LIFE PROCESSES AND ORGANIZATION 



91 



ASEXUAL LIFE CYCLES 



.;.; Diplophase .;.;.;.;.;.;.;.;.;. 



adult 



Haplophase 



(n):: 



veg. 



reprod 



Figure 6.7 The asexual life cycles. 



distributed throughout protistans and plants but is 

 more typical of plants. 



LIVING STRUCTURE 



As already stated, the lower levels of the life spec- 

 trum deal with the structural organization of proto- 

 plasm in living creatures. In the order of increasing 

 complexity this structure includes incompletely dif- 

 ferentiated protoplasm, "single cells," cells, tissues, 

 organs, organ systems, and organisms. However, this 

 structural spectrum should not be interpreted to 

 mean that all organisms are composed of protoplasm 

 organized into cells, the cells into tissues, the tissues 

 into organs, the organs into organ systems, and the 

 organ systems into a single organism. Such an ar- 

 rangement is found only in animals of complex struc- 

 tural form. Blue-green algae, bacteria, protozoans, 

 fungi, and sponges have no features more complex 

 than cells, yet all are organisms. In addition, there 

 are plants and animals whose organization does not 

 go beyond that of tissues or organs, and only certain 

 animals have organ systems. 



PROTOPLASM 



Protoplasm is living substance and is found in all 

 living structures. When not found in specialized cells, 

 it usually is a translucent or transparent, usually 

 grayish, slimy, jellylike, complex chemical substance. 

 Between 95 and 99 per cent of protoplasm is com- 

 posed of the elements carbon, hydrogen, oxygen, and 

 nitrogen. The remaining small percentage consists 



mostly of the elements sulfur, phosphorus, potassium, 

 iron, magnesium, calcium, manganese, sodium, chlo- 

 rine, zinc, boron, copper, and molybdenum — a fact 

 that serves to emphasize its complexity. 



The structure of bacteria and blue-green algae is 

 most like that of undifferentiated protoplasm. In 

 other words, these organisms lack true cells. For this 

 reason it is convenient to emphasize bacteria and 

 blue-green algae structure as protoplasmic. 



CELLS 



Cells are the fundamental structural unit, or build- 

 ing block, in most organisms. They are as variable 

 in shape as are the sources from which they are de- 

 rived. Ideally, perhaps, each cell should be a sphere, 

 but the forces of compression and adhesion holding 

 them together in a multicellular body mold their 

 shape; and cells of special function usually express 

 their function in a particular shape modification that 

 is an adaptation for greater efficiency. 



Cells, then, are variable. They vary in structure 

 not only when they are the components of a multi- 

 cellular organism, but also when one appears to con- 

 stitute the entire body of an apparently single-celled 

 (acellular or unicellular) organism. Although struc- 

 tural differences do exist, all cells — protistan, plant, 

 and animal — are usually composed of a cell membrane, 

 cytoplasm, and nucleus, and are similar. The cell mem- 

 brane is a living structure. It provides some protec- 

 tion and support, especially to animal cells; more 

 important, it regulates the movement of materials 

 between the inner parts of the cell and the outer en- 

 vironment. Cytoplasm consists of essentially undif- 

 ferentiated protoplasm and specialized structures 

 that contribute to cellular metabolism. The nucleus 

 is the coordinating or directing center of all cell ac- 

 tivity, including reproduction, and contains the he- 

 reditary units (genes) that are usually borne on 

 bodies called chromosomes. 



The principal differences between plant and animal 

 cells (Figure 6.8) stem from three additional struc- 

 tures in plant cells: a cell wall, plastids, and a large 

 central vacuole in the cytoplasm. A cell wall is com- 

 posed of nonliving material, mostly cellulose; it is a 

 somewhat elastic but tough and strong, protective 

 and supportive, outer layer of the cell. This structure 

 is mostly impervious to the passage of materials; how- 

 ever, holes in the cell wall allow the free passage of 

 substances into and out of the cell. Plastids are often 



