Reading Practice 33

[1] Everyone has heard of Charles Darwin and his theory of evolution, and many have heard of Gregor Mendel’s laws of inheritance. However, predating these two famous figures in the history of biology are several inventors and biologists whose work on microscopes and the study of cells laid the foundation for much of the work of biology that followed them.

[2] Of course, the study of cells would not have been possible without certain advances in technology. It was the microscope in particular that facilitated the original discovery of cells and the formulation of cell theory. The discovery of the magnification properties of glass goes back to the Romans in the first century. But it wasn’t until the 12th century that glass was made to fit over the eye with the purpose of magnifying objects. Another 400 years passed before Zacharias Jansen built the first compound microscope. This was an impressive achievement in 1595, but it took another 70 years of improvements before microscopes were powerful enough for Robert Hooke to discover the cell in 1665.

[3] Although Hooke named what he saw in his rudimentary microscope “cells,” he had no idea of their structure, their function, nor their internal makeup. In fact, he didn’t even believe that these tiny units of matter were alive. It was Anton van Leeuwenhoek who, 10 years after Hooke’s discovery, discovered motile units of life, which he named “animalcules.” With an improved microscope capable of 270x magnification, van Leeuwenhoek made some impressive observations, even witnessing the fertilization process.

 

[4] From the first discovery of cells, it still took more than 150 years for science to explain the basic tenets of cell theory, a theory which – with some changes and additions – forms the basis of modern microbiology. In 1838, Theodor Schwann and Matthias Schleiden were discussing their respective studies of cells. Schwann, who had been studying animal tissues, was amazed by Schleiden’s description of plant cells with nuclei. A year later, and without acknowledging any help from Schleiden, Schwann published his original work on cell theory.

 

[5] In its original form, cell theory rested on three tenets. The first is that the cell is the basic unit of organization of all living things. The second is that living organisms are composed of one or more cells. And thirdly, cells form spontaneously through crystallization. It took only a dozen years before Robert Remak, Rudolf Virchow, and Albert Kolliker refuted this third tenet, with Virchow revising it to a more accurate and familiar principle: that all cells arise only from pre-existing cells. Interestingly enough, how ideas are made is more confusing than how cells are made, and many historians claim that it was Remak who did the intellectual work that Virchow took credit for.

 

[6] Modern cell theory still includes those two original tenets and the third revised principle. However, with further study and the discovery of mitochondria and DNA, as well as the ability to manipulate cellular material, the theory has expanded to include three more ideas. The first is that cells contain hereditary information (encoded in DNA) that is passed on from cell to cell during reproduction. This cellular reproduction occurs either through meiosis or mitosis. Meiosis involves the mixing of chromosomes through sexual reproduction. It takes place in animals, plants, and fungi. Mitosis is asexual reproduction in which a cell divides into two. Single-celled organisms use mitosis to reproduce. Another modern tenet of cell theory is that all cells of organisms of similar species have basically the same chemical composition. This discovery has come through biochemical comparison of cells of different organisms. One final principle is that energy flow occurs within cells. Energy flow, or metabolism, involves the twin process of catabolism – the breaking down of organic molecules for energy – and anabolism – the building of new molecules required by the organism.