390 LIFE : OUTLINES OF GENERAL BIOLOGY 



catalytic agent which is not an enzyme, namely, iron, which is 

 represented in all cells, and probably aids greatly in their oxidations. 



Perhaps it may be said that all this is too chemical, and not 

 biological enough. Yet the discovery of glutathione is of the highest 

 biological interest. Under certain conditions in the cells of plants 

 and animals this substance reacts with the oxygen of the air, and 

 is able to pass this on to bring about the oxidation of food-materials 

 on which the supply of energy depends. It is the presence of this 

 "oxygen-transporter" that makes combustion possible within the 

 living cell at a temperature much lower than that of the burning 

 candle. This was our problem, and part of the solution is summed up 

 in the term enzyme and part in the word glutathione. 



In 1925 Keilin discovered in many kinds of cells another sub- 

 stance that is capable of combining with the oxygen of the air and 

 transferring it to the protoplasm. But this time it is a pigment 

 to which has been given the name Cytochrome. It is allied to the 

 reddish pigment of the blood (haemoglobin) and it is significant that 

 its molecule contains iron. 



The Internal Pressure of the Cell.— Familiarity with 

 cellular structure is apt to dull us to the physiological problems it 

 presents, as in the multitude of units which retain their relative 

 independence even when closely integrated, which absorb and lose 

 fluids without necessarily becoming swollen or shrunken, and which 

 so often have a substantial strength, although they may consist 

 chiefly of water, as in the stem of a succulent plant. Minute com- 

 partments with delicate walls enclosing a complex mixture of 

 materials in a colloid state, the seat of diverse chemical reactions 

 which are often very rapid and intricate — microscopic systems with 

 no little differentiation, such as is expressed in nuclei, centrosomes, 

 and mitochondria — how do all these retain their stability? There 

 must be a constant and delicately adjusted balance of conflicting 

 strains and forces; and as an illustration of these we may take the 

 conditions regulating the internal pressure of the cell. 



In the first place, the contents of the cell have an osmotic pressure, 

 due to the fact that there is a strong solution in water of organic 

 substances and mineral salts. If we prepare a pot of porous clay, 

 and bring about within its pores a deposit of a layer of copper 

 ferrocyanide, we obtain a semi-permeable membrane; that is to say, 

 a membrane which will allow certain materials to pass through, but 

 not others. In the simple case we have chosen, the membrane will 

 allow water to pass through; but it will prevent the passage of, for 

 instance, molecules of cane sugar dissolved in the water. If the semi- 

 permeable pot of clay be now filled with sugar solution and immersed 

 in pure water, it is found that water enters the pot through the 

 walls, tending to dUute the solution within. The level of the liquid 



