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The study of the activities, functions, properties, and structures of cells. Cells were discovered in the middle of the seventeenth century after the microscope was invented. In the following two centuries, with steadily improved microscopes, cells were studied in a wide variety of plants, animals, and microorganisms, leading to the discovery of the cell nucleus and several other major cell parts. By the 1830s biologists recognized that all organisms are composed of cells, a realization that is now known as the Cell Doctrine. The Cell Doctrine constitutes the first major tenet upon which the contemporary science of cell biology is founded. By the late 1800s biologists had established that cells do not arise de novo, but come only by cell division, that is, division of a preexisting cell into two daughter cells. This is the second major tenet upon which the modern study of cells is based.
By the end of the nineteenth century chromosomes had been discovered, and biologists had described mitosis—the distribution at cell division of chromosomes to daughter cells. Subsequent studies showed that the chromosomes contain genes and that mitosis distributes a copy of every chromosome and hence every gene to each daughter cell during cell division. This established the basis of cell heredity and ultimately the basis of heredity in multicellular organisms. See Chromosome, Mitosis
Microscope studies established that some kinds of organisms are composed of a single cell and some, such as plants and animals, are made up of many cells—usually many billions. Unicellular organisms are the bacteria, protozoa, some fungi, and some algae. All other organisms are multicellular. An adult human, for example, consists of about 200 cell types that collectively amount to more than 1014 cells.
All modern research recognizes that in both unicellular and multicellular organisms the cell is the fundamental unit, housing the genetic material and the biochemical organization that account for the existence of life. Many millions of different species of cells exist on Earth. Cells as different as a bacterium, an ameba, a plant leaf cell, and a human liver cell appear to be so unrelated in structure and life-style that they might seem to have little in common; however, the study of cells has shown that the similarities among these diverse cell types are more profound than the differences. These studies have established a modern set of tenets that bring unity to the study of many diverse cell types. These tenets are: (1) All cells store information in genes made of deoxyribonucleic acid (DNA). (2) The genetic code used in the genes is the same in all species of cells. (3) All cells decode the genes in their DNA by a ribonucleic acid (RNA) system that translates genetic information into proteins. (4) All cells synthesize proteins by using a structure called the ribosome. (5) Proteins govern the activities, functions, and structures in all cells. (6) All cells need energy to operate; all use the molecule adenosine triphosphate (ATP) as the currency for transfer of energy from energy sources to energy needs. (7) All cells are enclosed by a plasma membrane composed of lipid and protein molecules. See Cell membranes, Genetics, Ribosomes
In the twentieth century the study of cells, which had been dominated for more than 200 years by microscopy, has been enormously expanded with many other experimental methods. The breaking open of a large mass of cells and the separation of released cell parts into pure fractions led to the discovery of functions contributed by different structures and organelles.
Contemporary research in cell biology is concerned with many problems of cell operation and behavior. Cell reproduction is of special concern because it is essential for the survival of all unicellular and multicellular forms of life. Cell reproduction is the means by which a single cell, the fertilized egg, can give rise to the trillions of cells in an adult multicellular organism. Disrupted control of cell reproduction, resulting in accumulation of disorganized masses of functionally useless cells, is the essence of cancer. Indeed, all diseases ultimately result from the death or misfunctioning of one or another group of cells in a plant or animal. The study of cells pervades all areas of medical research and medical treatment. Great advances have been made in learning how cells of the immune system combat infection, and the nature of their failure to resist the acquired immune deficiency syndrome (AIDS) virus. See Acquired immune deficiency syndrome (AIDS), Cell senescence and death
The development of methods to grow plant and animal cells in culture has provided new ways to study cells free of the experimental complications encountered with intact plants and animals. Cell culture has greatly facilitated analysis of abnormal cells, including transformation of normal cells into cancer cells. Cultured cells are also used extensively to study cell differentiation, cell aging, cell movement, and many other cell functions.