Listen to part of a discussion in a biology class.

P: A little later we'll start exploring the characteristics and functions of various types of cells within the human body, skin cells, blood cells, muscle cells, etc. But before we do that, today we know quite a bit about cells, their structure, their function. That's not to say there's still not a ton we don't know. But just to put things in perspective, let's take a few minutes to turn back the clock and talk about some of the stuff from your assigned reading. So what first enabled us to become aware of the existence of cells? Michael? S: The invention of the microscope.

P: Right, in 1665 the curator of experiments at the Royal Society of London was a man named Robert Hooke, H-O-O-K-E. Hooke didn't invent the microscope, but in 1665 using a candle, a mirror and a lens, he put together his own little primitive microscope, and one of the many things Hooke viewed under the microscope were thin slices of cork. And where does cork come from? Michael? S: From Cork trees, from the bark of cork trees, actually.

P: That's right. And with his crude little microscope, Hooke was able to see the cells in the cork tissue, right? So Hooke was the first to see and describe cells. What did they look like to him? You know his first impression? S: Well, like columns of tiny square boxes all connected together. He named the boxes cells because they reminded him of the little rooms monks lived in in monasteries.

P: Right! And Hooke guessed that these cells explained the properties of cork, why it floats, for example, but it never occurred to him that these cells might also play a much more important role, not only in cork, but in all living things.

Okay, so now let's fast forward to 1838 and a scientist named Theodore Schwann. In 1838 Schwann publishes what he calls the cell theory, which was based both on his work and the work of a fellow researcher. In his cell theory, Schwann presents three tenets about cells over time, two of his three tenets have been revised a bit, and the third tenet has been disproved, and a few more tenets have been added as we've gained more and more understanding of cells.

But Schwann cell theory was a major breakthrough. It revolutionized the way scientists thought about anatomy and physiology, and it established the basis for modern cell theory. Okay, so what's one of the tenets of modern cell theory? Karen?

S: The cell is the basic unit of structure in all living things.

P: Okay, for more than 150 years after Hooke described the cork cells, it was thought that only plants had cells. S: Why?

P: Well, during that time, all cells were thought to have either a square shape or an egg-like shape, and a lot of animal cells don't look like that. Also, it was easier to see plant cells under a microscope because plant cells have rigid, clearly defined cell walls. Animal cells don't have cell walls. They have what's called an extra cellular matrix, which we'll talk about later.

P: Okay, Karen, what's another modern cell theory tenet? S: Cells arise only from pre-existing cells.

P: Right! Cells arise only from pre-existing cells. Okay, remember I mentioned that the third tenet of Schwann cell theory was later disproved? Well, Schwann third tenet was that cells were created spontaneously from non-living material, much the way crystals are formed. This tenet was actually a version of a theory that had been widely accepted since the time of the ancient Greeks and Romans.

Okay, enter a young French chemist by the name of Louis Pasteur. In 1859 Pasteur did an experiment, he took a glass flask, and he heated the neck of the flask with a flame until it became pliable, then bent it into the shape of an S, and then he poured some broth, kind of like a soup, into the flask, and then he boiled the broth to kill any microorganisms in it. So now there were no living microorganisms in the broth that could grow or reproduce.

And the question was, could new life forms, new microorganisms spontaneously come into existence in this broth? If so, the microorganisms would cause the broth to spoil. Well, the broth did not spoil, because, as we now know, for it to spoil, microorganisms would have to be introduced from somewhere else. And although the curved neck of the flask did allow for air to enter the flask, any microorganisms in the air stuck to the sides of the curves and couldn't get anywhere near the broth. At its core then this experiment affirmed that living organisms could only be generated by other pre-existing organisms.