text: 3 Major Functions of a Cell
  From single-celled organisms such as bacteria to humans, cells are the basic structural and functional units of life. An adult human body consists of more than 37 trillion cells, according to an estimate published in the November-December 2013 issue of the "Annals of Human Biology." In complex organisms like humans, there are hundreds of different cell types, organized into body system, organs and tissues 1. Each cell type has specialized functions that, working together, allow the body to function effectively. Considered broadly, however, several general functions are common to virtually all cells.
  Living cells exist in a perpetually active biological state. All cells require energy to conduct the activities necessary to stay alive. This makes energy generation the most essential and fundamental of all cellular functions. Human cells generate energy using chemicals derived from food as fuel. Mitochondria -- often called the powerhouses of cells -- serve as the final site of energy production within human cells. Using molecules derived from the digestion and processing of sugars, proteins and fats from the diet, a series of complex chemical reactions in the mitochondria generate a molecule call adenosine triphosphate (ATP). ATP molecules capture the chemical energy from food, which the cells then use to power their biological functions.
  Each cell is surrounded by a membrane that delineates its boundaries and acts as a gatekeeper, controlling the movement of molecules into and out of the cell. This molecular transport serves the essential role of maintaining a healthy internal environment so the cell can survive and perform its specialized functions. Some small molecules, such as oxygen, carbon dioxide and water, cross human cell membranes without expending energy. However, the transport of many important molecules depends on energy-utilizing cell membrane pumps. For example, sodium and potassium membrane pumps are essential for the transmission of nerve impulses and maintaining water balance in the human body. Molecular transport also allows specialized cells that produce important chemicals -- such as hormones -- to release them so they can be utilized elsewhere in the body.
  All living things, including cells, have a limited lifespan. Therefore, reproduction is an essential function for cells. Cellular reproduction in humans enables the body to replace dying, diseased or damaged cells -- and in the case of pregnancy, to perpetuate the human species. Most human cells reproduce through a process called mitosis. Using this reproductive process, the cell produces an exact copy of its genetic material before splitting into two genetically identical cells.
  Meiosis is a special cellular reproductive process used by eggs and sperm in organisms that employ sexual reproduction. Human genetic material is organized into 23 pairs of chromosomes, one each from the father and the mother. With meiosis, rather than the eggs and sperm having two copies of each chromosome, each contains only a single copy. When the egg and sperm join at the time of fertilization, the chromosomes pair up. Subsequent divisions of the fertilized egg occur via mitosis, with each new cell containing the newly formed pair of chromosomes.
  Reviewed by: Tina M. St. John, M.D.
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  - Annals of Human Biology: An Estimation of the Number of Cells in the Human Body
  - Molecular Biology of the Cell, 4th Edition; Bruce Alberts, et al.
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