A cell ,among othe things, is any small compartment and this is just what Robert Hooke had in mind in 1665 when he applied the term to the empty microscopic compartments he saw in cork. More particularly, Hooke was referring to the nonliving cellulose walls - the shell of live. The idea of this empty space held sway until Dujardin in 1835 underscored the contents of the cell rather than the walls. He called the living stuff "sarcode", a term to be replaced 11 years later by Hugo von Mohl's protoplasms. Today we think of cell as a circumsribed mass of protoplasm with a nucleus.

Cells are of various shapes and size, each kind designed and equipped for a particular job. Surface cells protect, muscle cells contract, nerve cells relay electrical messages, and so on. The smallest cells are as tiny as 0.1 micrometer in diameter, and the largest are the size of the largest bird's egg. Human cells have an average diameter somewhere in the vicinity of 10 micrometer. The ovum, the largest human cell, is about the size of the dot over an "i".

All cells are circumsribed by a cell membrane and plant cells are further circumsribed by an outer nonliving cell wall. The fundamental parts of the cell are the nucleus, cell membane, and cytosome. Most cells contain a single nucleus, but indeed there are notable exceptions. Some cells are without a distinct nucleus at all, whereas other are multinucleated. The classic idea of cell individuality has many exceptions, since in some tissues the cell membrane dissolve away, leaving one huge cytosome with myriad nuclei through-out its substance

Cell Division

The human body grows as a consequence of cell division, or mitosis. In this complex process a single cell gives rise to two identical daughter cells that contain the same number of chromosomes as the parent cell. A chromosome is a linear body of the cell nucleus containing deoxyribonucleic acid (DNA), a chemical substance responsible for the determination and transmission of hereditary characteristics. Specifically, the DNA molecule is blocked off into segments called genes, each of which is responsible for a specific hereditary feature. Every species of plants and animals has it own characteristic chromosome number. The human chromosome number is 46.

A very special type of cell division, called meiosis, occurs in sexually reproducing organisms. In this division the daughter cells become reproductive cells, each containing onehalf the species number of chromosomes. But this certainly makes biologic sense because when reproductive cells unite in the process of fertilization, the species chromosome number is restored. In human this means that each reproductive cell-that is, each sperm and each ovum- contain 23 chromosomes. For convenience, the species chromosome number is referred to as the diploid number, and the reproductive cell number is referred to as the haploid number. Thus the human species has the diploid number 46 and the haploid number 23.

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