PROTOPLASM \ 5 



to chemical stimuli, and special groups of cells in the skin respond to 

 changes in temperature or pressure. In lower animals such specialized 

 cells may be absent, but the whole organism responds to any one of a 

 variety of stimuli. Single-celled animals such as the ameba will respond 

 by moving toward or away from heat or cold, certain chemical substances, 

 or the touch of a microneedle. Indeed, many of the cells of higher 

 animals have a similar generalized sensitivity. 



Movement, A third characteristic of living things is their ability 

 to move. The movement of most animals is quite obvious— they wiggle, 

 swim, run or fly. The movement of plants is much slower and less obvi- 

 ous, but is present nonetheless. A few animals— sponges, corals, hydroids, 

 oysters, certain parasites— do not move from place to place, but most of 

 these have microscopic, hairlike, cytoplasmic projections from the cells, 

 called cilia or flagella, to move their surroundings past their bodies 

 and thus bring food and other necessities of life to themselves. The 

 movement of an animal body may be the result of muscular contraction, 

 of the beating of cilia or flagella, or of the slow oozing of a mass of 

 protoplasm (known as ameboid motion). 



Metabolism. AH living things carry on a wide variety of chemical 

 reactions, the sum of which we call metabolism. There is no way of 

 observing the occurrence of most of these chemical reactions without 

 the aid of special apparatus such as respirometers to measure oxygen 

 utilization and carbon dioxide production and thermometers to measure 

 heat production. Elaborate physical and chemical equipment and sub- 

 stances labeled with radioactive or stable isotopes are used to trace in 

 detail the paths of metabolism and their respective quantitative im- 

 portance to the animal or plant under investigation. Such studies have 

 shown that the protoplasm of all cells is constantly taking in new sub- 

 stances, altering them chemically in a multitude of ways, building new 

 protoplasm, and transforming the potential energy of some of the mole- 

 cules taken in into kinetic energy and heat. The large molecules taken 

 in— proteins, fats, carbohydrates and others— are broken down stepwise 

 to yield energy and simpler substances. This constant release and utiliza- 

 tion of energy is one of the unique and characteristic attributes of living 

 things. The rate of metabolism is affected by temperature, age, sex, 

 general health and nutrition, by hormones, and by many other factors. 



Those metabolic processes in which simpler substances are combined 

 to form more complex substances and which result in the storage of 

 energy and the production of new protoplasm are termed anabolic. The 

 opposite processes, in which complex substances are broken down to 

 release energy and which result in the wearing out of protoplasm, are 

 called catabolic. Both types of metabolism occur continuously and are 

 intricately interdependent so that they become, in practice, difficult to 

 distinguish. Complex compounds of one sort may be broken down and 

 their parts recombined in new ways to yield new compounds. Further- 

 more, the synthesis of most molecules requires energy, so that some 

 catabolic processes must occur to supply the energy to drive the anabolic 

 reactions of these syntheses. 



Growth. Both plants and animals grow; nonliving things do not. 



