The first glimpses biologists had of cells came with the invention of the microscope. Two German scientists, Matthias Schleiden and Theodor Schwann, were the first to describe the cell in scientific papers. Schleiden discovered that plant tissues are made of cells, and Schwann discovered that animal tissues are made of cells. Together they correctly hypothesized that all living things are made of cells.

Since all organisms are composed of cells, it follows that the cell is the basic unit of life. We can think of cells as biological factories, each with the equipment, assembly lines, and transportation systems to make whatever it needs for energy or to repair its structures. Cells also need a way to share their raw materials and finished products. We'll learn more about this as we study cells in detail.

Biologists soon began asking the question: Where do cells come from? Until the early nineteenth century, most people believed in spontaneous generation: the idea that living organisms – and by extension, their cells – arise from nonliving matter. For example, people thought rotting meat produced maggots, which turned into flies.

In the mid-nineteenth century, another German scientist, Rudolf Virchow, proposed a radical idea that directly challenged the concept of spontaneous generation: cells come from preexisting cells. Schleiden and Schwann are often credited with developing the cell theory, but Virchow's contribution is equally valuable. The cell theory states that: All living things are made of cells. Cells are the basic units of life. Cells come from preexisting cells.

If, according to the cell theory, cells come from preexisting cells, then spontaneous generation can't occur. The French scientist Louis Pasteur settled the debate in 1862. Pasteur boiled meat broth in a flask and bent the neck of the flask into an S shape. Air could enter the flask, but airborne microorganisms could not – they would settle by gravity in the neck. As Pasteur predicted, no microorganisms grew. When Pasteur removed the neck of the flask so that any airborne particles could enter, the broth rapidly became cloudy with life. Pasteur had successfully disproved the theory of spontaneous generation.

Although cells share many features, they come in many shapes and sizes because of their unique functions. Red blood cells carry oxygen through the body. They're biconcave, meaning they have two depressions on opposite sides of a sphere. Nerve cells need to transmit electrical impulses. They have extensions called dendrites and axons that participate in this process. In some animals, the axons are covered by accessory cells. Axons can be a meter long or more! Finally, plant cells are usually much larger than animal cells. Their plasma membrane is surrounded by a rigid cell wall to provide structural support for the plant.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education