LIFE PROCESSES AND ORGANIZATION 



85 



Organisms also have movements that are identical 

 to those of nonliving things, but such motions are 

 not strictly autonomous. They are the consequences 

 of external forces being applied to an object. For 

 example, both a rock and an animal may roll dovi/n a 

 hill as a result of the force of gravity. 



NUTRITION 



The nutrition of organisms is of two main types, 

 autotrophic and heterotrophic . Both types provide the 

 same thing, materials that organisms must have for 

 energy, growth, and repair, the sources of all life 

 processes. Autotrophic organisms need only rel- 

 atively simple inorganic materials for these functions; 

 heterotrophic creatures require more complex organic 

 compounds. 



Autotrophs are of two types, chernosynthetic and 

 pholosynlhetic. Chernosynthetic autotrophs build all 

 their nutrients from environmental materials without 

 the aid of chlorophyll. Photosynthetic autotrophs 

 have chlorophyll which traps sunlight, most fre- 

 quently, to allow the combination of carbon dioxide 

 and water into simple sugar in the process called 

 photosynth''sis . Some photosynthetic autotrophs use 

 compounds other than carbon dioxide or water. 



Heterotrophs are of six types. Saprophytes need rel- 

 atively simple organic compounds that they get from 

 dead or decaying matter. Scavengers require complex 

 organic compounds that likewise are obtained from 

 dead or decaying organic remains. Parasites often 

 use simple organic compounds in the form in which 

 they are taken from the host; however, some parasites 

 are similar to predators (carnivores) in that both in- 

 gest complex organic compounds which must be 

 digested before metabolism. The remaining nutrient 

 types (carnivores, meat-eaters; herbivores, plant-eaters; 

 and omnivores, mixed plant and animal eaters) are 

 often grouped as holozoic. Often the scavenger and 

 holozoic organisms are bulk feeders and other feeding 

 types extract only certain chemicals from their 

 source of food. 



REPRODUCTION 



Racial perpetuation is not simply of two major 

 types, sexual and asexual, but it is convenient to 

 consider two types. In sexual reproduction there is 

 fusion of two nuclei, each from a dififerent specialized 

 sex cell called a gamete. The diagnostic feature of 



sexual reproduction is the fusion of two nuclei within 

 a single cell called a zygote. Asexual reproduction, 

 then includes any reproductive process not including 

 fusion of nuclei from two parents. 



Unfortunately the terminology of reproduction was 

 developed before much was really known about the 

 subject. Therefore, designations for reproductive 

 structures and functions often are loosely applied. 

 The glossary shows much of this unfortunate multi- 

 plicity in term meanings. In the following discus- 

 sion an attempt is made to avoid this pitfall to learn- 

 ing. 



ASEXUAL REPRODUCTION 



Asexual means of racial perpetuation are mech- 

 anisms limited to individual cells or organisms (Fig- 

 ure 6.2). Interparental crosses do not exist. For 

 this reason offspring are of much the same appear- 

 ance as their parents; generally an individual has 

 the same gene composition as its parent. The types of 

 asexual reproduction are similar whether they occur 

 in a cell or an organism, but the diagnosis of types is 

 related, in part, to cell or organism reproduction. 



Fission and Fragmenfaiion. Fission is cell division 

 whether it occurs in the cell of a multicellular or- 

 ganism or in the cell-like structure, perhaps cell, of 

 an entire organism as in the Monera and many Pro- 

 tista. Fission produces two essentially equal daughter 

 cells from a single cell. When fission stops at this 

 point it is called binary fission, but if each of the 

 daughter cells also undergo fission, the process is 

 called multiple fission. Multiple fission in its simplest 

 form produces four cells from one, but cell division 

 often will continue in a prolonged sequence until a 

 great many cells, still called daughter cells, are 

 formed. 



Fission does not imply any mechanism of cell di- 

 vision, but two types of fission occur, mitosis and 

 amitosis. Mitosis is nuclear division diagnosed by 

 complex chromosomal movement and exact duplica- 

 tion of each chromosome of the original cell. (Chro- 

 mosomes originate in a cell's nucleus and bear the 

 units of heredity, genes.) In this simple cell division 

 the single parent cell goes through a process whereby 

 the original two sets of chromosomes (In) or perhaps 

 the single parental set (n) are each duplicated, dou- 

 bling the original chromosome number, and then the 

 parent cell divides, producing two daughter cells, 

 each cell with the same number of chromosomes as its 



