ACTIVITY 7 



is a change in the external environment, or it may occur without 

 apparent cause. The first, examples of which are the shaking of 

 the head of a dog when the hairs in its ear are touched and the 

 folding and falling of the leaf of a sensitive plant when it is 

 knocked, has been called irritabihty, and the second, which ma\- 

 be illustrated by the beating of the heart, automatism. In man\- 

 cases a distinction is difficult. Irritabihty differs from mere 

 mechanical change produced by external circumstances, such as 

 the melting of ice, in that the magnitude of the response, as 

 measured by the energy involved, bears no relation to the 

 magnitude of the stimulus. It follows from this, and from the 

 first law of thermodynamics, that the energy for the response 

 must come from within the organism. 



ABSORPTION AND ASSIMILATION 



Growth requires the intake of new material, and response 

 needs energy, which depends on the breakdown or conversion of 

 chemical substances. The incorporation of food is therefore 

 characteristic of living matter. Two distinct processes may be 

 recognised in incorporation — absorption and assimilation. Before 

 it can be absorbed the food of animals has generally to undergo 

 a preliminary process of digestion, whereby solid or indiffusible 

 nutriment which it contains is made soluble and diffusible. The 

 food must always contain the following materials : (i) water, 

 which is of the highest importance both as an essential con- 

 stituent of the living matter (protoplasm) and also because it is 

 used in the body for transporting substances in solution, as in 

 the blood and urine, (2) certain inorganic ions, such as chloride 

 and phosphate and those of sodium, potassium, and calcium, 

 (3) the very complex compounds known as proteins. A protein 

 is a colloid substance consisting of carbon, hydrogen, oxygen, 

 and nitrogen, with sometimes small quantities of sulphur and 

 phosphorus. A familiar example is the albumen which, mixed 

 with water, forms white of egg. Proteins are very complex 

 linkages of amino-acids, that is, compounds which contain both 

 the basic radicle — NHg and the acid group -COOH. A simple 

 example is aminoacetic acid or glycine, CHo.NHg.COOH. Thus 

 in the complicated chemistry of the body proteins are able to 

 exercise the power, which amino-acids have, of uniting cither 

 with acids or with bases ; and on final disintegration they always 



