Cells & Cell Theory

​Cells make up all living things, including your own body. This picture shows a group of kidney cells, but not all cells look alike. Cells can differ in shape and sizes, and the different shapes usually indicate different functions.

​A cell is the smallest structural and functional unit of an organism. Some organisms, like bacteria, consist of only one cell. Big organisms, like humans, consist of trillions of cells.

Compare a human to a banana. On the outside, they look very different, but if you look close enough you’ll see that their cells are actually very similar.

​In 1858, after using microscopes much better than Hooke’s first microscope, Rudolf Virchow developed the hypothesis that cells only come from other cells. For example, bacteria, which are single-celled organisms, divide in half (after they grow some) to make new bacteria.

In the same way, your body makes new cells by dividing the cells you already have. In all cases, cells only come from cells that have existed before. This idea led to the development of one of the most important theories in biology, the cell theory.

Cell theory states that:

1. ​All organisms are composed of cells.

2. ​Cells are alive and the basic living units of organization in all organisms.

3. ​All cells come from other cells.