386 LIFE : OUTLINES OF GENERAL BIOLOGY 



plants and another lor animals, while the protein with which it is 

 combined is different in every species of organism. 



The acid dyes— eosin, orange G, picric acid, and so on— combine 

 with the fixed cytoplasm, while others of the group (methyl blue, 

 acid fuclisin) have a special affinity for cytoplasm which has become 

 toughened in the specialisation of connective cells. In some cases 

 fixation is as informative as staining, not that this is saying very 

 much. Thus osmic acid blackens and alcohol dissolves fat-like sub- 

 stances. On this is based the conclusion that both mitochondria 

 and the Golgi apparatus consist of compounds of lipoids with 

 proteins. 



CHEMICAL PROCESSES WITHIN THE CELL 



In the previous section account has been taken of the chemical 

 vuiterials — the proteins, carbohydrates, fats, and so on, which 

 are present in the cell; the question now rises: What chtmical 

 changes are of common occurrence? All cells are laboratories in 

 which chemical reactions supply energy, which may do work as 

 in muscular activity, or may produce "animal heat", or may 

 facilitate some other associated chemical reaction. In nearly 

 all cells the energ}'-yielding reactions are of the nature of com- 

 bustions or oxidations, as Lavoisier first made clear when he empha- 

 sised the analogy between the life of an animal and the burning of 

 a candle. When organic carbon compounds are burned, most of 

 the carbon combines with oxygen, and carbon dioxide results ; most 

 of the hydrogen combines with oxygen, and water results; but 

 nitrogen, if it is present, as in proteins, is not easily oxidised; it 

 sometimes takes the form of ammonia, as in the burning of feathers. 

 Similar changes go on in the living cell. 



An organic molecule is said to be oxidised when oxygen is added 

 to it, even if the process does not go so far as its complete "burn- 

 ing away". If oxygen is taken from the organic molecule, it is said 

 to be reduced. In these processes there is an evident antithesis 

 between oxygen and hydrogen, for if hydrogen is added to a mole- 

 cule, it will require more oxygen for complete combustion, while if 

 hydrogen is taken away less oxygen will be required. Therefore the 

 removal of hydrogen is also called oxidation, and the addition of 

 hydrogen is called reduction, even if no oxygen enters into the 

 reaction at all. 



The student must be patient with the recall of these elementary 

 facts, for they lead to an important idea in the understanding of 

 the biochemistry of the cell, namely the ptTpetual tug-of-war 

 between oxygen and hydrogen, between oxidation and reduction — a 

 tug-of-war whose result depends on the relative strength of the two 

 sides. An oxidising agent is one that will readily give up oxygen, or 



