PRINCIPLES OF PHYSIOLOGY 79 



before it can be absorbed into the cell. Holozoic organisms must find 

 and catch other organisms; this has required the evolution of a variety 

 of sensory, nervous and muscular structures to find and catch food, and 

 some sort of digestive system to convert the food into molecules small 

 enough to be absorbed. Animals that feed chiefly upon plants are termed 

 herbivores, those that eat other animals are called carnivores and those 

 that eat both plants and animals are known as omnivores. The morphol- 

 ogy and mode of functioning of the digestive system in different kinds 

 of animals are correlated with the nature of food eaten, peculiarities of 

 the manner of life, and so on. Carnivores, for example, characteristically 

 have strong proteolytic (protein-digesting) enzymes; whereas herbivores 

 have weak proteolytic, but strong carbohydrate-splitting action. 



Although such familiar protozoa as amebas and paramecia do ingest 

 food particles, many protozoa, as well as yeasts, molds and most bacteria, 

 cannot ingest solid food. Instead, the required organic nutrients are 

 absorbed through the cell membrane as dissolved molecules. Plants and 

 animals with this type of heterotrophic nutrition are known as sapro- 

 phytic and saprozoic, respectively. Saprophytes can grow only in an 

 environment which contains decomposing animal or plant bodies, or 

 plant or animal by-products which will supply the necessary dissolved 

 organic substances. 



A third type of heterotrophic nutrition, parasitism, occurs when one 

 organism (the parasite) lives on or within the body of another living 

 organism (the host) and obtains its food from it. Almost every animal 

 is the host for one or more parasites; these obtain their nutrients either 

 by ingesting and digesting solid particles from the host, or by absorbing 

 organic molecules through their cell walls from the surrounding body 

 fiuids or tissues of the host. Some parasites cause little or no harm to 

 the host. Others harm the host by destroying cells, by robbing it of 

 nutrients or by producing toxic waste products, and produce definite 

 symptoms of disease. Some parasites have lost all traces of a digestive 

 system and get nutrients only by absorbing organic substances through 

 their body wall. Any given parasite is usually restricted to one or a tew 

 species of hosts; thus, most of the parasites that infect man will not infect 

 other animals. In the course of evolution, the parasite becomes adapted 

 to the specific conditions of temperature, pH, and the concentration of 

 salts, vitamins and other nutrients found in one particular host, and 

 cannot survive elsewhere. 



26. Ingestion, Digestion and Absorption 



The protozoa have no digestive system and most protozoans have no 

 specialized structure for taking in food. Amebas capture food by extrud- 

 ing two lobes of protoplasm, called pseudopods, which surround the 

 prey (Fig. 5.1). The pseudopods meet around the prey and form a food 

 vacuole containing the particle to be eaten. Digestive enzymes are se- 

 creted by the protoplasm into this food vacuole, the food particle is 

 digested, and the molecules of digested food are absorbed through the 

 wall of the vacuole into the cytoplasm, where they are metabolized to 



