CHEMICAL STRUCTURE AND METABOLISM 



81 



Thus we see that the elements carbon, hydrogen, oxygen, nitrogen, and 

 a few others combine to form proteins, carbohydrates, and fats. Proteins, 

 carbohydrates, and fats combine to form protoplasm. What does proto- 

 plasm form? In almost all living things protoplasm is built into structural 

 units called celJs (Fig. 5.1 ). These are usually of microscopic size; they are 



vacuole 



nucleus 



CELL 



V^ 



V 



PROTOPLASM 



r 



CARBOHYDRATES 



cell 

 membrane 



centrosome 

 chromatin 



y 



A 



FIG. 5.1. Diagram of the manner in which carbon, hydrogen, oxygen, and nitrogen 

 enter into formation of carbohydrates, fats, and proteins, and the latter in turn combine 

 to form protoplasm, which is aggregated into cells. 



the "building blocks" of which the tissues of plants and animals are con- 

 structed. An organism may consist of one cell (see Chap. 4), of several or 

 many cells, or of millions of cells. In the latter instance the body is com- 

 posed of specialized tissues, some serving purposes of digestion, some of 

 secretion, some of sensory perception, some of conduction of nervous im- 

 pulses, some of locomotion, and so on. But all of these specialized tissues 

 are composed of cells. The cells vary in shape and other properties, but 

 they are all so similar in fundamental plan that we can draw a diagram 

 of a "typical" cell (Fig. 5.1) stressing the points shared in common. In 



