Lecture II. 17 



a very large molecular weight may be built up. The formation 

 and dissolution of this linkage, which is called the peptide link- 

 age, possibly indicates one of the frequent changes occurring in 

 the metabolism of protoplasm. 



The other substances such as lipoids (fat-like bodies), carbo- 

 hydrates and salts are probably present in the protoplasm not as 

 essential constituents but as stores from which the much more 

 complex amino-acids and proteins are built up and renewed, and 

 from which, as we shall see later, energy is derived. 



The protoplasm of cells is continually incorporating external 

 materials in its substance and eliminating substances from itself 

 into its surroundings. It is constantly building itself up and at the 

 same time disintegrating, and, as we have seen, the compounds 

 within its own substance continue to interact. As long as these 

 changes continue the protoplasm is "alive". In fact, protoplasm 

 is the living part of the cell and the phenomena exhibited by 

 protoplasm are life. It has been called the " physical basis of 

 life ". When protoplasm is killed, e.g. by heat, constructive 

 metabolism comes to an end. With certain exceptions its 

 external and internal reactions cease, and it becomes an inert 

 mass of proteins, etc. It is in this form often called proteid, and 

 as we have seen it both loses its liquid nature and becomes 

 permeable. 



PRACTICAL WORK. 



Mount a small piece of the skin of the leaf of Tradescantia virginiana in a 

 small drop of water. Put on a cover- glass. If the space under the cover- 

 glass is filled with the drop draw off a little water by applying a piece of 

 blotting-paper to the edge of the cover. Now apply a drop of liquor iodi and 

 let it run in under the cover. See that the upper surface of the cover-glass 

 is quite clean and place the slide on the stage of the microscope. Observe 

 how the cells of the tissue change in appearance, and those first which are 

 at the edge of the specimen. The protoplasm takes on a brownish hue and 

 minute granules become apparent in it. The nucleus stands out clearly. 

 The iodine in solution has permeated the cell-walls and brought about these 

 changes within the cell. 



Mount a thin section of beetroot, or a thin flap cut from the crimson petal 

 of a rhododendron flower, in a small drop of water. Make a sketch, using 

 the high power, of a group of three or four uninjured cells near the edge of 

 the specimen. 



Focus for the middle of one of the cells, and neglecting the hazy shadows 

 of the parts above and below this level, sketch just what you see clearly 

 defined. This is an "optical section" of the cell. The wall will appear as 

 a very narrow band running round the cell defined by a fine line on the 

 outside and inside. The area enclosed by this band at first sight appears 

 uniformly rose coloured, but very careful observation will show that there is 

 an exceedingly inconspicuous layer of material the protoplasm between the 



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