What Is a Cell?
Cells are the fundamental structural and functional units within living organisms. All living organisms consist of one or more cells. With the exception of bacteria, all organisms are made of eukaryotic cells, which have a membrane-enclosed nucleus and organelles (e.g., mitochondria, endoplasmic reticulum, and ribosomes). The nucleus within each cell contains the hereditary information for the entire organism, encoded within DNA.
In multi-cellular organisms, cells differentiate and specialize. Specialized cells organize into tissues (e.g., muscle, blood, bone, fat, nerve), which make up organs (e.g., kidneys, heart, stomach, lung), which, in turn, comprise organ systems (e.g., respiratory, digestive, excretory). Genes that do not pertain to the functioning of each individual cell become inactive, or "turn off." For example, a kidney cell uses only the DNA needed to be a kidney cell. The remaining information is "turned off," but it is still present. There are more than 200 different types of cells (nerve cells, muscle cells, epithelial cells, blood cells, bone cells, etc.) among the human body's estimated 100,000,000,000,000 total cells.
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