48 Inside the Living Cell 



in chemical operations and these include the manufacture of all the 

 proteins which are necessary to the life of the cell. The cell thus not 

 only has a power station, but it is also continually manufacturing new 

 parts for its own use including the power station 'engines', besides 

 many other mechanisms. The main outcome of all the chemical 

 operations is thus to produce more of the instruments, such as 

 enzymes, which bring about these operations. It is this feature which 

 gives life its militant and aggressive character — living things are 

 always changing their environment, as far as they can, into more 

 living material. 



It is obvious that all this chemical activity requires a great deal of 

 organization. However, only very recently has much been learnt about 

 the structure of the cytoplasm in which many of these activities 

 occur, and the way in which the multiple chemical activities are 

 organized so that they are in harmony with each other. The fact 

 that a dead organism soon destroys itself by enzyme degradations 

 is a proof of the existence of co-ordinating factors in living cells. 



One of the first explorations of the constituents of cytoplasm was 

 carried out by Dr A. Claude.^ The walls of the cells were broken by 

 gentle rubbing processes in such a way as to liberate the contents 

 without damaging them severely. The different structures present in 

 the broken cell 'mush' could then be separated according to their size 

 and density by careful centrifuging at different speeds. 



The first structure to be deposited is the nucleus, together with cell 

 wall debris, which can be removed. Next come large granules from 

 the cytoplasm, known as mitochondria. Then at a still higher speed 

 we can separate submiscroscopic particles, known as microsomes. 

 This leaves a 'cell sap' containing some still smaller particles and also 

 proteins and compounds of proteins and nucleic acid, with molecular 

 weights up to a few millions. 



This structure of the cell has been confirmed by electron microscope 

 studies of cell sections. The cells are first 'fixed' with a staining 

 material of a heavy metal, osmium, which makes the cell structures 

 visible when the sections are examined in the electron microscope. 

 The cells are then impregnated with a plastic substance, which sets 

 hard and can be cut into very thin slices. Plates 5 and 6 are examples of 

 a cell section, which shows the main structures present. What we have 

 referred to as the microsome appears here as membranes lying 

 parallel with each other in pairs with small massive particles (sub- 

 microsomes) attached to their inner surfaces (see er in upper part 

 of Plate 7). Fig. 12 gives a diagrammatic representation of the con- 

 stitution of a typical cell according to these findings. 

 1 Earlier studies had been made bv R. R. Bensley and Robert Chambers. 



