forms of life are built of many cells. That is why cells are called the basic unit of life.
The human body is made of billions upon billions of cells are probably the bacteria. These one-celled forms of life can barely be seen even with a microscope. Nerve cells are probably the largest cells. Some of them are more than 3 feet long.

The microscope has shown that all cells have three main parts: the cell membrane, the cytoplasm, and the nucleus. The cell membrane is the outside surface of the cell. Inside the membrane is the cytoplasm. This is a mass of colorless material, which hold many other tiny cell parts. The nucleus is usually near the center of the cell. It is somewhat round in shape and has an outer membrane of its own.

There are many kinds of cells. Usually each kind does one special job. For example, in animals some cells expand and contract; these are muscle cells. Other cells are sensitive to light; these cells form the retina of the eyes.

Modern microscopes show that cells are far from simple. They contain many complicated structures. And there is constant activity going on in cells.
Food and oxygen pass steadily into the cell through the membrane. The cell changes them into energy. It uses the energy to do various kind of work. The cell also makes new cell material moiré living matter. Some waste matter is left over from these processes. It passes out of the cell through the membrane. So do worn-out cell materials.

With few exception, a single cell does not live very long. But if it divides, the two new cells start life afresh. That is, intead of aging and dying, the old cell becomes two new cells.
Plants and animals grow because of cell division continues. New cells replace old or worn out cells. As cell division slows, the body ages.

Although plant and animals cells differ from each other in some ways, in a broad sense they are alike. And so we say that the cell is the unit of life.