The Cell Essay -- essays research papers

The cell is the fundamental structural unit of all living organisms. Some cells are complete organisms, such as the unicellular bacteria and protozoa; others, such as nerve, liver, and muscle cells, are specialized components of multi-cellular organisms. Cells range in size from the smallest bacteria-like mycoplasmas, which are 0.1 micrometer in diameter, to the egg yolks of ostriches, which are about 8 cm (about 3 in) in diameter. Although they may differ widely in appearance and function, all cells have a surrounding membrane and an internal, water-rich substance called the cytoplasm, the composition of which differs significantly from the external environment of the cell. Within the cell is genetic material, deoxyribonucleic acid (DNA), containing coded instructions for the behavior and reproduction of the cell and also the chemical machinery for the translation of these instructions into the manufacture of proteins. Viruses are not considered cells because they lack this transla tion machinery; they must parasitize cells in order to translate their own genetic code and reproduce themselves. Cells are of two distinctly different types, prokaryotes and eukaryotes; thus, the living world is divided into two broad categories. The DNA of prokaryotes is a single molecule in direct contact with the cell cytoplasm, whereas the DNA of eukaryotes is much greater in amount and diversity and is contained within a nucleus separated from the cell cytoplasm by a membranous nuclear envelope. Many eukaryotic cells are further divided into compartments by internal membranes in addition to the nuclear envelope, whereas prokaryotic cells never contain completely internal membranes. The prokaryotes include the mycoplasmas, bacteria, and blue-green algae. The eukaryotes comprise all plant and animal cells. In general, plant cells differ from animal cells in that they have a rigid cell wall exterior to the plasma membrane; a large vacuole, or fluid-filled pouch; and chloroplasts that convert light energy to chemical energy for the synthesis of glucose. Structure and Function Cells are composed primarily of oxygen, hydrogen, carbon, and nitrogen, the elements that make up the majority of organic compounds. The most important organic compounds in a cell are proteins, nucleic acids, lipids, and polysaccharides (carbohydrates). The "solid" structures of the cell are co... ...t of cells and cell products. Thus, a whole organism could be understood through the study of its cellular parts. In 1858 the German pathologist Rudolf Virchow's theory, that all cells come from preexisting cells, led to the development of ideas about cell division and cell differentiation. The development in the late 19th century of techniques for staining cell parts enabled scientists to detect tiny cell structures that were not actually seen in detail until the advent of the electron microscope in the 1940s. The development of various advanced optical techniques in the 20th century also increased the detection power of the light microscope for observations of living cells. The study of cells is not limited to describing structures. A central concept in modern cytology is that each structure has a function that may be understood as a series of biochemical reactions. The understanding of these functions has been greatly aided by the development of cell fractionation techniques, using an ultracentrifuge to separate specific intracellular structures from the rest of the cell. Another technique is tissue culture, by which specific kinds of cells can be isolated and grown for study. The Cell Essay -- essays research papers The cell is the fundamental structural unit of all living organisms. Some cells are complete organisms, such as the unicellular bacteria and protozoa; others, such as nerve, liver, and muscle cells, are specialized components of multi-cellular organisms. Cells range in size from the smallest bacteria-like mycoplasmas, which are 0.1 micrometer in diameter, to the egg yolks of ostriches, which are about 8 cm (about 3 in) in diameter. Although they may differ widely in appearance and function, all cells have a surrounding membrane and an internal, water-rich substance called the cytoplasm, the composition of which differs significantly from the external environment of the cell. Within the cell is genetic material, deoxyribonucleic acid (DNA), containing coded instructions for the behavior and reproduction of the cell and also the chemical machinery for the translation of these instructions into the manufacture of proteins. Viruses are not considered cells because they lack this transla tion machinery; they must parasitize cells in order to translate their own genetic code and reproduce themselves. Cells are of two distinctly different types, prokaryotes and eukaryotes; thus, the living world is divided into two broad categories. The DNA of prokaryotes is a single molecule in direct contact with the cell cytoplasm, whereas the DNA of eukaryotes is much greater in amount and diversity and is contained within a nucleus separated from the cell cytoplasm by a membranous nuclear envelope. Many eukaryotic cells are further divided into compartments by internal membranes in addition to the nuclear envelope, whereas prokaryotic cells never contain completely internal membranes. The prokaryotes include the mycoplasmas, bacteria, and blue-green algae. The eukaryotes comprise all plant and animal cells. In general, plant cells differ from animal cells in that they have a rigid cell wall exterior to the plasma membrane; a large vacuole, or fluid-filled pouch; and chloroplasts that convert light energy to chemical energy for the synthesis of glucose. Structure and Function Cells are composed primarily of oxygen, hydrogen, carbon, and nitrogen, the elements that make up the majority of organic compounds. The most important organic compounds in a cell are proteins, nucleic acids, lipids, and polysaccharides (carbohydrates). The "solid" structures of the cell are co... ...t of cells and cell products. Thus, a whole organism could be understood through the study of its cellular parts. In 1858 the German pathologist Rudolf Virchow's theory, that all cells come from preexisting cells, led to the development of ideas about cell division and cell differentiation. The development in the late 19th century of techniques for staining cell parts enabled scientists to detect tiny cell structures that were not actually seen in detail until the advent of the electron microscope in the 1940s. The development of various advanced optical techniques in the 20th century also increased the detection power of the light microscope for observations of living cells. The study of cells is not limited to describing structures. A central concept in modern cytology is that each structure has a function that may be understood as a series of biochemical reactions. The understanding of these functions has been greatly aided by the development of cell fractionation techniques, using an ultracentrifuge to separate specific intracellular structures from the rest of the cell. Another technique is tissue culture, by which specific kinds of cells can be isolated and grown for study.