The Cell

Cell is the basic unit of all living matter; all plants and animals are composed of cells. An adult human body contains about 10 trillion (10,000,000,000,000) cells. Most cells only measure a few thousandths of a millimeter in diameter.     A Nerve cell: The transmission of
                                             impulses between nerve cells is
                                             achieved with the help of outgrowths
                                             (dendrites, Axon) from each cell body.

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Each human being starts life as a single cell, a fertilized egg, which divides into more cells during the embryonic development. In the course of this duplication, cells begin to differentiate into muscle cells, skin cells, nerve cells, and so on. Cells continue to divide and differentiate throughout a person's life.

                                        Q: How do cells reproduce?
A: Most human cells reproduce through the process called mitosis. In mitosis, a cell divides and forms into two identical daughter cells. Each daughter cell then doubles in size and becomes capable of dividing. During the period between divisions, the cell grows and carries on its normal activities, which include a duplication of its chromosomes, a process which precedes mitosis.
                                        Q: What makes one cell different from another?
A: Scientists are not exactly sure how cells establish their different functions, but they think that DNA (deoxyribonucleic acid), the hereditary material carried by the chromosomes, controls differentiation partly by directing the production of certain enzymes. As these enzymes appear in a cell, their chemical reactions cause the cell to become specialized.
The variations between cells reflect the tasks of different cells. Some of the cells nearest to the "basic" design include the hexagonal liver cells, which perform many complex chemical reactions. Other simple cells are those that provide support and lining in many tissues. Often these column-shaped cells produce the sticky substance, mucus, and are edged with minute hair-like projections (cilia), which can move substances along. The fat cells found beneath the skin and around many organs are simple cells whose cytoplasm is packed with globules of fat. The fat is used to provide insulation and energy. Red blood cells, which carry oxygen and carbon dioxide within the circulation, are unusual in having no nuclei in their mature form. In contrast, many of the white blood cells, part of the body's defense system against disease, have very large nuclei.
Three of the most specialized sorts of cells are muscle cells, nerve cells, and reproductive cells. Muscle cells are greatly elongated and have the power of contraction made possible by special proteins that can slide over one another.
Because muscle contraction is energy-intensive, muscle cells have huge numbers of mitochondria.

Nerve cells are also elongated but have specialized membranes for transmitting the electrical impulses of nerve messages. Each nerve cell ends in a cell body bearing projections that lie close to similar projections on adjacent nerve cells.

Messages "jump the gaps" with the aid of chemicals made in the nerve cells.
Reproductive cells (sperm from the male and eggs from the female) are unique in containing only half the usual number of chromosomes.
                                        Q: What are the components of a cell?
A: A thin covering called the cell membrane or plasma membrane encloses the cell and separates it from its surroundings. The cell has two main parts: (1) the nucleus and (2) the cytoplasm.