Eukaryotic Cells

Cells of the eukaryotes contain true nuclei and are much larger and more complex internally than are those of prokaryotes. The nucleus of a cell contains most of its DNA and is separated from the cytoplasm by membranes. Within the cytoplasm are various organelles with characteristic structures. These include mitochondria, lysosomes, peroxisomes, and centrioles. Eukaryotic cells come in so many sizes and shapes and with so many specialized features that it is impossible to say what is typical. Nevertheless, Fig. 1-6 is an attempt to portray some sort of “average” cell, partly plant and partly animal. As can be seen from Table 1-2, which lists the diameters and volumes of several roughly spherical cells, there is a great variation in size. However, a diameter of 10–20 µm may be regarded as typical for both plants and animals. For growth of a large cell such as the ovum, many adjacent cells assist in synthesis of foodstuffs which are transferred to the developing egg cell. Plant cells are often large but usually 90% or more of the cell is filled with a vacuole or tonoplast, 36
which is drawn unrealistically small in Fig. 1-6. The metabolically active protoplasm of lant cells often lies in a thin layer at their peripheries. Many cells are far from spherical; for example, human red blood cells are discs 8 x 8 x 1 to 2 µm with a volume of 80 µm3. Plant fiber cells may be several millimeters in length. Nerve cells of animals have long extensions, the axons, which in the human sometimes attain a length of a meter. Muscle cells fuse to give very long multinucleate fibers.