The Cell

3265 Words Sep 19th, 2000 14 Pages
The cell is the fundamental structural unit of all living organisms. Some cells are complete organisms, such as the unicellular bacteria and protozoa; others, such as nerve, liver, and muscle cells, are specialized components of multi-cellular organisms. Cells range in size from the smallest bacteria-like mycoplasmas, which are 0.1 micrometer in diameter, to the egg yolks of ostriches, which are about 8 cm (about 3 in) in diameter. Although they may differ widely in appearance and function, all cells have a surrounding membrane and an internal, water-rich substance called the cytoplasm, the composition of which differs significantly from the external environment of the cell. Within the cell is genetic material, deoxyribonucleic acid (DNA), …show more content…
In addition, the plasma membrane contains receptors that selectively receive nerve and hormone signals and transmit them to the interior of the cell. Finally, direct cell-to-cell interactions can occur through specialized regions of the plasma membrane known as junctions. Organs such as the skin and the small intestine consist of cells held together by tight junctions and local thickenings, or desmosomes, which constitute another type of junction. Cells can communicate electrically through a third type of junction, called a gap junction, that consists of tiny protein "tunnels" between two cells, through which tiny "message" molecules and ions may be passed. When the plasma membranes of two cells are continuous, an actual bridge of cytoplasm forms between them; in plants these bridges are called plasmodesmata.
Cell Walls
Exterior to the plasma membrane of most plant cells and bacteria is a cell wall, a cell product made largely of complex polysaccharides. In higher plants the polysaccharide is cellulose. The presence of a cell wall makes these cells rigid and sturdy, but it poses special problems for the transport of substances into and out of the cell.
Cytoplasm
The cytoplasm is the water-rich matrix within a cell that contains and surrounds the other cellular contents. It is more like a viscous gel than a watery solution, but it liquefies when shaken or stirred. Such gel-to-sol transitions are thought by some cell biologists