2.2The Sizes and Diversity of Cells

Fig. 2-2. Sizes of cells, organelles, and biomolecules

Living organisms include unicellular organisms living as only one cell, and multicellular organisms whose bodies are made of many cells. Various kinds of unicellular organisms exist, from bacteria such as the 1-2-μm-long Escherichia coli (E. coli) to protists such as the 200-μm-long paramecia (Fig. 2-2). Multicellular organisms, e.g., humans, are made of a great variety of cells. Cells have various shapes, including flat cells (e.g., skin epithelial cells), disc-shaped cells (e.g., red blood cells), and long, narrow, extended cells (e.g., nerve cells). Cell sizes vary from a diameter of about 7 μm in red blood cells to a dendrite length of up to 1 m in nerve cells. Plant cells also have a variety of shapes and sizes, including rectangular compartmentalized cells (e.g., cork cells), cells resembling jigsaw puzzle pieces (e.g., spongy cells), and cells elongated from a few millimeters to several dozen centimeters in length (e.g., pollen tube cells).

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