Ribosome
Some living organisms, such as bacteria, consist of a single cell and can be observed only under the microscope. Other organisms are multicellular and may be composed of billions of specialized cells. In each example, the cell is the basic unit of life. Throughout history, two major types of cells have evolved. The more primitive prokaryotic cell lacks a defined nucleus and specialized structures called organelles. Eukaryotic cells make up most of the multicellular plants and animals, including humans. They have organelles and a nucleus, which contains the genetic material of the cell and acts as a control center. Other organelles include lysosomes, Golgi apparatus, vacuoles, endoplasmic reticula, and mitochondria. One of the most numerous and important organelles within the cell is the ribosome. A growing bacteria cell may have 15,000 ribosomes, and a eukaryotic cell may contain many times more. Each ribosome is a small circular structure that produces proteins from smaller amino acids.
The way in which ribosomes are distributed within a cell seems to relate to the types of protein molecules that are being assembled. For example, in a cell that makes proteins for its own use, such as an embryonic cell, the ribosomes are found free in the cytoplasm. In a cell that makes digestive enzymes, however, the ribosomes are found along the internal membrane called the endoplasmic reticulum. The enzymes can then be transported through the cell and excreted.
Despite the large numbers of ribosomes in the cell, these organelles were not discovered until recently. Robert Brown and Walther Flemming used light microscopes to first reveal the nucleus in the cell and the chromosomes within the nucleus. Later, with the higher magnification of Vladimir Zworykin's (1889-1982) electron microscope, a revived interest in cell biology occurred.
Most scientists focused their research on the larger mitochondria. George Emil Palade (1912-) concentrated on the smaller bodies initially referred to as microsomes. These microsomes were originally believed to be nothing more than pieces of mitochondria, but he used a transmission electron microscope to show that these microsomes were actually independent structures with chemical compositions unique from mitochondria. By 1956, Palade had determined that microsomes contained ribonucleic acids (RNA) and promptly renamed them ribosomes. Further studies showed how ribosomes manufacture proteins. Due in large part to Palade's work, we have a better understanding of how the cell operates and how life itself functions. The disciplines of cellular physiology and molecular biology have been closely related ever since. This valuable research was recognized in 1974, when Palade was awarded a share of the Nobel Prize in physiology and medicine.
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