Bio 101 Introduction To Biology

BIO 101 INTRODUCTORY BIOLOGY I THE CELL A cell may be defined as he standard unit of biological activity bounded by a membrane, and able to reproduce itself independently of any other living system. All living organisms, large and small, plant and animal, fish and fowl, man and microbe, are made up of cells. All cells are basically similar to each other, having many structural features in common. Organisms may be composed of only one cell, when we describe them as being unicellular, or of many cells when we say they are multicellular. With the exception of eggs, which are the largest cells (in volume) known, cells are small and mostly invisible to the unaided eye. Consequently, our understanding of cells paralleled technical advances in the resolving power of microscopes. Englishman Robert Kooke first saw the remains of dead cells in 1665 in a piece of cork as he was using his newly invented microscope and he coined the work cell= to describe the tiny structures, thinking that they resembled the unadorned cells occupied by the monks. In 1838 Mathias Schleiden, a German botanist, announced that all plant tissues were composed of cells. A year later one of his countrymen, Theodor Schwann, described animal cells as being similar to plant cells. Schleiden and Schwann are thus credited with the unifying cell theory. Some 20 years after the announcements of Schleiden and Schwann, Rudolf Virchow, a great German physician, made another important generalization, cells come only from pre-existing cells