50 



LIFE: ITS BEGINNINGS AND NATURE 



one another and so the entire emulsion has 

 a more or less fluid consistency ( Fig. 2-20 ) . 

 In the half-way or critical condition the 

 soap molecules act like those in a solid 

 state, so that any change in shape of the 

 emulsion must involve the rending and tear- 

 ing of the soap film. In other words, the en- 

 tire mass holds its shape; fragile though it 

 is, it acts like a jelly. In protoplasm a sim- 



5ol 



Gel 



Fig. 2-20. A schematic explanation of how sois and gels 

 form. In a sol the elongated molecules flow smoothly 

 past one another in a more or less fluid state. In a 

 gel a latticework effect is produced by the gelation 

 of the emulsion. Such a physical arrangement of the 

 particles produces a semi-solid material. 



ilar situation exists; here, the large elongate 

 protein and polysaccharide molecules tend 

 to interlock, forming a cotton-like meshwork 

 in which the water and crystalloidal com- 

 ponents are trapped. This is called gelation, 

 and the resulting emulsion is a gel. The 

 emulsion can quickly change from the gel 

 to the fluid or sol condition under the vary- 

 ing factors of the protoplasm itself. For ex- 

 ample, such factors as hydrogen ion concen- 



tration or temperature directly determine 

 the condition of the protoplasmic emulsion, 

 and metabolic products constantly change, 

 the condition of the emulsion changing 

 with them. 



Myosin, the protein of muscle tissue, be- 

 haves like a gel, which is probably respon- 

 sible for its ability to contract. In such a gel 

 the large interlinked molecules can bring 

 about contraction by folding upon them- 

 selves or upon other particles. The chemical 

 and physical changes that go on in this proc- 

 ess have been learned only recently, and 

 they mark a milestone in the study of cellu- 

 lar physiology. 



The protruding pseudopod of the amoeba 

 is a result of a phase reversal from gel to 

 sol; the clotting of blood is the opposite 

 reaction, from sol to gel. Such changes are 

 going on continually in protoplasm; it is 

 hoped that the above discussion may give a 

 little better understanding of how it occurs. 



Limiting membranes 



Protoplasm is confined within containers 

 which are composed of molecules similar to 

 those found in the rest of the emulsion, al- 

 though they have different properties. They 

 have the composition of a gel, that is, they 

 are semi-rigid in order to contain the more 

 fluid material within. Furthermore, they al- 

 low certain substances to pass through them 

 in both directions in order that material 

 essential for life can enter and leave. These 

 membranes are thus selectively permeable. 

 Let us see, at least in part, how this selec- 

 tive permeability is accomplished. 



The large protein and lipid molecules 

 float freely unless they are forced to gel by 

 the relative proportions of calcium and po- 

 tassium ions present within the protoplasm 

 and the outside fluid world. The propor- 

 tions of these ions are just right in the nor- 

 mal environment of a cell because they 

 bring about the formation of the gelatinous 

 membrane. Any change in the concentra- 

 tions of these ions, either within or without, 

 drastically affects the membrane. For exam- 



