PHYSICOCHEMICAL AND CHEMICAL PHASES 



167 



the process of diffusion are such that materi- 

 al scattered ahiiost in "trace" concentrations 

 will steadily diffuse into the environmental 

 niches from which they are being with- 

 drawn by organisms. Sea water contains a 

 number of chemicals in extremely dilute so- 

 lutions; of these, silicon is normally present 

 in amounts ranging from 0.02 to 4.0 mg. 

 per kilogram of sea w^ater, and is extracted 

 from this dilute concentration by diatom? 

 and various sponges, among other organ- 

 isms. Glass sponges (Hexactinellida) give 

 interesting examples of animals that form 

 extensive and heavy skeletons from silicon 

 hydrate. These sponges reach their greatest 

 development at the bottom of the ocean 

 in depths from 500 to 1000 meters. They 

 commonly grow to be from 10 to 30 

 cm. long and may even reach a length of 

 a meter. A museum specimen of Eiiplectella, 

 23 cm. long weighs 5.3 gm. Diffusion must 

 be an important factor in this large concen- 

 tration of siHcon. 



Copper, present in sea water in quanti- 

 ties of between 0.001 to 0.01 mg. per kilo- 

 gram is concentrated to become an impor- 

 tant constituent in hemocyanin, the respira- 

 tory pigment of Limidus and of many other 

 marine invertebrates. Similarly, iron, an im- 

 portant part of hemoglobin, the respiratory 

 pigment in fishes and other vertebrates and 



Table 11. The Relative Concentration of Differ- 

 ent Ions in Pond Water and in Nitella in 

 Millimoles* 



' A "mole" or molar solution of any sub- 

 stance equals the number of grams correspond- 

 ing to the molecular weight of the substance, 

 dissolved in distilled water, and made up to a 

 liter of solution. A normal solution contains 1 

 grammolecular weight of solute divided by its 

 hydrogen equivalent and made up to a liter of 

 solution; a normal solution contains 1 gram- 

 equivalent of the solute in 1 liter of solution. 



in the red-blooded marine invertebrates, 

 must be concentrated from dilutions in sea 

 water of the same order of magnitude as 

 copper. It is not meant that the indi\idual 

 animal necessarily concentrates such dilute 

 materials from the sea directly, but at some 

 stage in the food chain, the concentration is 

 accomphshed, and diffusion plays a large 

 and vital role whenever it happens. More 

 striking instances are known. 



Nitella is an alga growing in fresh water. 

 The concentrations of different ions in pond 

 water and in Nitella are shown in Table 11. 

 Nitella concentrates CI" some 100 times 

 and H2PO4- 1800 times the density found 

 in pond water. It is able to do so because 

 the ions continue to diffuse toward the 

 point of reduced density produced by the 

 action of the alga (data from Krogh, 1939). 

 These concentrations are carried on directly 

 by Nitella; they are not automatic, since 

 NO3" is excluded, and the other ions are 

 taken up in differing proportions. 



OSMOSIS 



Living organisms are surrounded and 

 subdivided by membranes through which 

 substances must pass when there is an ef- 

 fective exchange of materials between or- 

 ganism and environment. Diffusion through 

 membranes is known as osmosis, though 

 usually in osmosis membranes are thought 

 of as being somewhat semipermeable. It is, 

 in part, an ecological process (cf. Shelford, 

 1929; Pearse, 1939). Various other phases 

 of osmosis are treated extensively in books 

 on physical chemistry and on general phys- 

 iology, and these should be consulted for 

 many details and refinements that cannot be 

 included here. 



A given membrane or set of membranes 

 (forming the surface of an organism) may 

 1)6 impermeable or almost completely per- 

 meable, or there mav be one or more com- 

 binations of a great variety of partial 

 permeabilities. When a membrane is per- 

 meable to water and approximately imper- 

 meable to solutes, it is said to be semiper- 

 meable. Completely semipermeable mem- 

 branes probably exist only in theory. If the 

 concentration of solutes is unequal on the 

 two sides of a so-called semipermeable 

 membrane, water passes through to the 

 more concentrated solution (the one in 

 which there is less water per unit volume) 

 until equihbrium is established. The pres- 



