The Cell and Tissue Biology

Abstract

The cell is the basic unit of life in all forms of living organisms, from the smallest bacterium to the most complex animal. On the basis of microscopic and biochemical differences, living cells are divided into two major classes: prokaryotes, which include bacteria, blue-green algae, and rickettsiae, and eukaryotes, which include yeasts and plant and animal cells. Eukaryotic cells are far more complex internally than their bacterial ancestors, and the cells are organized into compartments or organelles, each delineated by a membrane (Fig. 2.1a, b). The DNA of the cell is packaged with protein into compact units called chromosomes that are located within a separate organelle, the nucleus. In addition, all eukaryotic cells have an internal skeleton, the cytoskeleton of protein filaments that gives the cell its shape, its capacity to move, and its ability to arrange its organelles and that provides the machinery for movement.