1.3 Eukaryotic Cell Structure and Function

Eukaryotes comprise all members of the plant and animal kingdoms as well as protozoans (proto, “primitive”; zoan, “animal”), which are exclusively unicellular and include fungi and amoebae. Eukaryotic cells are commonly about 10–100 μm across, generally much larger than bacteria. A typical human fibroblast, a connective tissue cell, is about 15 μm across, with a volume and dry weight some thousands of times those of an E. coli cell. An amoeba, a single-celled protozoan, can have a cell diameter of approximately 0.5 mm, more than 30 times that of a fibroblast.

Eukaryotic cells, like prokaryotic cells, are surrounded by a plasma membrane. However, unlike prokaryotic cells, most eukaryotic cells (the human red blood cell is an exception) also contain extensive internal membranes that enclose specific subcellular compartments, the organelles, and separate them from the cytoplasm (Figure 1-12). The cytosol, the organelle-free part of the cytoplasm, contains water, dissolved ions, small molecules, and proteins. Plant cells and most fungal cells are surrounded by a cell wall that gives the cell a rigid shape and also allows for rapid cell expansion.

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FIGURE 1-12 Subcellular organization of eukaryotic cells. (a) Schematic overview of a “typical” animal cell (top) and plant cell (bottom) and their major substructures. Not every cell type will contain all the organelles, granules, and fibrous structures shown here, and other substructures can be present in some cell types. Cells also differ considerably in shape and in the prominence of various organelles and substructures. (b) Electron micrograph of a plasma cell, a type of white blood cell that secretes antibodies, showing some of the larger organelles.
[Part (b) courtesy of I. D. J. Burdett and R. G. E. Murray.]

All eukaryotic cells have many of the same organelles and other subcellular structures. Many organelles are surrounded by a single phospholipid membrane, but the nucleus, mitochondrion, and chloroplast are enclosed by two membranes. Each organelle membrane and each space in the interior of an organelle has a unique set of proteins that enable it to carry out its specific functions, including enzymes that catalyze requisite chemical reactions. The membranes defining these subcellular compartments contain proteins that control their internal ionic composition so that it generally differs from that of the surrounding cytosol as well as that of the other organelles. Here we describe the organelles common to all eukaryotic cells as well as several that are found only in certain types of eukaryotes. We begin with the proteins that give eukaryotic cells their shapes and organize the organelles.