14 GENERA I CONCEPTS 



Cytoplasm 



Pia.sma membrane 



Golgi bodies 



Centriole 



Vactxole 



Mitochondria 



Nucleus 



Cliromatin 



Nucleolus 



Nuclear 



membrane 



Cylopiasm 



Figure 2.1. Schematic drawing of a generalized animal cell. 



5. Characteristics of Living Things 



Organization. Each kind of living organism is recognized by its 

 characteristic form and appearance; the adult organism usually has a 

 characteristic size. Nonliving things generally have much more variable 

 shapes and sizes. The fundamental structural and functional unit of 

 living things, both animals and plants, is the cell. It is the simplest bit 

 of living matter that can exist independently and exhibit all the char- 

 acteristics of life. A typical cell, such as a liver cell (Fig. 2.1), is polygonal 

 in shape, with a plasma membrane separating the living substance, or 

 protoplasm, from the surroundings. Almost without exception, cells 

 have a nucleus, a specialized part of the protoplasm typically spherical 

 or ovoid in shape and separated from the rest of the protoplasm by a 

 nuclear membrane. The protoplasm that makes up the nucleus is known 

 as nucleoplasm, that outside the nucleus as cytoplasm. The nucleus, as 

 we shall see later, has a major role in controlling and regulating the 

 cell's activities. It contains the hereditary units or genes. A cell experi- 

 mentally deprived of its nucleus usually dies in a short time; even if 

 it survives for several days it is unable to reproduce. 



Irritability. Living things are irritable; they respond to stimuli, to 

 physical or chemical changes in their immediate surroundings. Stimuli 

 which are effective in evoking a response in most animals and plants are 

 changes in light (either in its color, intensity or direction), temperature, 

 pressure, sound, and in the chemical composition of the earth, water, 

 or air surrounding the animal. In man and other complex animals, 

 certain cells of the body are highly specialized to respond to certain 

 types of stimuli: the lods and cones in the retina of the eye respond to 

 light, certain cells in the nose and in the taste buds of the tongue respond 



