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# Introduction to Cells ## The Origins of Cell Theory Many biologists contributed to the development of the cell theory. This concept evolved gradually in Western Europe during the 19th century, as a result of the steadily accelerating pace of developments in microscopy and biochemistry. ### Fig...

# Introduction to Cells ## The Origins of Cell Theory Many biologists contributed to the development of the cell theory. This concept evolved gradually in Western Europe during the 19th century, as a result of the steadily accelerating pace of developments in microscopy and biochemistry. ### Figure 1.2. Early Steps in the Development of the Cell Theory - **Robert Hooke (1662)**, an expert mechanic and one of the founders of the Royal Society in London, was fascinated by microscopy. He devised a compound microscope, and used it to observe the structure of cork. He described and drew cork cells, and also measured them. He was the first to use the term "cells." - **Anthonie van Leeuwenhoek (1680)** was born in Delft. Despite no formal training in science, he developed a hobby of making lenses. He mounted them in metal plates to form simple microscopes. Magnifications of x240 were achieved, and he observed blood cells, sperms, protozoa with cilia, and even bacteria (among many other types of cells). His results were reported to the Royal Society, and he was elected a fellow. - **Robert Brown (1831)**, a Scottish botanist, observed and named the cell nucleus. He also observed the random movements of tiny particles (pollen grains, in his case) when suspended in water (Brownian movement). - **Matthias Schleiden (1838) and Theodor Schwann (1839)**, German biologists, established cells as the natural unit of form and function in living things: "Cells are organisms, and entire animals and plants are aggregates of these organisms arranged to definite laws." - **Rudolf Virchow (1856)**, a German pathologist, established the idea that cells arise only by division of existing cells. - **Louis Pasteur (1862)**, a brilliant French microbiologist, established that life does not spontaneously generate. The bacteria that "appear" in broth are microbes freely circulating in the air, which contaminate exposed matter. ## Introducing Animal and Plant Cells No "typical" cell exists - there is a very great deal of variety among cells. However, we will see that most cells have features in common. Viewed using a compound microscope, the initial appearance of a cell is of a simple sac of fluid material, bound by a membrane, and containing a nucleus. ### Figure 1.3. Animal and Plant Cells from Multicellular Organisms - **Canadian Pondweed (Elodea)** grows submerged in fresh water. - **Human** #### Photomicrograph of a leaf cell of Elodea (*x*400) - Large permanent vacuole, surrounded by a membrane - Cellulose cell wall - Pit, where the cytoplasm of cells connects - Chloroplasts (with starch grains) - Junction between walls (the middle lamella) #### Photomicrograph of a human cheek cell (*x*800) - Cytoplasm - Plasma membrane - Secretory granules - Temporary vacuoles - Nucleus - Centrioles ## Pasteur's experiment In this experiment, broth was sterilized (1), and then either exposed to air (3) or protected from air-borne spores in a swan-necked flask (2). Only the broth in 3 became contaminated with bacteria.