398 LIFE : OUTLINES OF GENERAL BIOLOGY 



In summary, it may be pointed out that two contrasting pro- 

 cesses take place in the plant: (a) the transformation of complex 

 molecules into carbon dioxide and water, with consumption of 

 oxygen and evolution of energy; and {b) the predominating process 

 of tlie conversion of water and carbon dioxide into complex mole- 

 cules, with the evolution of oxygen and the consumption of the 

 energy of sunlight. In animal cells, except in one or two green 

 Protozoa, or in those which harbour symbiotic green Alga, the 

 latter process does not occur. Once a certain stage is reached, how- 

 ever, where syntheses take place unaccompanied by reduction, as 

 when sugars form glycogen in animals or starch in plants, or when 

 amino-acids form proteins, plant and animal cells become much 

 more alike in their properties. Some power of synthesis is character- 

 istic of all cells, a particularly important example being the synthesis 

 of enzymes, of which we know almost nothing save that it must occur. 



Summary on Cell-chemistry. — In what has preceded we have 

 briefly considered some of the fundamental properties of living 

 matter, such as its irritability and contractility, its power of 

 secreting and growing; and the central fact about the chemistry of 

 living matter is that it does not display these properties unless it 

 is in a partially oxidised condition. In typical cells this condition 

 is brought about by contact of the protoplasm with oxygen; and 

 there are constituents of the protoplasm which are particularly 

 able to unite with molecular oxygen and to pass it on to less readily 

 oxidised constituents. The final products of the oxidation of organic 

 compounds are carbon dioxide, water, and simple nitrogenous sub- 

 stances. To replace the materials used up in these reactions animals 

 ingest food and green plants synthesise organic compounds from 

 carbon dioxide and water, using the energy of sunlight in so doing. 

 Processes of building up and breaking down of organic molecules 

 go on in the cell, partly by way of preparation for the energy- 

 yielding oxidation reactions, partly to form specific substances 

 which the cell makes use of either as reagents (enzymes, pigments, 

 glutathione, and so on), or to repair or enlarge its own structure in 

 growth and reproduction. 



There are, indeed, living cells which flourish in the absence of 

 oxygen. In the higher animals which have adapted themselves to 

 this mode of existence the essential reactions of the cell are incom- 

 plete combustions of organic material. Among anaerobic bacteria 

 are those which are concerned chiefly with nitrogen, with sulphur, 

 and with iron. Most important are those symbions which form 

 nodules on the roots of leguminous plants and are able to convert 

 the gaseous nitrogen of the air into organic nitrogen-containing 

 compounds which the plant can make use of. In these cases it may 

 be difficult to demonstrate that the energy of the cell is derived 

 from oxidations, though in the widest chemical sense of that 



