It is not possible to describe a typical cell, because cells vary so greatly in size, shape, content, and function. We can, however, consider a hypothetical composite cell that includes many known cell structures (fig. 3.3).
A cell consists of three major parts—the nucleus (nuckle-us), the cytoplasm (sicto-plazm), and the cell membrane. The nucleus is innermost and is enclosed by a thin membrane called the nuclear envelope. The cytoplasm is a mass of fluid that surrounds the nucleus and is itself encircled by the even-thinner cell membrane (also called a plasma membrane). Within the cytoplasm are specialized structures called cytoplasmic organelles that perform specific functions. The nucleus directs the overall activities of the cell by functioning as the hereditary headquarters, housing the genetic material (DNA).
Cells with nuclei, such as those of the human body, are termed eukaryotic, meaning "true nucleus." In contrast are the prokaryotic ("before nucleus") cells of bacteria. Although bacterial cells lack nuclei and other membrane-bound organelles and are thus simpler than eukaryotic cells, the bacteria are nevertheless quite a successful life form — they are literally everywhere, and have been for much longer than eukaryotic cells. A third type of cell, termed archaea, lack nuclei but have many features like those of eukaryotic cells.
U Give two examples to illustrate how the shape of a cell makes possible its function.
^9 Name the major parts of a cell.
^9 What are the general functions of the cytoplasm and nucleus?
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