Grafts et al. — 66 — Water in Plants 



Furthermore, account should be taken of the fact that the water content 

 may vary between wide limits. Some evidence suggests that changes of 

 physiological significance occur only at certain critical contents. For in- 

 stance Sponsler, Bath and Ellis (1940) state that the distance between 

 polypeptide chains of gelatin increases markedly at 33 per cent but remains 

 constant from that water content up to one of about 90 per cent. Prob- 

 ably most remarkable is the fact that the thin layer of protoplasm of a 

 highly vacuolated cell may control the movement of salts and hence of 

 water against gradients of a hundred fold or more where the protoplasm 

 itself is so highly hydrated. 



Chemically, protoplasm is a heterogeneous mixture of water, proteins, 

 phosphatides, lipids, sugar, salts, and other compounds. Protein makes up 

 about two-thirds of the non-aqueous substances. 



Proteins are composed of amino acids united through peptide linkages. 

 A peptide chain may contain several hundred amino acid residues arranged 

 somewhat as follows : 



R' H O R"' H O 



I I II I I ii 

 C NHC C NHC 



/|\/\|/\/|\/\l/\/ 



HC C NHC C N 



O R" H O R'^ H 



In the protoplasm these chains are probably coiled or folded into vari- 

 ous shapes and sizes. When spread in a film they are stretched out more 

 nearly straight. The R residues consist of, or bear, both hydrophihc and 

 hydrophobic groups. Oxygen and nitrogen atoms coordinate water by 

 virtue of their ability to form hydrogen bonds. Each oxygen may hold 2 

 water molecules, and the NH groups a like number. OH and NH2 groups 

 potentially hold three molecules each and COOH groups four. Further- 

 more, layers of water molecules may form bridges between the backbones 

 of adjacent polypeptide chains to produce a vein of strongly bonded water 

 separating the two chains by the width of a water molecule. Hydration 

 centers on the side chains are fewer and attractive forces somewhat weaker 

 than on the backbone. 



From x-ray data on gelatin (Sponsler, Bath and Ellis, 1940), the 

 spacing between backbone layers is constant at 4.4 A up until a hydration 

 of about 33 per cent is reached, at which time the spacing increases to 7.0 A, 

 which holds up to a water content of 90 per cent. Other properties of 

 proteins that change as water content passes through the critical range of 

 30 to 35 per cent are heat of imbibition, imbibition pressure, freezing 

 point, expansion upon freezing, etc. When less than 30 per cent water is 

 present in proteins the freedom of movement of water molecules is appar- 

 ently restricted. 



There is speculation as to the grouping of the chains of a protein, the 

 shape of the particles or fibrils formed, the degree of continuity, and the 

 forces holding them together. Sponsler (1940) and Sponsler and Bath 

 (1942) picture an enormous number of molecular and submicroscopic par- 

 ticles to be present in protoplasm, grading in size up to visibility. These 

 are composed mostly of protein displaying various shapes, monolayers, 

 packets, etc. The chain is the fundamental unit of structure just as in 

 cellulose, the principal difference being that the protein molecule is looped 

 or folded. These units are aggregated into (a) super molecules, small 



