58 



LIFE: ITS BEGINNINGS AND NATURE 



brane, plant cells are enclosed by a cell 

 wall, composed of a non-living carbohy- 

 drate called cellulose which lends to the 

 cell a definite, rigid shape. This cell wall is 

 porous enough to allow the passage of sol- 

 uble substances through it. In animal cells 

 the cell wall is usually absent, the boundary 

 therefore being the plasma membrane. 



Several characteristic structures are pres- 

 ent in the cytoplasm of most but not all 

 cells. Most animal cells possess a region 

 near the nucleus known as the centre- 

 sphere, inside of which two small bodies, 

 the centrosomes (centrioles) are sometimes 

 found. Both of these structures are absent 

 in higher plant cells, although they are 

 found among the lower plants, such as some 

 of the algae. There is some doubt as to their 

 function in the life of the cell. Small bodies 

 called plastids are frequently present in the 

 cytoplasm in both animal and plant cells. 

 In the former they are usually colorless, 

 whereas in the latter they may contain 

 chlorophyll, the green pigment essential in 

 photosynthesis. Fiber-like bodies floating 

 in the cytoplasm are the mitochondria, 

 which have recently been identified as bun- 

 dles of enzymes important in the metabo- 

 lism of the cell. The irregularly shaped 

 Golgi apparatus is scattered through the 

 cell or collected near the nucleus, very often 

 in the region of the centrosphere. The func- 

 tion of this structure is unknown, although 

 it is thought to be associated with secretion 

 in some cells. Vacuoles are spaces in the 

 cytoplasm that usually contain gases, solids, 

 or liquids. In addition to these more or less 

 regularly occurring parts of the cell, there 

 may be present also certain lifeless bodies 

 floating haphazardly in the cytoplasm. They 

 are referred to as cell inclusions. They may 

 be stored starch or fat, or undigested bits of 

 organisms which have been taken in as food. 

 .The nucleus is the vital part of the cell 

 because it contains the genes, carriers of 

 heredity factors. Genes are localized in the 

 chromatin granules that are visible under a 

 light microscope. The chromatin is confined 



to discrete bodies called chromosomes, 

 about which we shall learn a great deal 

 more in a later chapter. The undifferenti- 

 ated protoplasm of the nucleus is named 

 nucleoplasm or nuclear sap. 



Although these are the essentials of most 

 cells, some possess additional structures 

 which are particularly designed to do spe- 

 cific jobs. Furthermore, cells vary tremen- 

 dously in shape, size, and special functions. 

 Size. There is as much difference in the 

 size of cells as there is between different 

 animals, an elephant and a mouse, for ex- 

 ample. It has been thought that certain 

 species of spherical bacteria, which have a 

 diameter of 1 micron or less, are the small- 

 est cells. Cells must not be confused with 

 viruses which are not organized into cells 

 and which, in fact, normally live at the 

 expense of cells. Bacteria can be seen only 

 with the best high magnification micro- 

 scopes. Recently they have been photo- 

 graphed through the electron microscope 

 which has made possible more detafled 

 studies of their internal anatomy (Fig. 2- 

 10). Most cells that compose the bodies of 

 animals are considerably larger than the 

 largest bacteria; on the average they are 

 about 7 microns in diameter. The nerve 

 cells are very long and tliin, particularly 

 those reaching from the tip of the toe to the 

 spinal cord in the back. In an elephant or 

 giraffe these cells are 6 or 7 feet long. The 

 largest animal cells are found among bird's 

 eggs, in which what is popularly called the 

 "yolk" is a single cell. The largest cell 

 known is the yolk of the ostrich egg which 

 reaches a diameter of 2 or more inches. 

 The large quantity of stored food (yolk) in 

 eggs is responsible for their great mass. 



Shape. The shape of cells depends a great 

 deal on their function (Fig. 3-2). If they 

 perform a tensile function, such as the cells 

 found in tendons, they are long and thin 

 because of the constant stretching action 

 placed upon them. If they are conductive, 

 as in the case of nerve cells, they must also 

 be long and thin. On the other hand, red 



