CELLS AND TISSUES 53 



fluid is said to be hypotonic to the cell. A solution of 0.9 per cent 

 sodium chloride, 0.9 gm. per 100 ml. of water, soinetimes loosely called 

 "physiological saline," is isotonic to human cells. 



A cell placed in a solution that is not isotonic with it may adjust 

 to the changed environment by undergoing a change in its water con- 

 tent, so that it eventually achieves the same concentration of solutes as 

 in the environment. Many cells have the ability to pump water or 

 certain solute molecules into or out of the cell and in this way can 

 maintain an osmotic pressure that differs from that of the surrounding 

 medium. Amebae, paramecia and other protozoa that live in pond water, 

 which is very hypotonic to their intracellular fluid, have evolved con- 

 tractile vacuoles (Fig. 3.1) which collect water from the protoplasm and 

 pump it to the outside. Without such a mechanism the cells would 

 quickly burst from the water entering the cell. 



The power of certain cells to accumulate selectively certain kinds of 

 molecules from the environmental fluid is truly phenomenal. Human 

 cells (and those of vertebrates in general) can accumulate amino acids 

 so that the concentration within the cell is 2 to 50 times that in the 

 extracellular fluid. Cells also have a much higher concentration of po- 

 tassium and magnesium, and a lower concentration of sodium, than the 

 environmental fluids. Certain primitive chordates, the tunicates (p. 384), 

 can accumulate vanadium so that the concentration inside the cell is 

 some 2,000,000 times that in the surrounding sea water, and sea weeds 

 have a comparable ability to accumulate iodine. The transfer of water 

 or of solutes in or out of the cell against a concentration gradient is 

 physical work and requires the expenditure of energy. Some active 

 physiologic process is required to perform these transfers, hence a cell 

 can move molecules against a giadient only as long as it is alive. If a 

 cell is treated with some metabolic poison, such as cyanide, it quickly 

 loses its ability to maintain concentration differences on the two sides 

 of its plasma membrane. 



17. Tissues 



In the evolution of both plants and animals one of the major trends 

 has been toward the structural and functional specialization of cells. 

 The cells which comprise the body of one of the higher animals are not 

 all alike, but are differentiated and specialized to perform certain func- 

 tions more efficiently than an unspecialized animal body could. This 

 specialization has also had the effect of making the several parts of the 

 body interdependent, so that an injury to, or the destruction of, cells in 

 one part of the body may result in the death of the whole organism. 

 The advantages of specialization are so great that they more than out- 

 weigh the disadvantages. The cells of the body which are similarly spe- 

 cialized are known as a tissue. A tissue may be defined as a group or 

 layer of similarly specialized cells which together perform certain special 

 functions. The study of the structure and arrangement of tissues is 

 known as histology. Each tissue is composed of cells which have a 

 characteristic shape, size and arrangement; the different types of tissue 



