84 



THE ORGANISM: 



LIVING STRUCTURES ORGANISM 



protoplasm 



cell 



ceils 



tissues 



organs 



organ systenns 



ORGANISMS 



protoplasnnic 



acellular (= unicellular) 



cellular 



tissue 



organ 



organ system 



population 

 comnnunity 



ORGANISMS plus ENVIRONMENT 



ecosystem 

 biosphere 



Figure 6.1 Levels of body and life organization. Organization of 

 living structure in on organism, organisms, and organisms plus 

 tfieir environment. 



ganism construction. Protoplasm is any living sub- 

 stance and the cell usually is the fundamental struc- 

 tural unit of protoplasm in most living creatures. 

 Certain organisms have cells grouped into tissues, 

 tissues into organs, and organs into organ systems; 

 however, other life may have protoplasmic, "single 

 celled," cellular, tissue, organ, or organ system 

 bases, or grades, of structure. Therefore, the first 

 six levels of the life spectrum are either possible 

 grades of plant and animal construction, or progres- 

 sively more complex structural units in an animal 

 with organ systems. One should realize that all of 

 these grades of structural organization really are 

 protoplasmic because all living structure is proto- 

 plasm, but protoplasmic organization here is limited 

 to cell-like structure that does not clearly display a 

 complete nucleus (see Monera, p. 127). 



The last four levels refer to ever more complex in- 

 terrelations between organisms and their environ- 

 ment. A population is a species or a portion of a 

 species. A community is all populations in a definite 

 geographic area. An ecosystem is a functional re- 

 lationship, specifically the complex of creatures in- 

 teracting with one another and their physical en- 

 vironment. It may be considered a community and 

 its environment and implies dvnamic activity within 



and between each. The biosphere is the part of the 

 earth containing all ecosystems, the zone of life. 



IRRITABILITY AND ADAPTATION 



Irritability is that property of an organism or cell 

 which causes an appropriate reaction to sudden 

 changes in the external or internal environment. It 

 includes sensitivity to the changes (stimuli), and 

 stimulation or excitation by them. When stimula- 

 tion takes place, the stimulus is transported to an 

 appropriate center of reaction, and the center brings 

 about a response which has some relation to the orig- 

 inal stimulus. This response is temporary and is 

 followed by a return to so-called normal conditions. 



Stimuli need not always cause responses. Repeated 

 responses to stimuli produce fatigue, a state in 

 which an organism can no longer react until its body 

 processes recover from overstimulation. Other in- 

 hibition of response can come from repetition of the 

 same stimulus, which soon is ignored although the 

 organism is physically capable of reacting. In con- 

 trast to these actions of life, nonliving objects 

 usually have the same response to a given stimulus 

 and do not display inhibition. 



Irritability in some measure overlaps adaptation, 

 which is segregated into two other phases of activity, 

 adjustment and specialization. Adjustment is the process 

 in which a plant or animal in its own lifetime 

 gradually builds up a functional balance with an 

 environment factor. In specialization, the species as 

 a whole through chance evolves a hereditary balance 

 with its environment. Although this contrast be- 

 tween adjustment and specialization is possible, 

 fundamentally both have a hereditary basis in the 

 past evolution of a species. If adjustment were 

 without any hereditary basis and came about by 

 chance alone, most organisms would perish at their 

 first contact with any sudden change in their sur- 

 roundings. 



AUTONOMOUS MOVEMENT 



Although commonly believed to be unique to 

 animals, self-controlled movement is found in other 

 life. For example, many bacteria, blue-green algae, 

 and protozoans are completely independent, freely 

 moving creatures; even certain vascular plants react 

 to a stimulus by moving their leaves and, sometimes, 

 their stems. 



