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Introduction to Cells

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Introduction to Cells

Lesson Objectives

  • Explain how cells are observed.
  • Define the three main parts of the cell theory.

Check Your Understanding

  • What are the five main characteristics of living things?

What are cells?

A cell is the smallest unit of an organism that is still considered living (see the onion cells in Figure below). Some organisms, like bacteria, are unicellular; consist of only one cell. Big organisms, like humans,are multicellular;  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.

The outline of onion cells are visible under a light microscope.

Observing Cells

Most cells are so tiny that you cannot see them without the help of a microscope. It was not until 1665 that English scientist Robert Hooke invented a basic light microscope and observed cells for the first time. He thought the individual boxes he was seeing in the cork he was lookin at looked like the individual cells in a monastary.  That's how cells got their names. You may use light microscopes in the classroom. You can use a light microscope to see cells.

How to Correctly Use a Microscope can be viewed at http://www.youtube.com/watch?v=jP9HtcAvGDk (1:43).

Cell Theory

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 are composed of only one cell (Figure below) and divide in half 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, 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.

As with other scientific theories, many hundreds, if not thousands, of experiments support the cell theory. Since Virchow created the theory, no evidence has ever contradicted it.

Lesson Summary

  • Cells were first observed under a light microscope.
  • Cell theory says that:
    • All organisms are composed of cells;
    • Cells are alive and the basic living units of organization in all organisms; and
    • All cells come from other cells.

Review Questions


1. What scientific tool was used to first observe cells?

2. What are the three main parts of the cell theory?

Think Critically

3. According to the cell theory, can we create a new cell in laboratory by putting different molecules together? Why or why not?

Further Reading / Supplemental Links

  • Baeuerle, Patrick A. and Landa, Norbert. The Cell Works: Microexplorers. Barron’s; 1997, Hauppauge, New York.
  • Sneddon, Robert. The World of the Cell: Life on a Small Scale. Heinemann Library; 2003, Chicago.
  • Wallace, Holly. Cells and Systems. Heinemann Library; 2001, Chicago.

Points to Consider

  • Do you think there would be a significant difference between bacterial cells and your brain cells? What might they be?
  • Do you think a bacterial cell and a brain cell have some things in common? What might they be?

Missouri Standards

3.1.E.a, 3.1.E.b, 3.1.C.a

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